Green Leaf Volatiles in the Atmosphere—Properties, Transformation, and Significance

Abstract

This review thoroughly covers the research on green leaf volatiles (GLV) in the context of atmospheric chemistry. It briefly takes on the GLV sources, in-plant synthesis, and emission inventory data. The discussion of properties includes GLV solubility in aqueous systems, Henry’s constants, partition coefficients, and UV spectra. The mechanisms of gas-phase reactions of GLV with OH, NO₃, and Cl radicals, and O₃ are explained and accompanied by a catalog of products identified experimentally. The rate constants of gas-phase reactions are collected in tables with brief descriptions of corresponding experiments. A similar presentation covers the aqueous-phase reactions of GLV. The review of multiphase and heterogeneous transformations of GLV covers the smog-chamber experiments, products identified therein, along with their yields and the yields of secondary organic aerosols (SOA) formed, if any. The components of ambient SOA linked to GLV are briefly presented. This review recognized GLV as atmospheric trace compounds that reside primarily in the gas phase but did not exclude their transformation in atmospheric waters. GLV have a proven potential to be a source of SOA with a global burden of 0.6 to 1 Tg yr⁻¹ (estimated jointly for (Z)-hexen-1-ol, (Z)-3-hexenal, and 2-methyl-3-buten-2-ol), 0.03 Tg yr⁻¹ from switch grass cultivation for biofuels, and 0.05 Tg yr⁻¹ from grass mowing.

Contents

1. Introduction	2
2. Sources of GLV	6
2.1. Reasons or Causes of Emissions	6
2.2. Synthesis in Plants	6
2.3. Emission Data	8
3. Physical Properties of GLV	15
3.1. Solubility in Water and Partition Coefficients	15
3.2. UV Spectra	21
4. Gas-Phase Chemistry	27
4.1. Products and Mechanisms	27
4.1.1. Reactions with OH	28
4.1.2. Reactions with NO3	35
4.1.3. Reactions with O3	38
4.1.4. Reactions with Cl	44
4.1.5. Gas-Phase Photolysis	51
4.2. Gas-Phase Kinetics	52
4.2.1. Gas-Phase Reactions with OH Radicals	53
4.2.2 Gas-Phase Reactions with NO3 Radicals	58
4.2.3. Gas-Phase Reactions with O3	61
4.2.4. Gas-Phase Reactions with Cl Radicals	64
4.2.5. Gas-Phase Photolysis	68
4.2.6. SAR and LFPR Methods	69
5. Aqueous-Phase Kinetic	69
6. Multiphase and Heterogeneous Transformation	72
6.1. Smog-Chamber Studies	72
6.2. Ambient Aerosols	83
6.3. Aqueous and Multiphase Mechanisms	83
7. Atmospheric Impact of GLV	89
7.1. Atmospheric Lifetimes of GLV	89
7.2. SOA Potential of GLV	93
8. Conclusions	94
Abbreviations	95
References	96

1. Introduction

Green leaf volatiles (GLV)—C₅ and C₆ aldehydes, alcohols, ketones, and esters—belong to inducible volatile organic compounds synthesized and emitted by green plants in response to mechanical wounding, herbivore attack, or abiotic stress [1,2,3,4,5,6,7,8]. They are synthesized rapidly via the lipoxygenase (LOX) pathway within minutes or seconds and can alert other plants fostering interplant communication [1,9]. Furthermore, they are responsible for the “green odor” of green leaves [10]. The first burst of volatiles often occurs when the leaves and stems are cut, followed by more intense and hours-long emissions from drying material [11]. Together with GLV, wounded plants emit methanol, acetaldehyde, acetone, butanone, and formaldehyde [12]. Plants also emit GLV upon the light-dark transition [13] and in high-light exposure [14].

The GLV include (Z)-3-hexenol, (Z)-3-hexenyl acetate, (E)-2-hexenal, (Z)-2-hexen-1-ol, (Z)-2-penten-1-ol and 1-penten-3-ol [12,13,15,16,17,18,19,20]. Jasmonic acid (JA), methyl jasmonate (MeJa), methyl salicylate (MeSa), and 2-methyl-3-butene-2-ol (MBO) also count as GLV [21,22]. However, they formally belong to other groups of plant volatiles [23,24].

Table 1. Key Green Leaf Volatiles detected in plant emissions ^a.

Name	Chemical Formula	MW g mol−1	Structure	References
pentan-1-ol	C5H12O	88.15		[9,26]
1-penten-3-ol	C5H10O	86.13		[9,16,18,26,27,28,29,30]
(Z)-2-penten-1-ol	C5H10O	86.13		[26,29,30,31]
(E)-2-pentenol	C5H10O	86.13		[28]
(E)-2-pentenal	C5H8O	84.12		[28,30]
1-penten-3-one	C5H8O	84.12		[9,16,27,28,29,30]
(Z)-2-pentenyl acetate	C7H12O2	128.17		[31]
n-hexan-1-ol	C6H14O	102.16		[16,26,28,32,33]
(E)-2-hexen-1-ol	C6H12O	100.16		[26,28,32]
(Z)-2-hexen-1-ol	C6H12O	100.16		[18]
(E)-3-hexen-1-ol	C6H12O	100.16		[28,32]
(Z)-3-hexen-1-ol (leaf alcohol)	C6H12O	100.16		[9,10,12,13,16,18,26,28,31,32,34,35,36,37]
n-hexan-1-al	C6H12O	100.16		[3,18,26,28,30,32,33,34,36,38]
(Z)-3-hexen-1-al	C6H10O	98.14		[9,12,13,18,26,28,32,33,34,37,38,39]
(E)-2-hexen-1-al (leaf aldehyde)	C6H10O	98.14		[9,10,18,26,28,31,32,33,35,36,38]
(Z)-2-hexenal	C6H10O	98.14		[16,28]
(E)-3-hexenal	C6H10O	98.14		[28,32,37]
hexyl acetate	C8H16O2	144.21		[33,37,40,41]
(Z)-2-hexenyl acetate	C8H14O2	142.20		[18]
(E)-2-hexenyl acetate	C8H14O2	142.20		[28,37]
(Z)-3-hexenyl acetate (leaf acetate)	C8H14O2	142.20		[12,13,16,18,26,28,31,32,34,37]
(Z)-3-hexenyl-propionate	C9H16O2	156.22		[28,30,42]
(Z)-3-hexenyl butanoate	C10H18O2	170.25		[30,37,40,41,42,43]
(Z)-3-hexenyl isobutanoate (isobutyrate)	C10H18O2	170.25		[30,40,42]
(E)-3-hexenyl butanoate	C10H18O2	170.25		[41]
(E)-2-hexenyl butanoate	C10H18O2	170.25		[37,42]
(Z)-3-hexenyl isopentanate	C11H20O2	184.27		[40,42,44]
(Z)-3-hexenyl 2-methyl-2-butenoate	C11H18O2	182.26		[40,42,44]
3-hexenyl hexanoate b	C12H22O2	198.3		[41]
2-methy-3-buten-2-ol	C5H10O	86.13		[45,46,47]
1-octen-3-ol	C8H16O	128.21		[18,28,48]
nonanal	C9H18O	142.24		[44]
jasmonic acid	C12H18O3	210.27		[26,39,49]
methyl jasmonate	C13H20O3	224.30		[26,49]
methyl salicylate	C8H8O3	152.15		[7,30,40]

^a for convenience of the audience, we used traditional GLV names rather than the latest IUPAC recommendations; ^b unspecified isomer.

lists the compounds we included in this review as GLV (Table 1). Approximate yearly defense-initiated and cutting-drying emission of GLV in North America ranges from 1.5 to 2.6 Tg C yr⁻¹ with estimated rates of 0.1 to 0.2 µg-C g⁻¹ h⁻¹ [25].

The global annual emission of all BVOC reached 1087 Tg yr⁻¹ estimated using the Community Land Model (CLM4) integrated with the MEGAN2.1 framework for estimating fluxes of biogenic compounds between terrestrial ecosystems and the atmosphere [50]. Isoprene was 535 Tg yr⁻¹, monoterpenes — 162 Tg yr⁻¹, and other compounds—390 Tg yr⁻¹. The modeling did not treat GLV as a separate group of compounds. It included some GLV in the stress VOC group (3-hexenal, 2-hexenal, 3-hexenol, 3-hexenyl acetate, hexanal, 1-hexenol, MeJa, and MeSa) and considered MBO individually. The emission of (Z)-3-hexenal was estimated as 4.9 Tg yr⁻¹, of (Z)-3-hexenol as 2.9 Tg yr⁻¹, and MBO as 2.2 Tg yr⁻¹. The annual emission of MBO calculated with MEGAN and averaged throughout 1980–2010 was 1.6 ± 0.1 Tg yr⁻¹ [51].

Although the global emission of GLV is relatively small, the significance of local and seasonal emissions of GLV due to such events as harvests or grass mowing can be high for local air quality and SOA burden.

The emission budget of GLV can increase further if the GLV treatments become introduced to the world of agriculture, horticulture, and forestry. Prospective solutions are being investigated and discussed. Treatment of tomato with (Z)-3-hexenol prepared it for better defense against Tomato yellow leaf curl virus transmitted by whitefly [52]. Spraying with (Z)-3-hexenyl acetate reduced the salt stress in peanuts by a complex genetic mechanism increasing the photosynthetic rate, plant height, and shoot biomass [53]. Post-harvest fumigation of strawberries with (E)-2-hexenal and (Z)-3-hexenal decreased the mold infection rate [54]. Maize seedlings treated with physiological amounts of (Z)-3-hexenyl acetate showed less damage from cold stress and increased growth compared to untreated seedlings [55]. (Z)-3-hexenyl butyrate successfully modified the pre-harvest ripening of grapes [56].

Once in the atmosphere, GLV immediately encounter several reactants like OH and NO₃ radicals or O₃, which are ready to interact with unsaturated carbonyl compounds and contribute to the formation of secondary organic aerosols (SOAs) [57,58].

The modeling, which is the only way to estimate the global production and burden of SOA, provides mixed results. The intercomparison of 31 global chemistry-transport and general circulation models showed that the global annual output of SOAs varied from 13 to 121 Tg yr⁻¹, with a median value of 19 Tg yr⁻¹ [59]. For the models which considered the semi-volatile character of SOAs, the global annual output of the SOAs varied from 16 to 121 Tg yr⁻¹ with a much higher median of 51 Tg yr⁻¹. The STOCHEM model predicted 46.4 Tg yr⁻¹ of SOAs, formed globally from sesquiterpenes (29 Tg yr⁻¹) and monoterpenes (162 Tg yr⁻¹) [60]. The corresponding SOA burden was 0.45 Tg. Approximately 0.010 Tg (2%) originated from isoprene, 0.188 Tg (44%) from monoterpenes, 0.187 Tg (41%) from both monoterpenes and sesquiterpenes, and 0.041 Tg (10%) from sesquiterpenes alone. Those results are consistent with GEOS-CHEM modeling based on the volatility basis set, which predicted 36.2 Tg yr⁻¹ and 0.88 Tg of SOAs, including 21.5 Tg yr⁻¹ and 0.53 Tg of biogenic SOA; as well as 14.7 Tg yr⁻¹ and 0.35 Tg of anthropogenic and biomass burning SOAs [61]. The SOA production increased when modeling covered: the semi- and intermediately volatile OC; new SOA yields from chamber experiments; wet and dry deposition of organic vapors; and the photolytic and heterogeneous loss of SOAs. The new production of 132.2 Tg yr⁻¹ included 8.8 Tg yr⁻¹ from anthropogenic and biomass burning emissions, 97.5 Tg yr⁻¹ from biogenic emissions, and 25.9 Tg yr⁻¹ from semi- and intermediately volatile OC emissions. However, the SOA burdens have not changed and included 0.08, 0.59, and 0.21 Tg, respectively. Gas-phase processing of isoprene alone was estimated to contribute 6–30 Tg yr⁻¹ of SOAs [62] with an additional 2 Tg yr⁻¹ from aqueous-phase processing [63]. The CESM model with CAM5.3 and CLM4 components predicted 55.7 Tg yr⁻¹ of isoprene SOA (1.07 Tg burden) for climate and emission conditions of 2000 [64]. Against all the quantities mentioned above, a less formal prediction of 1–5 Tg C yr⁻¹ of SOA produced from GLV [65] seems overestimated and needs re-evaluation.

Major reviews published in the last decade provide little information on a GLV role in atmospheric chemistry and SOA formation. A special issue of Trends in Plant Science (“Induced plant volatiles: from genes to climate change”) [66] discussed the BVOC emissions and their general relations with the environment with a focus on emissions [8,67,68]. The review on molecular identification of organic compounds in the atmosphere [69] mentioned the possibility of SOA formation from GLV based on four references. The extensive evaluation of the tropospheric aqueous-phase chemistry [70] included the rate constants for the aqueous-phase reactions with a few GLV, based on one work. The review on the atmospheric chemistry of oxygenated volatile compounds [71] provided the lifetimes of a few GLV due to the gas-phase reactions with OH, based on one reference. The opinion piece on plant-derived secondary organic material in the air [72] mentioned GLV as SOA precursors but did not go into any details.

The purpose of this review is to provide a practical background framework for designing future research on the role of GLV in atmospheric processes and specifically in the formation of SOAs.

2. Sources of GLV

2.1. Reasons or Causes of Emission

Plants release GLV when attacked by animals, insects, and microbes or exposed to abiotic stress or mechanical wounding [16,27,35,43,48,73,74]. For instance, trees and shrubs emitted (E)-2-hexenal upon wounding and in response to bacterial pathogenesis [6]. Emissions of Pinus sylvestris saplings infected with large pine aphid (Cinara pinea Mordviko) contained methyl salicylate and (Z)-3-hexenyl acetate [75]. Plants infected with pathogens probably emit methyl salicylate to activate disease resistance in neighboring plants and healthy tissues of the emitting plant [7]. Blande et al. [44] detected significant emission of MeSa from Betula pendula and Alnus glutinosa trees infested with Euceraphis betulae (Koch.) or Pterocallis alni (De Geer) aphids. The emission of volatiles depended quantitatively on the stress dose for both abiotic and biotic stressors [76].

Plants stimulated with GLV rapidly produced jasmonic acid (JA) and emitted anti-herbivore volatiles [34]. Plants pre-treated with GLV had more JA and sesquiterpenes when attacked by herbivores. The released GLV can inform the neighboring plant ecosystem of possible danger and prompt it to prepare its defense [4,5,6,34,77]. Experiments showed that intact undamaged corn seedlings produced JA and emitted sesquiterpenes when treated with GLV [34]. The GLV-primed defense initially suppresses the growth of plants, but after a few days, the growth significantly increases [78].

GLV can attract carnivores or parasitoids of herbivores that attack plants [79]. For instance, (Z)-3-hexenal attracted a chalcidoic wasp Encarsia formosa, a natural parasitoid of whitefly Bemisia tabaci, a herbivore pest of tomato plants [80]. Products of GLV reactions with ozone in the atmosphere also attract predators [81].

GLV emitted by plant seedlings can decrease their acceptability by herbivores (e.g., snails) [82]. Zhang et al. [41] showed that Ips typographus antennae responded strongly to green leaf alcohols (Z)-3-hexen-1-ol, 1-hexanol, and (E)-2-hexen-1-ol emitted by non-host plants such as Betula pendula, B. pubescens, and Populus tremula. Thus, the purpose of emission may be to direct the host-searching insect away.

(Z)-3-hexenyl butyrate controls the stomata closure to regulate CO₂ and water vapor transfer under abiotic stress like drought [56].

(Z)-3-hexenol, (Z)-3-hexenal, and (Z)-3-hexenyl acetate were also emitted by velvet mosquito plants upon the light-dark transition, probably due to the dark activation of 13-lipoxygenase enzymes in chloroplasts [13].

2.2. Synthesis in Plants

The GLV listed in Table 1 differ in the pathways by which they are synthesized [23,83]. Compounds traditionally considered to be GLV—C₆ aldehydes, alcohols, ketones, and esters, along with C₅ compounds and MeJa—are produced via the LOX pathway. MBO is a hemiterpene synthesized by the mevalonate or DMAPP pathway [24]. MeSa is an aromatic compound synthesized by the shikimic acid pathway [23]. (Z)-3-hexenyl-propionate, (Z)-3-hexenyl butyrate, and (Z)-3-hexenyl isobutyrate are esters probably synthesized from (Z)-3-hexenol by AAT [30,74].

The LOX pathway converts free fatty acids (linolenic, hexadecatrienoic, and linoleic acid) into C₆ GLV. Scheme 1 shows the reactions for linolenic acid [1,20,35,73,84]. This acid is oxidized to 13-hydroperoxodecatrienoic acid by molecular oxygen at position 13 in a reaction catalyzed by a non-heme iron-containing enzyme lipoxygenase (13-LOX). Then, hydroperoxyl lyase (HPL) converts the hydroperoxo acid into C₆ aldehyde (Z)-3-hexanal and a C12 oxo acid (Z)-9-traumatin. The aldehyde is converted to (Z)-3-hexenol by alcohol dehydrogenase (ADH). The latter is converted to (Z)-3-hexenyl acetate by alcohol acyltransferase (AAT). In an alternative pathway, isomerase converts (Z)-3-hexanal to (E)-2-hexanal. The ADH enzyme converts the latter isomer to (E)-2 hexenol, which in turn is converted to (E)-2-hexenyl acetate by AAT. Hexadecatrienoic acid follows a similar pathway, including (Z)-7-dinortraumatin as a counterpart to oxo acid (not shown here).

Interestingly, hexenals can also form directly from membrane lipids, without their conversion to fatty acids by lipase [1,85]. Linoleic acid follows a path similar to Scheme 1 with hexanal and hexanol produced (not shown here).

Scheme 2 explains the formation of C₅ GLV from 13-hydroperoxodecatrienoic acid [9]. First, lipoxygenase abstracts the OH group from OOH. The radical formed undergoes C₅–C₁₃ β-scission to give 1-penten-3-ol. The latter is converted to 1-penten-3-one by ADH.

13-hydroperoxodecatrienoic acid is also a precursor of JA and MeJa (Scheme 3) [9,32,49]. First, it is converted to alkene oxide by allene oxide synthase (AOS). Then, the oxide is cyclized to 12-oxophytodienoic acid by allene oxide cyclase (AOC). Finally, that acid is reduced and β-oxidized three times to give JA.

2.3. Emission Data

The variety and quantity of emitted GLV vary between plant species [84]. The emission rates range from values below the detection limits to 100 μg g⁻¹ fresh weight. The fraction of some components increased with the total amount of emitted GLV, without any general patterns.

Table 2. Emission and emission rates of atmospherically relevant GLV.

GLV	Emission or Emission Rate
pentan-1-ol	0.2 and 8.8 ng g DW−1 h−1 rape and blossoming rye, resp. (field enclosure) [86] a; 7.15 ± 0.46 pmol m−2 s−1 L. pulmonaria thalii: (intact); 18.4 ± 0.6, 28.0 ± 3.7, 56.3 ± 7.8 pmol m−2 s−1 after heat shock of 37, 46, and 51 °C, resp.; 68.6 ± 13.9 pmol m−2 s−1 (wounded) [87].
1-penten-3-ol	0.2 kg C ha−1 yr−1 switchgrass plantations (growth and harvest); 18 ± 9 ng g DW−1 h−1 (6 ± 3 pmol m−2 s−1) mean before cutting; 91 ± 114 ng g DW−1 h−1 (31 ± 39 pmol m−2 s−1) max after cutting; 48 ± 22 ng g DW−1 h−1 (17 ± 17 pmol m−2 s−1) mean after cutting (including isoprene) b [88]. 0.2, 0.3, 1.2 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16]; 17.4, 22.0, 4.8, 5.9, 1.9, and 37.7 ng g DW−1 h−1 rape, blossoming rape, rye, blossoming rye, grassland, and grassland after mowing, resp. (in a field enclosure) [86].
(E)-2-penten-1-ol	0.2 and 1.9 ng g DW−1 h−1 rape and blossoming rye (in a field enclosure) [86].
3-pentanone	0.2, 0.5, 2 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16].
1-hexanol	(3.5 ± 4.0), (18.2 ± 4.9), (0.5 ± 1.1) ng g DM−1 Betula pendula, B. pubescens, Populus tremula, resp. (dry leaves, June) [41]. < 0.3 and 2.3 pmol m−2 s−1 Bel-W3 tobacco, before and after exposure to O3 in a lab reactor [89]. 0.1, 0.2, 0.3 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16]. 0.27 ± 0.04 pmol m−2 s−1 L. pulmonaria thalli: (intact); 0.27 ± 0.6, 0.33 ± 0.03, 0.52 ± 0.06 pmol m−2 s−1 after heat shock of 37, 46, and 51 °C, resp.; 0.46 ± 0.05 pmol m−2 s−1 (wounded) [87]. 58.2 and 31.1 ng g DW−1 h−1 rye and blossoming rye (in a field enclosure) [86]. 1.13 ± 1.13 ng g DW−1 h−1 Betula pendula [44]. 0.6, 0.3 μg C g DW−1 h−1 rabbit brush, Metasequoia glyptostroboides (cut branches) [90]. (0.4 ± 0.4)–(0.5 ± 0.5) ng g DW−1 h−1 Pinus sylvestris (various seedlings) [91].
(E)-2-hexen-1-ol	(4.6 ± 1.4 | 1.3 ± 1.0), (4.2 ± 1.8 | 1.3 ± 1.0), (1.5 ± 1.0 | 0.6 ± 0.5), (- |1.0 ± 0.8) ng g DW−1, (June | August) Betula pendula, B. pubescens, Populus tremula, and Sambucus nigra, resp. (dry leaves) [41]. 0.1–1.2 μg C g DW−1 h−1 various trees (cut branches) [90].
(Z)-3-hexen-1-ol	1.21 ± 0.06 μg m−2 mowed lawn [92]. 9.6 and 163 pmol m−2 s−1 Bel-W3 tobacco, before and after exposure to O3 in a lab reactor [89]. 0.10 ± 0.03, 0.039 ± 0.006 pmol m−2 s−1 intact apple and grape foliage day mean, resp. [93]. (108 ± 53), (2736 ± 546) ng/30 min from caterpillar-infested corn plant and its cut leaves, resp. (in a chamber) [34]. 0.1, 0.3, 0.7 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16]. (3.4 ± 6.7), (30 ± 46) μg m−2 h−1 (1992), (1993), resp., from a test grassland in Argonne June–August average at variable locations, noon, sunny days [94]. (0.85 ± 2.2), (54 ± 110) μg m−2 h−1 (1992), (1993), resp., from a test grassland in Argonne June-August average at fixed location, noon, sunny days [94]. (29.0, 2.4), 20.1, 20.3, 33.9, 11.8, 0.5, 31.5, 48.4, 6.8, (0.8, 6.1, 7.5) and 23.6 ng g DW−1 h−1 (grape with fruits), rape, blossoming rape, rye, blossoming rye, beech, hornbeam, birch, oak, (grassland), grassland after mowing, resp. (in a field enclosure) [86]. 0.2, 0.2, 0.06, 0.03, 0.05, 0.2, 0.07, 0.1, 0.06, 0.8, 1.3, 0.3, 0.3, 0.3, and 0.5 μg g DW−1 h−1 alfalfa, almond, apricot, bean, cotton, grape, nectarine, olive, orange, peach, pistachio, plum, walnut, valley oak, and whitehorn, resp. (in flow-through field enclosures) [95]. 380 ± 180 ng μL−1 (24 h)−1 brussels sprout leaves infested by caterpillars [77]. 0, (2.5 ± 0.9) ng g DW−1 h−1 from intact and moth-damaged cabbage, resp. [81]. 0.6 ± 0.1 μg g−1 h−1 cut sugarcane in a chamber [48]. (55.5 ± 14.6 | 26.1 ± 20.1), (202 ± 68 | 62.7 ± 34.5), (8.8 ± 3.8 | -); (9.0 ± 15.2 | -) ng g DW−1 (June | August), Betula pendula, B. pubescens, Populus tremula, Sambucus nigra, resp., (dry leaves) [41]. (247.08 ± 172.86)/(27.18 ± 6.67/16.95 ±6.97)/(218.05 ± 110.23) ng gDW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed; resp.) [44]. (43.22 ± 14.97) / (190.16 ± 47.64) ng g DW−1 h−1 Betula pendula (control/aphid infested for 21 days) [44]; (14.42 ± 4.63) / (57.58 ± 31.53) ng g DW−1 h−1 Alnus glutinosa (control/aphid infested for 21 days) [44]. 0.1–13.9 μgC g DW−1 h−1 various trees (cut branches) [90]. 2.29–7.7 ng g (foliage)−1 h−1 five tree species [40].
(Z)-3-hexenol + (E)-3-hexenol + (E)-2-hexenol	0.1 kg C ha−1 yr−1 from switchgrass plantations (growth and harvest); 6 ± 3 ng g DW−1 h−1 (1 ± 0.5 pmol m−2 s−1) before cutting; 466 ± 144 ng g DW−1 h−1 (133 ± 35 pmol m−2 s−1) after cutting; 149 ± 14 ng g DW−1 h−1 (42 ± 4 pmol m−2 s−1) mean [88].
Hexanal	7.3, (25.3, 2.1), 10.4, 26.9, 19.2, 7.3, (6.4, 4.8), 6.3, 22.2, 13.6, (1.0, 3.1, 2.1), and 4.6 ng g DW−1 h−1 grape, (grape with fruits), rape, blossoming rape, rye, blossoming rye, (beech), hornbeam, birch, oak, (grassland), grassland after mowing, resp. (in a field enclosure) [86]. 2 and 5 nmol g−1 fresh weight of intact and homogenized cucumber leaves [38]. 4.7 ± 5.3, 1.3 ± 3.0 ng g−1 Betula pendula, B. pubescens (dry leaves), June [41]. 0.6, 0.1 μgC g DW−1 h−1 rabbit brush, Populus tremuloides (cut branches) [90].
(Z)-2-hexenal	4.25 ± 0.64 pmol m−2 s−1 L. pulmonaria thalli (intact); 7.19 ± 1.98, 6.61 ± 0.16, 10.0 ± 1.9 pmol m−2 s−1 after heat shock of 37, 46, and 51 °C, resp.; 8.84 ± 1.31 pmol m−2 s−1 (wounded) [87] b. 0.2, 0.3, 0.5 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16].
(E)-2-hexen-1-al	8.6 nmol m−2 s−1 mountain grassland, cut [96]. <0.4 and 9.8 pmol m−2 s−1 Bel-W3 tobacco, before and after exposure to O3 in a lab reactor [89]. 8.7, 0.8, 8.9 ng g DW−1 h−1 oak, grassland, and grassland after mowing (in a field enclosure) [86]. 40, 5 nmol m−2 s−1 from Phragmite leaves exposed to 45 °C in normal and O2-deprived atmosphere; 5, 6.8 nmol m−2 s−1 from Phragmite leaves and from Arabidopsis NPQ1 plants exposed to high light, resp. [14]. 8, 28 nmol g−1 fresh weight of intact and homogenized cucumber leaves [38]. (8.6 ± 11.1) / (0.7 ± 1.4) ng g DW−1 Betula pendula dry leaves, June/August [41].
(Z)-3-hexen-1-al	0.5 mg C g−1 of drying aspen leaves [33]. 50 μgC g−1 h−1 cut and drying Festuca rubra (red fescue) [45]. 1.6 and 49.3 pmol m−2 s−1 Bel-W3 tobacco, before and after exposure to O3 in a lab reactor [89]. 356 ± 95, 4872 ± 633 ng/30 min from caterpillar-infested corn plant and its cut leaves, resp. (in a chamber) [34]. 2500 nmol g−1 fresh weight of homogenized Arabidopsis leaves [39]. 25, 41 nmol g−1 fresh weight of intact and homogenized cucumber leaves, resp. [38].
(Z)-3-hexenal + (E)-2-hexenal	0.2 kg C ha−1 yr−1 from switchgrass plantations (growth and harvest); 4 ± 6 ng g DW−1 h−1 (0.6 ± 2 pmol m−2 s−1) mean before cutting; 2666 ± 4787 ng g DW−1 h−1 (731 ± 1315 pmol m−2 s−1) max after cutting; 486 ± 903 ng g DW−1 h−1 (133 ± 247 pmol m−2 s−1) mean after cutting [88].
Hexyl acetate	(14.4 ± 4.0 | 2.3 ± 4.5), (47.4 ± 13.6 | 13.9 ± 6.7) ng g DM−1 (June | August), Betula pendula, B. pubescens, resp. (dry leaves) [41]. 0.43–3.16 ng g (foliage)−1 h−1 five tree species [40].
(Z)-3-hexenyl acetate	8.66 ± 0.08 μg m−2 mowed lawn [92]. <0.3 and 39.6 pmol m−2 s−1 Bel-W3 tobacco, before and after exposure to O3 in a lab reactor [89]. 0.21 ± 0.06, 0.27 ± 0.02 pmol m−2 s−1 intact apple and grape foliage day mean, resp. [93]. 595 ± 569, 3720 ± 1052 ng/30 min from caterpillar-infested corn plant and its cut leaves, resp. (in a chamber) [34]. 0.1, 0.2, 2 nmol m−2 s−1 Couepia longipendula Pilg. (detached leaves over Amazon Forest canopy, February, peak values at 7:30, 12:40, 16:40, resp.) [16]. 10.9, (114.1, 9.4), 37.5, 117.3, 11.1, (3.4, 4.1), 50.9, 95.0, 5.4, (1.8, 13.8, 13.1), and 29.6 ng g DW−1 h−1 grape, (grape with fruits), rape, blossoming rape, blossoming rye, (beech), hornbeam, birch, oak, (grassland), grassland after mowing, resp. (in a field enclosure) [86]. 0.7, 1.9, 1.1, 0.4, ≤ 0.2, (0.8, 0.5), 0.1, 0.6, 0.2, 0.06, 3.4, 1.6, 0.4, 0.3, 0.2, and 3.3 μg g DW−1 h−1 alfalfa, almond, apricot, cherry, cotton (grape), lemon, nectarine, olive, orange, peach, plum, sorghum, walnut, valley oak, and whitehorn, resp. (in flow-through field enclosures) [95]. 3608 ± 2076, 490 ± 220, 340 ± 200 ng μL−1 (24 h)−1 brussel sprout leaves infested by caterpillars, non-infested artificially damaged leaves from the infested plant, and undamaged leaves, resp. [77]. 0, 8.4 ± 3.8 ng g DW−1 hr−1 from intact and moth-damaged cabbage, resp. [81]. 1.22 ± 0.18 μg g−1 hr−1 cut sugarcane in a chamber [48]. 7.35 ± 1.02 pmol m−2 s−1 L. pulmonaria thalli (intact); 7.36 ± 1.41, 12.1 ± 1.1, 17.5 ± 2.5 pmol m−2 s−1 after heat shock: 37 °C, 46 °C, 51 °C; 20.5 ± 1.4 pmol m−2 s−1 (wounded) [87]. (130 ± 42.8/44.4 ± 40.0), (501 ± 246 / 26.4 ± 16.3), (67.6 ± 18.6/-), (14.3 ± 6.0/-ng) g DW−1 Betula pendula, Populus tremula, B. pubescens, Sambucus nigra, resp. (dry leaves), June/August [41]. (41.18 ± 20.87)/(8.96 ± 1.61)/(6.90 ± 2.38)/(34.89 ± 13.66) ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed) [44]. (15.99 ± 5.08)/(62.38 ± 16.54) ng g DW−1 h−1 Betula pendula (control/aphid infested for 21 days) [44]. (21.44 ± 6.11)/(43.91 ± 20.52) ng g DW−1 h−1 Alnus glutinosa (control/aphid infested for 21 days) [44]. 0.3–19 μgC g DW−1 h−1 various trees (cut branches) [90]. 6.28–71.39 ng g (foliage)−1 h−1 five tree species [40]. (75.5 ± 5.7) − (178.1 ± 31.3) ng g DW−1 h−1 Pinus sylvestris (various seedlings) [91].
(E)-2-hexenyl acetate	(4.0 ± 2.1 / 0.4 ± 0.8); (38.7 ± 22.2/-) ng g−1 Betula pendula, B. pubescens, resp. (dry leaves), June/August [41]. 0.1, 0.2 μg C h−1 g DW−1 ironwood (Accacia), slippery elm (Ulmus), resp. [90]. 0.12 ± 0.07, 0.35 ± 0.17 ng g (foliage)−1 h−1 blue ash, white fingertree [40].
(Z)-3-Hexenyl acetate + (E)-2-hexenyl acetate	3 ± 2 ng g DW−1 h−1 (0.5 ± 0.3 pmol m−2 s−1) switchgrass, before cutting; 35 ± 50 ng g DW−1 h−1 (6 ± 8 pmol m−2 s−1) after cutting; 13 ± 18 ng g DW−1 h−1 (2 ± 3 pmol m−2 s−1) mean [88].
(Z)-3-hexenyl butanoate (butyrate)	0.0306 ± 0.0008 pmol m−2 s−1 intact apple foliage day mean [93]. (-| 20.8 ± 10.1), (0.7 ± 1.1 | -) ng g DW−1 (June | August), Betula pubescens, Populus tremula, resp. (dry leaves) [41]. (12.84 ± 8.86)/(0.48 ± 0.32)/(0.37 ± 0.37)/(4.45 ± 1.71) ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed) [44]. (0.19 ± 0.19)/(2.42 ± 1.04) ng g DW−1 h−1 Betula pendula (control/aphid infested for 21 days) [44]. 0.2–5.2 μg C g DW−1 h−1 various trees (cut branches) [90]. (0.9 ± 0.9) − (1.3 ± 0.9) ng g DW−1 h−1 Pinus sylvestris (various seedlings) [91].
(E)-3-hexenyl butanoate	(7.3 ± 2.1 | -), (33.8 ± 11.3 | 7.3 ± 8.5), (4.8 ± 1.4 | -) ng g DW−1 (June | August), Betula pendula, B. pubescens, Populus tremula, resp. (dry leaves) [41].
(Z)-3-hexenyl 3-methylbutanoate (isovalerate)	(0.46 ± 0.46)/(0.30 ± 0.30)/(0 ± 0)/(5.50 ± 1.70) ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed) [44]. 0.49 ± 0.49 ng g DW−1 h−1 Betula pendula [44]. 0.3, 0.8 μg C g DW−1 h−1 Artemisa tridentata, Betula alleghaniensis (cut branches) [90].
(Z)-3-hexenyl 2- methyl-2-butenoate (tiglate)	(1.21 ± 0.54)/0.13 ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days [44].
3-hexenyl hexanoate c	2.9 ± 2.3 ng g DW−1 Populus tremula (leaves), June [41].
(Z)-3-hexenyl isobutyrate	0.03 ± 0.01, 0.40 ± 0.15, 0.03 ± 0.01 ng g−1 h−1 black ash, white fingertree, and olive tree, resp. (foliage) [40].
(Z)-3-hexenyl butyrate	0.15–0.66 ng g−1 h−1 five tree species (foliage) [40].
(Z)-3-hexenyl isovalerate	0.06–1.34 ng g−1 h−1 five tree species (foliage) [40].
(Z)-3-hexenyl isotiglate	0.02–0.38 ng g−1 h−1 four tree species (foliage) [40].
hexenyl type	140, 50, 0.06 μg (C) g DW−1 h−1 grass, clover (both cut and slashed), and over mowed grassland, resp [18].
1-octen-3-ol	9.0 ng g DW−1 h−1 blossoming rye in a field enclosure [86]. (1.0 ± 0.6) − (44.6 ± 40.4) ng g DW−1 h−1 Pinus sylvestris (various seedlings) [91].
2-methyl-3-buten-2-ol	3.93 ± 1.70, 2.69 ± 1.24 nmol m−2 s−1 Pinus ponderosa, drought-stressed and watered [97]. 6.933, 0.522 nmol m−2 s−1 mean, Pinus ponderosa and Pinus nigra, de novo synthesis [98]. 1.6, 0.77 nmol m−2 s−1 (max, daily mean, resp.) Pica abies flux over mountain forest [99]. 0.4–1.5 mg m−2 h−1 over mixed coniferous Roosevelt National Forest, Colorado (the lowest and peak values) [46]. 76, 530 μg C m−2 h−1 Pinus taeda (plantation canopy level), mid-spring to summer and fall to early spring, resp. [47]. 1.1 mg C m−2 h−1 Pinus ponderosa (plantation canopy level average), July [100]. 1.37/0.01 mg C m−2 h−1 Pinus ponderosa (plantation average canopy level), daytime/nighttime, July to September [101]. 1.0 mg C m−2 h−1 subalpine forest in the Rocky Mountains (canopy level average), July–August [102]. 0.3–1.4 mg C m−2 h−1 Pinus taeda (canopy level, plantation near Chappel Hill) July [103]. 2 mg C m−2 h−1 Pinus ponderosa (plantation canopy level average), July August; 18 μg C g−1 h−1 (max. at leaf level), at 30 °C and 1000 mmol m−2 s−1 PAR [104]. 18 μg ± 7.4 C g−1 h−1 mean Pinus ponderosa at 30 °C and high irradiance [98]. 25 μg C g−1 h−1 average Pinus ponderosa 1-year-old needles, intact [105]. Over 20 mg C g−1 h−1 from leaves exposed to sunlight at 30 °C [45]. 0.2, 0.3 μg C g DW−1 h−1 Picea engelmanni, Pinus contorta (cut branches) [90].
Nonanal	(12.94 ± 0.95)/(12.57 ± 1.78)/(23.50 ± 3.09)/(25.12 ± 2.53) ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed) [44]. (6.51 ± 1.61)/(3.94 ± 2.76) ng g DW−1 h−1 Betula pendula (control/aphid infested for 21 days) [44]. (3.92 ± 0.87)/(0.79 ± 0.50) ng g DW−1 h−1 Alnus glutinosa (control/aphid infested for 21 days) [44].
Methyl salicylate	Up to 0.25 mg m−2 h−1 over walnut agroforest after cold night (7.5 °C) [106]. <0.2 and 123 pmol m−2 s−1 tobacco Bel-W3 before and after exposure to O3 in a lab reactor [89]. 40 ± 17 pmol m−2 s−1 from drought-stressed Alnus glutinosa in a chamber; 63 ± 17 pmol m−2 s−1 with additional insect treatment; 14 ± 5 pmol m−2 s−1 after re-watering [107]. 2.8 ng cm−2 h−1 from aphid-infested Pinus sylvestris saplings [75]. (26.3 ± 22.0 | 1.2 ± 1.7), (9.2 ± 11.8 | -), (14.9 ± 3.8 |-), (0.7 ± 1.7 | -) ng g DW−1, (June | August), Betula pendula, B. pubescens, Populus tremula, Sambucus nigra, resp. (dry leaves) [41]. (25.45 ± 10.08)/(370.82 ± 99.35)/(551.32± 99.35)/(387.82 ± 77.14) ng g DW−1 h−1 Betula pendula (control/aphid infested for 10 days/aphids removed/washed) [44]. (172.09 ± 120.115)/(721.63 ± 204.91) ng gDW−1 h−1 Betula pendula (control/aphid infested for 21 days) [44]. (1.39 ± 1.39)/(137.98 ± 45.16) ng g DW−1 h−1 Alnus glutinosa (control/aphid infested for 21 days) [44]. 0.5, 0.3 μg C g DW−1 h−1 Fagus grandfolia, Quercus velutina (cut branches) [90]. 0.27–2.76 ng g−1 h−1 five tree species (foliage) [40].
Hexenals	0.07–0.417 μmol g DW−1 various cut grasses [108]. 4 × 10−7 g m−2 s−1 hay after cutting and drying [109]. 100–240 μg g DW−1 hay drying in a cuvette; 40–50 mg m−2 h−1 hay cutting in a chamber; 0.2–0.4 mg m−2 h−1 hay drying in a chamber; 0.1–1.5 mg m−2 h−1 1st day of hay drying on a pasture in St. Johann, Tirol [110].
Pentenols and 2- methyl butanal	15 μg g DW−1 hay drying in a cuvette; 0.1–0.9 mg m−2 h−1 hay cutting in the chamber; 0.07–0.1 mg m−2 h−1 hay drying in the chamber; 0.1–0.7 mg m−2 h−1 1st day of hay drying on a pasture in St. Johann, Tirol [110].
Pentenols and 3- methyl butanal	8 × 10−8 g m−2 s−1 hay cutting and drying [109].
Hexanal and (E)-2-hexenol	8 × 10−8 g m−2 s−1 hay cutting and drying [109].
Hexenols and hexanal	0.07–0.211 μmol g DW−1 various cut grasses [108]. 30–60 μg g DW−1 hay drying in a cuvette; 1–3 mg m−2 h−1 hay cutting in the chamber; 0.1–0.3 mg m−2 h−1 hay drying in the chamber; 0.1–0.3 mg m−2 h−1 1st day of hay drying on a pasture in St. Johann, Tirol [110].
Hexenyl acetates	0.10–0.844 μmol g DW−1 various cut grasses [108].
Hexyl acetates	0.003–0.015 μmol g DW−1 various cut grasses [108]. 2.71 ± 0.05, 0.29 ± 0.07 pmol m−2 s−1 intact apple and grape foliage day mean, resp. [93].

^a leaf (or plant material) dry weight (mass); ^b isoprene fragment was observed pre-cut and more 1-penten-3-ol fragment post-cut on MS; ^c unspecified isomer.

collects emission values and emission rates of GLV and their mixtures measured in the field and laboratory experiments. Emission measures were a GLV mass per mass of leaves or plant material, dry or fresh, or a GLV mass per land area. Quantities that refer to the leaf area are used in quantifying the VOC emission from living vegetation and modeling atmospheric chemistry and aerosol formation. Values referring to the mass of plant material help describe events like harvesting or grass mowing and compare VOC emission between species.

Table 3. Ambient and headspace concentrations of GLV.

GLV	Concentrations and References
1-penten-3-ol	17–54 ng dm−3 over freshly harvested tall fescue of various types in a headspace chamber [28].
(E)-2-penten-1-ol	236–1529 ng dm−3 over freshly harvested tall fescue of various types (in a headspace chamber); 1–9 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28].
(E)-2-pentenal	1–4 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28].
1-penten-3-one	7–55 ng dm−3 over freshly harvested tall fescue of various types (in a headspace chamber); 3–9 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28].
1-hexanol	1 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28]. 0.04–0.12 ppbv at the canopy level (30 m) October-September 2010 in Amazon Rainforest [16].
(E)-2-hexen-1-ol	11–42 ng dm−3 over freshly harvested tall fescue of various types (in a headspace chamber); 0–1 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28].
(Z)-3-hexen-1-ol	958–3176 ng dm−3 over freshly harvested tall fescue of various types (in a headspace chamber); 2–5 ng dm−3 over sun-cured hay of tall fescue of various types (in a headspace chamber) [28]. Max ~1000 ppbv, during cutting and drying grass and clover; 10–100 ppbv from red fescue grass; 5–100 ppbv from white clover leaves and stems, when cut and drying [12]. 732 pptv mixing ratio over the canopy of tropical rainforest in Surinam [65].
(E)-3-hexenol	3–31 ng dm−3 over freshly harvested tall fescue of various types (in a headspace chamber) [28].
Hexanal	5–28 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber [28].
(E)-2-hexenal	22–458 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber; 4–12 ng dm−3 over sun-cured hay of tall fescue of various types, in a headspace chamber [28]. 0.50–0.78 ppbv in 24 L of air at charcoal production workplace [111].
(Z)-2-hexenal	11–72 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber; 0–1 ng dm−3 over sun-cured hay of tall fescue of various types, in a headspace chamber [28].
(E)-3-hexenal	26–165 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber; 1–2 ng dm−3 over sun-cured hay of tall fescue of various types, in a headspace chamber [28].
(Z)-3-hexenal	18–698 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber; 0–1 ng dm−3 over sun-cured hay of tall fescue of multiple types, in a headspace chamber [28]. <100 pptv, ~16–20 ppbv, before and after lawn mowing [112]. Max ~1000 ppbv, during cutting and drying grass and clover; 10–1000 ppbv from red fescue grass; 10–100 ppbv from white clover leaves and stems when cut and dried in a lab chamber [12]. Up to 650 ppbv from cut aspen leaves in a chamber [33]. Up to 160 ppbv over cut drying bluegrass leaves (25–30 °C) [29].
(E)-2-hexenyl acetate	49–102 ng dm−3 over freshly harvested various types of tall fescue in a headspace chamber [28].
(Z)-3-hexenyl acetate	10,195–24,816 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber; 0–2 ng dm−3 over sun-cured hay of tall fescue of various types, in a headspace chamber [28]. 165.3–353.7 pptv on a spring day; 213.3–790.2 pptv in spring night, over flowering orange tree [113]. 1–10 ppbv from red fescue grass, 0.5–9 ppbv from white clover leaves and stems, when cut and dried in a lab chamber [12].
(Z)-3-hexenyl propionate	1–5 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber [28].
1-octen-3-ol	6–18 ng dm−3 over freshly harvested tall fescue of various types, in a headspace chamber. 1–2 ng dm−3 over sun-cured hay of tall fescue of various types, in a headspace chamber [28].
2-methyl-3-buten-2-ol	19 ppbv max., 0.2–2 ppbv avg. Pinus ponderosa plantation canopy level [104]. 0–7 ppbv Pinus ponderosa plantation in the Sierra Nevada, canopy level, July to September [101]. 50–200 pptv day (mixing ratio) Niwot Ridge, Colorado, June 1991 [114].
Methyl salicylate	Mixing ratio up to 14 ppbv over walnut agroforest after cold night (7.5 °C) [106].

shows ambient concentrations of GLV and headspace concentrations measured in laboratory or field chambers.

Several researchers developed models that predict emission fluxes of volatile compounds, including GLV. The most comprehensive is the Model of Emission of Gases and Aerosols from Nature version 2.1 (MEGAN) [50,115], which aimed to estimate fluxes of biogenic compounds between terrestrial ecosystems and the atmosphere. MEGAN divided BVOC into 19 classes. There is no separate GLV class, but the “Stress VOC” class includes 3-hexenal, 2-hexenal, 3-hexenol, 3-hexenyl acetate, hexanal, 1-hexenol, MeJa, and MeSa. 2-Methyl-3-buten-2-ol is a separate class alone. MEGAN requires the input of meteorological, radiation, and land cover data, including the emission factors for 15 predefined classes of vegetation.

Heiden et al. [116] studied the emission dynamics of (Z)-3-hexenol, (Z)-3-hexenal, (E)-3-hexenol, (E)-2-hexenol, (E)-2-hexenal, hexanal, and 1-hexanol produced by various plants (tobacco, corn, pine, tomato, sunflower, broadleaf bean) after exposure to various stresses (ozone, pseudomonas infection, wounding) in a laboratory reactor. They obtained a general correlation:

$$\frac{\varphi }{{\varphi }_{max}}=A\left[\exp \left(-k_{1}t\right)-\exp \left(-k_{2}t\right)\right]$$

(1)

where ϕ is the actual emission rate, ϕ_max is the maximum emission rate in mol cm⁻² s⁻¹, A is the scaling factor to force ϕ = ϕ_max at the correct experimental time, t is time in h, k₁ and k₂ measured in h⁻¹ are empirical factors. For sunflower and broadleaf bean, the equation without the k₁ term provided a good approximation. The parameters in Equation (1) are available in the original publication.

Kirstine and Galbally [19] developed a model for estimating BVOC emission from uncut and cut grass in urban environments. They estimated that hexenyl-type compounds (C₆ GLV alcohols and aldehydes) constituted more than 70% of total BVOC emission upon initial wounding of grass and 22–40% during drying of cut material. For Sydney and Melbourne, Australia, the total emissions were 2.9 × 10⁹ g yr⁻¹ and 2.9 × 10⁹ g yr⁻¹, respectively, from uncut grass, 4.5 × 10⁹ g yr⁻¹ and 5.4 × 10⁹ g yr⁻¹ from cut grass, and 1.2 × 10¹¹ g yr⁻¹ and 2.0 × 10¹¹ g yr⁻¹ from other biogenic sources. The diurnal variation of BVOC flux varied from 0 in the night to peak values between 2 and 3 pm. Maximum emissions occurred in November and December.

Karl et al. [46] provided several model correlations for estimating MBO fluxes over vegetation, based on sensible heat (ωT) and latent heat fluxes (ωq), temperature (T), and two linear models (GLM1, GLM2) using temperature and light (PAR). The correlations utilized VOC fluxes measured at the Niwot Ridge AmeriFlux site in the Roosevelt National Forest in the Rocky Mountains of Colorado, USA, using the eddy covariance approach with PTR-MS instrument as a VOC sensor.

Geron et al. [47] provided a formula estimating the temperature dependence of MBO emission on temperature:

$$M=M_S{e}^{\left[\beta \left(T-T_s\right)\right]}$$

(2)

where M is the emission rate of MBO in mg C m⁻² h⁻¹ at temperature T in °C, M_s is the emission rate at standard temperature T_s = 30 °C, β is the temperature response factor (average β = 0.17, for the growing season β = 0.1).

Harley et al. [98] developed a model that described the diurnal emission of MBO and other BVOC from trees at varying temperatures and incident photosynthetic photon flux densities based on several fitted parameters like energies of activation and deactivation, and temperature and light scalars.

3. Physical Properties of GLV

3.1. Solubility in Water and Partition Coefficients

Henry’s constants, 1-octanol/water partition coefficients, and 1-octanol/air partition coefficients are collected in

Table 4. Henry’s constant, 1-octanol/water partition coefficient (K_OW), and 1-octanol/air partition coefficient (K_OA) for GLV in water and aqueous solutions.

GLV	Henry’s Constant				Octanol/Water Partition Coefficient		Octanol/Air Partition Coefficient
	H	T	I	Ref.	Log (KOW)	Ref.	Log (KOA)	Ref.
	M atm−1	K	M
pentan-1-ol	87.7 ± 8.9	298	0	[120] c	1.5	[122]	4.7	[123]
	84.4 ± 7.0 46.1 ± 2.1 11.9 ± 0.3 3.8 ± 0.1	298 306 323 343		[124] a	1.34	[125]	f 4.8	[117] i
	71.9 ± 3.6	298		[117] g	1.3	[117] h	4.6	[117] i
1-penten-3-ol	72.5 ± 40.6	298		[117] g	0.84	[126]	4.5	[117] i
					0.81	[125]
					1.1	[117] h
(Z/E)-2-penten-1-ol	119.8 ± 34	298	0	[117] g	1.1	[117] h	4.4	[117] i
(E)-2-pentenal	17.3 ± 12.3	298		[117] g	1.1	[117] h	3.6	[117] i
1-penten-3-one	28.5 ± 23.2	298		[117] g	0.9	[117] h	3.8	[117] i
(Z)-2-pentenyl acetate	2.8 ± 0.8	298		[117] g	2.1	[117] h	3.8	[117] i
n-hexan-1-ol	61.8 ± 16.3	298	0	[120] c	1.84	[125]	5.2	[123]
	59.6 ± 2.8 32.7 ± 1.1 8.0 ± 0.3 2.53 ± 0.01	298 306 323 343		[124] a	2.03	[122]	5.0	[117] i
	55.5 ± 3.5	298		[117] g	1.8	[117] h
n-hexan-1-al	3.2 ± 0.4	295	0	[127]	1.78	[122]	4.4	[123]
	5 ± 1	298		[128]	1.8	[117] h	f 3.8, 3.9	[117] i
	5.2 ± 0.5 4.4 ± 0.4 3.4 ± 0.3 3.6 ± 0.2 3.2 ± 0.1		freshwater 25% seawater 50% seawater 75% seawater 100% seawater	[129]
	4.5 ± 1.0		0	[120] c
	3.9 ± 1.0			[117] g
hexyl acetate	1.5 ± 0.2	298	0	[127]	2.59	[126]	4.6	[123]
	1.4 ± 0.2			[117] g	2.8	[117] h	4.5 f, 4.4	[117] i
hexenol b	25	298	0	[33]
(E/Z)-2-hexen-1-ol	94.4 ± 35.2			[117] g	1.6	[117] h	4.8	[117] i
(Z)-3-hexen-1-ol	113 ± 7.1 140 ± 18 132 ± 11 83.4 ± 8.3 62.7 ± 3.0	298 303 308	0 0.01 1 0 0	[130]	1.6	[117] h	4.8	[117] i
	129.9 ± 65.6	298		[117] g
(E)-2-hexenal	14.5 ± 1.7	298	0	[127]	1.6	[117] h	4.3 f, 4.0	[117] i
	20			[128,131]
	13.6 ± 7.3			[117] g
(Z)-3-hexenal	6			[33]	1.6	[117] h	3.7	[117] i
	4.1 ± 1.9			[117] g
hexenyl acetate b	1			[33]
(E/Z)-2-hexenyl acetate	1.8 ± 0.7			[117] g	2.6	[117] h	4.2	[117] i
(Z)-3-hexenyl acetate	3.1 ± 0.4	298	0	[127]	2.6	[117] h	4.2	[117] i
	3.62 ± 0.22 3.29 ± 1.1 2.32 ± 0.17 3.21 ± 0.17 2.56 ± 0.19	298 303 308	0 0.01 1 0 0	[130]
	3.3 ± 2.4	298		[117] g
(E)-3-hexenyl acetate	3.3 ± 0.4	298	0	[127]
	3.3 ± 2.4			[117] g
(Z)-3-hexenyl- propionate	2.4 ± 1.6			[117] g	3.1	[117] h	4.6	[117] i
(E/Z)-3-hexenyl butyrate	1.8 ± 1.1			[117] g	3.6	[117] h	4.9	[117] i
(Z)-3-hexenyl isobutyrate	1.3 ± 0.6			[117] g	3.5	[117] h	4.9	[117] i
(E)-2-hexenyl butanoate	1.2 ± 0.3			[117] g	3.6	[117] h	4.9	[117] i
(Z)-3-hexenyl 3-methylbutanoate	1.1 ± 0.6			[117] g	4.0	[117] h	5.2	[117] i
(Z)-3-hexenyl 2-methyl-2-butenoate	1.8 ± 0.6			[117] g	3.9	[117] h	5.4	[117] i
3-hexenyl hexanoate	1.0 ± 0.5			[117] g	4.6	[117] h	5.7	[117] i
2-methyl-3-buten-2-ol	73 ± 3 38 ± 7 48 ± 26	296 ± 2	0 40% wt H2SO4 55% wt H2SO4	[132]	1.1	[117] h	4.2 f, 4.5	[117] i
	48 a 52.9 ± 5.1	298	0 0	[119]
	38.7 ± 2.2 21.8 ± 4.4 40.2 ± 5.4 31.7 ± 2.2	303 308	0.01 1 0 0	[130]
	65 ± 3.5	303	0	[128,133]
	62 ± 0.8	303	22.7 mM KNO3 + 7.42 mM CaSO4	[133]
	59.8 ± 58.6 d			[117] g
1-octen-3-ol	13.2	>378 K	0	[120,134]	2.6	[117] h	5.6	[117] i
	44.5 ± 1.7			[117] g
nonanal	1.3 ± 0.2	298	0	[120] c	3.3	[117] h	f 4.8, 5.0	[117] i
	1.2 ± 0.1 0.51 ± 0.03 0.38 ± 0.03	298	Freshwater 50% seawater 100% seawater	[129]
	1.6 ± 0.4			[117] g
Jasmonic acid e					2.5	[117] h	9.7	[117] i
methyl jasmonate e	5081 ± 1003	298	0	[106]	2.8	[117] h	7.5	[117] i
	8091 ± 1121 5454 ± 520 3869 ± 261 6716 ± 1272 4837 ± 272	298 298 298 303 308	0 0.01 1 0	[130]
methyl salicylate e	33.5 ± 4.0	298	0	[106]	2.6	[117] h	5.0 f, 6.3	[117] i
	37.9 ± 2.1 26.7 ± 3.4 20.1 ± 1.6 16.4 ± 0.9 10.0 ± 4.2	298 298 298 303 308	0.01 1 0	[130]

^a recalculated from the original data; ^b unspecified structure; ^c mean of experimental values summarized by Sander 2015; ^d EPI suite could not estimate the value using group estimation method, so the difference between bond estimation value and VP/WSol resulted in large std. deviation; ^e the predicted KH values are unreasonable and therefore not included ^f Log KOA (octanol/air): estimated within EPI suite using one of the experimental values; ^g mean of bond est., group est., and VP/Wsol est. values by EPI suite (HENRYWIN v3.20) for which standard deviation is provided as error; ^h calculated with EPI suite HENRYWIN v3.20, Log KOW (version 1.68 estimate); ⁱ calculated with EPI suite HENRYWIN v3.20, Log Octanol-Air (KOAWIN v1.10).

. Along with the experimental values, the table contains estimates obtained here using the EPI suite HENRYWIN v3.20 freely available from the US EPA [117]. EPI estimates Henry’s constants using three different methods (the bond estimation method, the group estimation method [118], and the VaporPressure/WaterSolubility estimation method. Estimates in Table 4 are the mean values from those three methods with standard deviations taken as uncertainties. The Log KOW procedure ver. 1.68 and Log Octanol-Air KOAWIN procedure ver. 1.1 were used to estimate the partition coefficients. The reviews of other estimation methods are provided elsewhere [119,120,121].

The octanol/water partition coefficient represents the equilibrium in a two-phase three-component system solute-water-octanol—it is a ratio of the solute concentration in the octanol-rich phase to the solute concentration in the water-rich phase [122]. It shows the balance between the lipophilicity and hydrophilicity of a solute. The octanol/air partition coefficient represents the equilibrium in a two-phase three-component system solute-air-octanol—it is a ratio of a solute concentration in octanol to the solute concentration in the air. It estimates the partitioning of a solute between air and various environmental matrices like soil, vegetation, and aerosol particles [135].

Table 5. Solubility in water and vapor pressure of GLVs at 298 K, experimental and estimated with EPI suite (MPBPVP (v1.43), mean of Antoine and Grain methods, and WSKOW v1.42 method, resp.).

GLV	Solubility		Vapor Pressure
	mmol dm−3	Ref.	Atm	Ref.
1-pentanol	245.5	[136]	2.9 × 10−3	[137]
	307	[138]	3.7 × 10−3	[138]
	257	[126]	3.49 × 10−3	[117] b
	236.98	[117] a
1-penten-3-ol	1047.1	[126]	1.20 × 10−2	[117] b
	1035.1	[125]
	525.48	[117] a
(Z/E)-2-penten-1-ol	530.83	[117] a	3.46 × 10−3	[117] b
(E)-2-pentenal	178.08	[117] a	2.43 × 10−2	[117] b
1-penten-3-one	260.34	[117] a	5.03 × 10−2	[117] b
(Z)-2-pentenyl acetate	11.23	[117] a	4.14 × 10−3	[117] b
n-hexan-1-ol	57.5	[136]	1.22 × 10−3	[139]
	61.3	[125]	1.16 × 10−3	[117] b
	67.39	[117] a
n-hexanal	56.3 c	[117] b	1.49 × 10−2	[139]
	49.9	[140]	1.39 × 10−2	[140]
	35.21	[117] a	1.26 × 10−2	[117] b
hexyl acetate	3.5	[141]	1.74 × 10−3	[142]
	8.9	[126]	1.91 × 10−3	[117] b
	2.14	[117] a
hexenol f	159.74	[117] a
(E/Z)-2-hexen-1-ol	159.74	[117] a	1.20 × 10−3	[117] b
(Z)-3-hexen-1-ol	162 ± 6	[130]	1.23 × 10−3	[117] b
	159.74	[117]
(E)-2-hexenal	53.61	[117] a	6.21 × 10−3	[117] b
(Z)-3-hexenal	53.61	[117] a	1.97 × 10−2	[117] b
(E/Z)-2-hexenyl acetate	3.38	[117] a	2.61 × 10−3	[117] b
(Z)-3-hexenyl acetate	3.12 ± 0.17	[130]	1.50 × 10−3	[117] b
	3.38	[117] a
(E)-3-hexenyl acetate	3.38	[117] a	1.50 × 10−3	[117] b
(Z)-3-hexenyl-propionate	1.02	[117] a	5.50 × 10−4	[117] b
(E/Z)-3-hexenyl butyrate	0.31	[117] a	2.05 × 10−4	[117] b
(Z)-3-hexenyl isobutyrate	0.35	[117] a	3.71 × 10−4	[117] b
(E)-2-hexenyl butanoate	0.31	[117] a	2.05 × 10−4	[117] b
(Z)-3-hexenyl 3-methylbutanoate	0.11	[117] a	1.39 × 10−4	[117] b
(Z)-3-hexenyl 2-methyl-2- butenoate	0.13	[117] a	7.53 × 10−5	[117] b
3-hexenyl hexanoate	0.03	[117] a	3.04 × 10−5	[117] b
2-methyl-3-buten-2-ol	1959 ± 36	[130]	3.08 × 10−2	[117] b
	565.89	[117] a
1-octen-3-ol	14.32	[117] a	3.13 × 10−4	[117] b
nonanal	0.7	[140]	4.87 × 10−4	[139]
	0.93	[117] a	5.13 × 10−4	[140]
			7.42 × 10−4	[117] b
Jasmonic acid	3.53	[117] a	2.41 × 10−8 d 1.79 × 10−7 e	[117] b
methyl jasmonate	4.52 ± 0.09	[130]	4.43 × 10−7 d 1.24 × 10−6 e	[117] b
	0.64	[117] a
methyl salicylate	5.11 ± 0.06	[130]	7.03 × 10−5	[117] b
	4.6 c	[141]
	12.32	[117] a

^a estimated with EPI suite (WSKOW v1.42); ^b estimated with EPI suite (MPBPVP (v1.43), mean of Antoine and Grain methods); ^c 308 K; ^d selected VP with modified grain method; ^e subcooled liquid VP with modified grain method; ^f unspecified structure.

presents the GLV solubility in water and equilibrium vapor pressure either determined experimentally or calculated using EPI MPBPVP and WSKOW suites [117].

The Henry’s constants in Table 4 attributed to Sander [120] are actually the mean values of the constants determined by several authors: for pentan-1-ol [118,119,138,143,144,145,146,147,148], hexanal [129,140,144,149,150], and nonanal [140,144,149,150]. One of the values for nonanal [149] seems an outlier compared to the other latest values and therefore was skipped in calculating the mean.

There are no experimental Henry’s constants for 1-penten-3-one (ethyl vinyl ketone). Still, the EPI estimate at 298 K in Table 4 is close to the mean empirical constant of (30.4 ± 10.1) M atm⁻¹ determined for the structurally similar methyl vinyl ketone [120]. Similarly, the EPI estimate of the Henry’s constant for (Z)-2-pentenyl acetate at 298 K (Table 4) is close to the (2.7 ± 0.4) and (2.3 ± 0.4) M atm⁻¹ values determined for the homolog amyl acetate and isoamyl acetate, respectively [120].

Hansel et al. [21] estimated Henry’s constant for several intermediate and final products of methyl jasmonate and methyl salicylate reactions with OH radicals (not shown here).

A few studies provided the temperature dependence of the GLV Henry’s constants, and most of them were for pentanol and hexanol. Gupta et al. [151] developed the classical van’t Hoff equations for dimensionless Henry’s constants based on empirical determination and valid between 313–363 K (Equations (3a) and (3b)):

$$ln\left(K_{H, pentanol}\right)=14.233-\frac{6559.6}{T}$$

(3a)

$$ln\left(K_{H, hexanol}\right)=11.705-\frac{5538.7}{T}$$

(3b)

where K_H was the ratio of the solute molar concentrations in the gas and aqueous phases.

Falabella et al. [152] correlated the experimental Henry’s constants determined in water and aqueous solutions of Na₂SO₄ for the temperature range 313–363 K using a modified theory of dilute solutions [153,154] (Equations (4a) and (4b)):

$$ln\left(K_{H, pentanol}\right)=ln\left(P_{H2O}^{sat}\right)+\frac{10.04}{T_r}-14.51\frac{{\left(1-T_r\right)}^{0.355}}{T_r}+2.72\frac{exp\left(1-T_r\right)}{T_r^{0.41}}+0.91x$$

(4a)

$$ln\left(K_{H, hexanol}\right)=ln\left(P_{H2O}^{sat}\right)+\frac{9.57}{T_r}-10.61\frac{{\left(1-T_r\right)}^{0.355}}{T_r}+0.64\frac{exp\left(1-T_r\right)}{T_r^{0.41}}+1.11x$$

(4b)

where K_H is in kPa, P^sat_H2O is the saturation pressure of water in kPa, T_r = T/T_critical is the reduced temperature, and x is the concentration of Na₂SO₄ in mol kg⁻¹ water.

Dohnal et al. [155] provided a correlation for the temperature dependence of the thermodynamic Henry’s constant for pentanol at 273–373 K (Equation (5)).

$$ln\left(K_{H, pentenol}^o\right)=78.7049-\frac{99.5059}{\tau }-97.8025 ln\left(\tau \right)+D\tau$$

(5)

where K⁰_H is in kPa and τ = T/298.15.

Equation (6) approximates the temperature variation of the Henry’s Law constant for MBO in water [132]:

$$ln\left[{\mathrm{H}}_{\mathrm{MBO}}\right]=\frac{\left(7230\pm 190\right)}{T}-\left(20.2\pm 0.7\right), T=275-295 \mathrm{K}$$

(6)

where H is in M atm⁻¹. The corresponding enthalpy of solvation is ΔH_sol = −(60 ± 7) kJ mol⁻¹. Equation (6) calculated the constants at 298 K and 303 K (58 and 38.9 M atm⁻¹ resp.) that were lower than the experimental values from other authors (Table 4).

Zhou et al. [129] determined the temperature dependence of the Henry’s constant for hexanal (Equations (7a) and (7b)) and nonanal (Equations (8a) and (8b)) in freshwater and seawater across the range of 283–318 K:

log H _{hexanal, seawater} = −8.35 + 2645/T

(7a)

log H _{hexanal, freshwater} = −8.76 + 2819/T

(7b)

log H _{nonanal, seawater} = −9.01 + 2555/T

(8a)

log H _{nonanal, freshwater} = −9.81 + 2929/T

(8b)

where H is in M atm⁻¹ and covers all possible hydrated forms of the solutes.

Liyana-Arrachchi et al. [156,157] simulated the behavior of MBO, MeSa, (Z)-3-hexen-1-ol, and (Z)-3-hexenylacetate at air-water interfaces using molecular dynamics approaches. The surface concentrations of GLV at the same bulk concentration decreased in the following order: (Z)-3-hexenyl acetate, (Z)-3-hexen-1-ol, MBO, and MeSa. The models accurately reproduced the experimental values of 1-octanol/water partition coefficients and the surface tension of solutions at 298 K and 0.01 MPa. The models indicated that all four GLV tended to remain at water-air interfaces with polar groups oriented towards the water. The observation indicated that the atmospheric reactions of those GLV may proceed at the aqueous interfaces rather than in the bulk gas or aqueous phases.

3.2. UV Spectra

UV spectroscopy is often used to follow the concentrations of reactants quantitatively in laboratory systems. The UV spectra of compounds quickly inform us about the possible photodegradation of these products. The energy flux of solar UV radiation is practically null below 290 nm [158], so compounds that do not absorb light above this value do not undergo photolysis in the atmosphere. The absorption cross-sections in the gas phase and the extinction coefficients in the aqueous phase were defined with Equations (9a) and (9b), resp. Formally, Equation (9c) converts both quantities between each other.

$$\sigma \left(\lambda \right)=-\frac{ln\left[I\left(\lambda \right)/I_0\left(\lambda \right)\right]}{Lc}$$

(9a)

$$\epsilon \left(\lambda \right)=-\frac{ln\left[I\left(\lambda \right)/I_0\left(\lambda \right)\right]}{Lc}=\frac{A\left(\lambda \right)}{Lc}$$

(9b)

$$\epsilon =\sigma N\times {10}^{-3}$$

(9c)

where σ (λ) is the absorption cross-section in cm² molecule⁻¹ at wavelength λ, ε(λ) is extinction coefficient in cm⁻¹ M⁻¹, L is the optical path in cm, C is the concentration of absorbing gas in molecule cm⁻³, c is the concentration of absorbing solute in M, I(λ) and I₀(λ) are the light intensities in the presence and absence of absorbing gas, resp., A(λ) is the absorbance of a solute at λ, N is Avogadro’s number.

Jiménez et al. [57] determined the absolute UV absorption cross-sections of pure gaseous 1-penten-3-ol (1.5–6.5 mmHg), 1-penten-3-one (1.1–7.4 mmHg), and (Z)-3-hexen-1-ol (0.2–0.4 mmHg) in a Pyrex absorption cell with quartz windows at 298 K (Figure 1 and Figure 2). Jiménez et al. [159] used a similar setup to determine the UV absorption cross-sections of pure vapors of hexanal (0.9–7.0 mmHg) and (E)-2 hexenal (0.6–3.5 mmHg) at room temperature (Figure 3 and Figure 4).

Xing et al. [160] determined the UV absorption cross-section of gaseous (Z)-3-hexenal (0.2–7.0 mmHg) at 298 K using a Horiba Jobin Yvon, Triax-320 spectrophotometric setup (Figure 5). Absorption followed the Beer’s Law in the range 240–340 nm.

O’Connor et al. [161] determined the UV absorption cross-sections of gaseous (E)-2-hexenal and hexanal in a Pyrex cell with quartz windows at 297 ± 3 K (Figure 3 and Figure 4). They used both a static and a dynamic method. The cell was either filled with a given amount of aldehyde or flushed continuously with a mixture containing 1.7 × 10¹⁶–8.8 × 10¹⁶ molecule cm⁻³ of aldehyde and nitrogen. The cell pressure varied between 1.0 and 5.8 mmHg. The dynamic method failed for (E)-2-hexenal, probably because of leaks. The absorption cross-sections were slightly lower than [159], but the main absorption peaks appeared at the same wavelength.

Kalalian et al. [162] determined the UV absorption cross-sections for gaseous (E)-2-pentenal (0.2–9 mmHg) and (E)-2-hexenal (0.2–3 mmHg) at 298 K (Figure 1 and Figure 4). Their results agree well with [159,161] in terms of peak wavelengths and with [159] in terms of intensities. Absorption below 245 nm resulted from the π-π * transition and the absorption band between 270 and 370 nm–to the n-π* transition of the carbonyl group absorption.

Richards-Henderson et al. [22] determined the UV absorption coefficients for (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, MeSa, MeJa, and 2-methyl-3-butene-2-ol in aqueous solutions of pH = 5.5 ± 03 at 298 K (Figure 2, Figure 6, and Figure 7). Their results agree well with [163] and this work regarding peak location, while differences in the intensities can result from experimental errors [163].

Canosa-Mas et al. [164] recorded a UV spectrum of MeSa vapors at equilibrium with liquid phase at 293 K (Figure 6d).

Sarang et al. [163] presented UV spectra of 1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al in aqueous solutions at 298 K and pH = 7 (Figure 1, Figure 4 and Figure 8). The spectra were recorded with a double-beam Lambda 900 UV/VIS/NIR spectrometer (Perkin Elmer Instruments). We also included unpublished spectra of (E)-2-hexenal, (Z)-2-hexen-1-ol, (Z)-3-hexenyl acetate, MBO, and MeSa in aqueous solutions recorded at 295 K with a double-beam Jasco V-570 UV-VIS-NIR spectrophotometer using water as a reference medium (Figure 4, Figure 6, Figure 7, Figure 8).

Rudich et al. [165] determined the UV absorption cross-section of MBO in the gas phase using a flow-through cell at 10 mmHg to minimize the influence of isoprene formed by the decomposition of MBO (Figure 7a).

Figure 1. Absorption cross-sections σ of (a) 1-penten-3-one, (b) 1-penten-3-ol [57], and (c) (E)-2-pentenal [162] in the gas phase at 298 K, and absorption coefficients ε of (d) 1-penten-3-ol in the aqueous phase at 298 K and pH = 7 [163].

Figure 1. Absorption cross-sections σ of (a) 1-penten-3-one, (b) 1-penten-3-ol [57], and (c) (E)-2-pentenal [162] in the gas phase at 298 K, and absorption coefficients ε of (d) 1-penten-3-ol in the aqueous phase at 298 K and pH = 7 [163].

Figure 2. (Z)-3-hexen-1-ol: absorption cross-sections σ in the gas phase [57] (a) and absorption coefficient ε in the aqueous phase at 298 K and pH = 5.5 [22] (b).

Figure 2. (Z)-3-hexen-1-ol: absorption cross-sections σ in the gas phase [57] (a) and absorption coefficient ε in the aqueous phase at 298 K and pH = 5.5 [22] (b).

Figure 3. Absorption cross-sections σ of hexanal in the gas phase (a) [159], (b) at 297 K [161], and (c) at room temperature [166].

Figure 3. Absorption cross-sections σ of hexanal in the gas phase (a) [159], (b) at 297 K [161], and (c) at room temperature [166].

Figure 4. (E)-2-hexenal: absorption cross-sections σ in the gas phase (a) at 298 K [159], (b) at 297 ± 3 K [161], ((c), top and middle plate) at 298 K [162], and (d) absorption coefficients ε in the aqueous phase at 298 K and pH = 5.5 Sarang [163] and this work.

Figure 4. (E)-2-hexenal: absorption cross-sections σ in the gas phase (a) at 298 K [159], (b) at 297 ± 3 K [161], ((c), top and middle plate) at 298 K [162], and (d) absorption coefficients ε in the aqueous phase at 298 K and pH = 5.5 Sarang [163] and this work.

Figure 5. Absorption cross-sections σ of (Z)-3-hexenal in the gas phase at 298 K [160].

Figure 5. Absorption cross-sections σ of (Z)-3-hexenal in the gas phase at 298 K [160].

Figure 6. Absorption coefficients ε of (a) (Z)-3-hexenyl acetate, (b) methyl jasmonate, and (c) methyl salicylate in aqueous solutions at 298 K and pH = 5.5 (Richards-Henderson [22]) and pH = 7 (this work), and (d) absorption cross-sections of methyl salicylate vapor at 293 K [164].

Figure 6. Absorption coefficients ε of (a) (Z)-3-hexenyl acetate, (b) methyl jasmonate, and (c) methyl salicylate in aqueous solutions at 298 K and pH = 5.5 (Richards-Henderson [22]) and pH = 7 (this work), and (d) absorption cross-sections of methyl salicylate vapor at 293 K [164].

Figure 7. 2-methyl-3-buten-2-ol: absorption cross-sections σ of 2-methyl-3-buten-2-ol in the gas phase [165] (a) and absorption coefficients ε in the aqueous phase at 298 K and pH = 5.5 (Richards-Henderson [22]) and pH = 7 (this work) (b).

Figure 7. 2-methyl-3-buten-2-ol: absorption cross-sections σ of 2-methyl-3-buten-2-ol in the gas phase [165] (a) and absorption coefficients ε in the aqueous phase at 298 K and pH = 5.5 (Richards-Henderson [22]) and pH = 7 (this work) (b).

Figure 8. (Z)-2-hexen-1-ol: absorption coefficients ε in the aqueous phase at 298 K and pH = 7 Sarang [163] and this work.

Figure 8. (Z)-2-hexen-1-ol: absorption coefficients ε in the aqueous phase at 298 K and pH = 7 Sarang [163] and this work.

4. Gas-Phase Chemistry

GLVs undergo reactions with oxidants such as hydroxyl (OH) radicals, nitrate (NO₃) radicals, ozone (O₃), and Cl atoms in the atmospheric air.

Atkinson and Arey [167,168] reviewed the literature on mechanisms and kinetics of gas-phase atmospheric reactions of VOC and BVOC but included no data on GLV except hexanal and hexenols. Section 4 discusses detailed mechanisms and products of GLV reactions with OH, NO₃, O₃, and Cl. This review did not aim to cover the theoretical modeling of atmospheric reactions. Interested readers may see an excellent review by Vereecken et al. [169].

4.1. Products and Mechanisms

Volatile organic compounds react with radicals X following the general Scheme 4 [168,170], which begins with the formation of an alkyl radical by adding X to a double C=C bond or by hydrogen abstraction. The alkyl radical reacts with oxygen to form the alkylperoxy radical RO₂. The fate of RO₂ depends on available reactants. RO₂ self-reacts with another RO₂ and gives a carbonyl and alcohol or an alkoxy radical. The reaction with HO₂ radicals leads to hydroperoxides, while the reaction with NO₂− to the peroxynitrate. The reaction of RO₂ with NO gives either the nitrate or the alkoxy radical, which reacts further to various products.

4.1.1. Reactions with OH

GLV reactions with OH radicals begin with the OH addition to the C=C bond or the hydrogen abstraction by OH. Almost all experimental works cited in this section illustrate their results using the addition mechanisms. However, Gai et al. [171] compared both channels in reactions of several hexenols with OH radicals at CCSD(T)/6-311++G(2d,2p)//BH&HLYP/6-31 G(d,p) level of theory. The hydrogen abstraction channel included 11 abstractions at all C sites and excluded the OH group. The addition channel generally appeared faster, and the branching ratios of the abstraction channel ranged from 0.16 to 0.59 (

Table 6. Theoretical rate constants (cm³ molecule⁻¹ s⁻¹) for reactions of hexenols with OH radicals through the OH addition and hydrogen abstraction channels [171].

Reactant	kOH addition	kH abstraction	ktotal	H-Abstraction Branching Ratio	ktotal Deviation from Experimental Values %
(E)-2-hexen-1-ol	4.97 × 10−11	9.76 × 10−12	5.95 × 10−11	0.16	26.4
(Z)-2-hexen-1-ol	5.34 × 10−11	2.19 × 10−11	7.53 × 10−11	0.29	11.7
(E)-3-hexen-1-ol	2.45 × 10−11	3.50 × 10−11	5.95 × 10−11	0.59	34.6
(Z)-3-hexen-1-ol	1.17 × 10−10	3.07 × 10−11	1.48 × 10−10	0.21	−46.5
(E)-4-hexen-1-ol	4.74 × 10−11	1.30 × 10−11	6.04 × 10−11	0.22	15.4
(Z)-4-hexen-1-ol	1.10 × 10−10	2.73 × 10−11	1.37 × 10−10	0.20	82.6

). The overall theoretical rate constants (abstraction + addition) differed by (−46) to 82% from the experimental rate constants determined by those authors (Section 4.2.1, Table 15). Table 7 and Table 8 show the products of GLV–OH reactions and the corresponding experiments.

On the contrary, Du and Zhang [172] performed quantum chemical modeling of the OH radical reaction with MBO. The reaction proceeded mainly through the equally fast additions of OH to two unsaturated C atoms in MBO and the formation of C₄ and C₃ adducts. Both intermediates reacted further with oxygen and finally gave two primary products: glycolaldehyde CH₂OHCHO and propanone CH₃COCH₃. The minor products, formaldehyde HCHO and 2,3-dihydroxy-3-methyl propanal (CH₃)₂COHCHO, formed through the C₄ adduct [172].

Among the experimentalists, only Davis et al. [173] considered the hydrogen abstraction channel in reactions of GLV aldehydes with OH (Scheme 5, Scheme 6, Scheme 7). First, an alkyl radical forms and reacts with molecular oxygen to give the peroxy radical (Scheme 5).

In the presence of NO_x, the peroxy radical reacts with NO₂ to produce a peroxynitrate or reacts with NO to afford an alkoxy radical by oxygen abstraction (Scheme 6). The alkoxy radical enters a series of reactions with O₂ and NO that lead to the formation of an aldehyde and alkyl glyoxal (Scheme 7).

We explain the principles of the OH addition mechanism using (Z)-3-hexen-1-ol as an example. The references quoted may refer to other GLV but includes the same mechanism principle. First, the OH radicals add to the C=C bond producing two isomeric alkoxy radicals. Those radicals react with molecular oxygen to form two isomeric peroxy radicals (Scheme 8) [95].

In the absence of NOx, the peroxy radicals undergo a self-reaction that produces isomeric alkoxy radicals (Scheme 9), as shown for similar hexenals [174,175,176] and MBO [177].

If NO is present, it reacts with peroxy radicals either to subtract one oxygen atom and produce two alkoxy radicals [95,173,174,175,176,177,178,179,180,181,182] or to afford two organic dihydroxy nitrates by addition [178,180] (Scheme 10).

The alkoxy radicals can isomerize [180,183], decompose [95,174,175,176,180], or undergo hydrogen abstraction [175,176] (Scheme 11). The products formed include respectively: (i) 3,4-dihydroxyhexanal, (ii) propanal and 3-hydroxypropanal, and (iii) two dihydroxyhexanones and one trihydroxyhexanone.

Propanal can react further with OH, O₂, and NO_x to give peroxynitrates, formaldehyde, and CO₂ [95].

MBO reacts with OH like (Z)-3-hexen-1-ol, and final products include glycolaldehyde, acetone, formaldehyde, hydroxymethylpropanal, and trihydroxybutanal. Scheme 12 shows the plausible reaction paths for MBO-derived alkoxy radicals [177,178,180,182].

Furthermore, the MBO-derived peroxy radicals can react with NO to give MW 165 dihydroxynitrates (Scheme 13) [180].

The OH radicals add to GLV aldehydes like they do to GLV alcohols and lead to aldehydes, alkylglycolaldehydes, glyoxal, and CO (Scheme 14) [173]. Yet, an alternative decomposition of the alkoxy radical leads to aldehyde, alkylglyoxal, and CO₂ (Scheme 7).

A saturated GLV pentan-1-ol reacts with OH by the hydrogen abstraction from a CH₂-group at position 2 and subsequent reaction with oxygen, leading to a β-hydroxyalkoxy radical. The latter can isomerize to 1,5-dihydroxy-2-pentanone, decompose to butanal, or get autoxidized to 1-hydroxy-2-pentanone [184]. Initial hydrogen abstraction at position 3 leads to 3-hydroxypropanal, and at position 4, to 5-hydroxy-2-pentanone and 4-hydroxypentanal.

Table 7. Products from gas-phase reactions of GLV with OH radicals.

GLV	Product	Yield %	Expt Table 8	Ref.
Pentan-1-ol	Pentanal	40.5 ± 8.2	X	[184]
	Butanal	16.2 ± 3.7
	Propanal	8.1 ± 1.9
	Etanal	18.1 ± 4.2
	Formaldehyde	25.1 ± 1.3
	5-hydroxy-2-pentanone	observed
	3-hydroxypropanal	observed
1-penten-3-ol	Formaldehyde	35 ± 4	I	[185]
	Glycolaldehyde	47 ± 6
(Z)-2-penten-1-ol	Formaldehyde	11 ± 2
	Propanal	91 ± 13
	Glycolaldehyde	87 ± 11
(E)-2-hexen-1-ol	Butanal	Main	II	[176]
(E)-3-hexen-1-ol	Propanal	Main	II	[176]
		37 ± 7	III	[175]
(Z)-3-hexen-1-ol	Propanal	Main	II	[176]
		58 ± 8	IV	[180]
		74.6 ± 6.7	VIII	[183]
	3-hydroxypropanal	101 ± 24	IV	[180]
		48 + 48/−24	VIII	[183]
	3,4-dihydroxyhexanoic acid	Observed	IV	[180]
	Dihydroxynitrates MW 179 (see Scheme 10)	Observed	IV, VIII	[180,183]
	Hydroxycarbonyl MW 132 (see Scheme 11)	Observed	VIII	[183]
(Z)-3-hepten-1-ol	Butanal	33 ± 3	III	[175]
(E)-2-hexenylacetate	Butanal	Observed	XI	[186]
2-methyl-3-buten-2-ol	Acetone	14.1 ± 0.2	V	[182]
		52 ± 5 molar	VI	[179]
		58 ± 4	VII	[178]
		observed	IV	[180]
		67 ± 5	IX	[177]
	Formaldehyde	9.3 ± 3.3	V	[182]
		35 ± 4 molar	VI	[179]
		29 ± 3	VII	[178]
		observed	IV	[180]
		33 ± 3	IX	[177]
	Formic acid	9.3 ± 3.3	V	[182]
	Glycolaldehyde	28.0 ± 2.8	V	[182]
		50 ± 5 molar	VI	[179]
		61 ± 9	VII	[178]
		78 ± 20	IX	[177]
		66 ± 2	XIII a	[187]
		29 ± 5	XIII a	[187]
	2-hydroxy-2-methylpropanal	19 ± 7	VII	[178]
		31 ± 4	IV	[180]
		31 ± 11	IX	[177]
		12 ± 2	XIII a	[187]
		37 ± 7	XIII b	[187]
	Dihydroxynitrates MW 165 (see Scheme 13)	observed	IV	[180]
	Organic nitrates	5 ± 2	VII	[178]
	Glyoxal	26	XIII a	[187]
		37	XIII b	[187]
	CO	observed	V	[182]
	CO2	observed	V	[182]
Nonanal	1-nitooxy octane	40	XII	[188]

Table 7. Products from gas-phase reactions of GLV with OH radicals.

GLV	Product	Yield %	Expt Table 8	Ref.
Pentan-1-ol	Pentanal	40.5 ± 8.2	X	[184]
	Butanal	16.2 ± 3.7
	Propanal	8.1 ± 1.9
	Etanal	18.1 ± 4.2
	Formaldehyde	25.1 ± 1.3
	5-hydroxy-2-pentanone	observed
	3-hydroxypropanal	observed
1-penten-3-ol	Formaldehyde	35 ± 4	I	[185]
	Glycolaldehyde	47 ± 6
(Z)-2-penten-1-ol	Formaldehyde	11 ± 2
	Propanal	91 ± 13
	Glycolaldehyde	87 ± 11
(E)-2-hexen-1-ol	Butanal	Main	II	[176]
(E)-3-hexen-1-ol	Propanal	Main	II	[176]
		37 ± 7	III	[175]
(Z)-3-hexen-1-ol	Propanal	Main	II	[176]
		58 ± 8	IV	[180]
		74.6 ± 6.7	VIII	[183]
	3-hydroxypropanal	101 ± 24	IV	[180]
		48 + 48/−24	VIII	[183]
	3,4-dihydroxyhexanoic acid	Observed	IV	[180]
	Dihydroxynitrates MW 179 (see Scheme 10)	Observed	IV, VIII	[180,183]
	Hydroxycarbonyl MW 132 (see Scheme 11)	Observed	VIII	[183]
(Z)-3-hepten-1-ol	Butanal	33 ± 3	III	[175]
(E)-2-hexenylacetate	Butanal	Observed	XI	[186]
2-methyl-3-buten-2-ol	Acetone	14.1 ± 0.2	V	[182]
		52 ± 5 molar	VI	[179]
		58 ± 4	VII	[178]
		observed	IV	[180]
		67 ± 5	IX	[177]
	Formaldehyde	9.3 ± 3.3	V	[182]
		35 ± 4 molar	VI	[179]
		29 ± 3	VII	[178]
		observed	IV	[180]
		33 ± 3	IX	[177]
	Formic acid	9.3 ± 3.3	V	[182]
	Glycolaldehyde	28.0 ± 2.8	V	[182]
		50 ± 5 molar	VI	[179]
		61 ± 9	VII	[178]
		78 ± 20	IX	[177]
		66 ± 2	XIII a	[187]
		29 ± 5	XIII a	[187]
	2-hydroxy-2-methylpropanal	19 ± 7	VII	[178]
		31 ± 4	IV	[180]
		31 ± 11	IX	[177]
		12 ± 2	XIII a	[187]
		37 ± 7	XIII b	[187]
	Dihydroxynitrates MW 165 (see Scheme 13)	observed	IV	[180]
	Organic nitrates	5 ± 2	VII	[178]
	Glyoxal	26	XIII a	[187]
		37	XIII b	[187]
	CO	observed	V	[182]
	CO2	observed	V	[182]
Nonanal	1-nitooxy octane	40	XII	[188]

Table 8. Experiments used to determine the rate constants listed in Table 7.

Expt	Photoreactor			Detection	Temp.	p	OH Source	Ref.
	Type	Vol. dm3	Material		K	atm
I	Chamber	47	Steel	LP FTIR	298	1	Ethyl nitrite	[185]
II	Bag	80	Teflon	GC-FID (SPME)	298	1	H2O2	[176]
III	Chamber	1080	Quartz	LP FTIR	298	1	H2O2	[175]
IV	Chamber	7000	Teflon	GC-FID, GC-MS, API-MS	296	0.97	Methyl nitrite	[180]
V	Chamber	480	Teflon	LP FTIR	295	0.97	H2O2	[182]
VI	Chamber	47	Steel	LP FTIR	295		Methyl nitrite	[179]
VII	Chamber	5800	Teflon	LP FTIR, GC-FID, GC-MS, API-MS/MS	298	1	Ethyl nitrite	[178]
VIII	Chamber	6500–7900	Teflon	GC-FID, GC-MS, API-MS	298	1	Methyl nitrite	[183]
IX	LISA CRAC EUPHORE			LP FTIR	298	1	HONO, n-propyl nitrite	[177]
X	Reactor	480	Duran	LF FIR, GC-PI	298	0.97	Methyl nitrite	[184]
XI	Bag	80	Teflon	GC-FID	298	1	H2O2	[186]
XII	Chamber	5000	Teflon	GC-FID	298	1	Isopropyl nitrite	[188]
XIII a	Chamber	28,000	Teflon	LIP	294–298		Cyclohexane	[187]
XIII b							Cyclohexane NOx

Table 8. Experiments used to determine the rate constants listed in Table 7.

Expt	Photoreactor			Detection	Temp.	p	OH Source	Ref.
	Type	Vol. dm3	Material		K	atm
I	Chamber	47	Steel	LP FTIR	298	1	Ethyl nitrite	[185]
II	Bag	80	Teflon	GC-FID (SPME)	298	1	H2O2	[176]
III	Chamber	1080	Quartz	LP FTIR	298	1	H2O2	[175]
IV	Chamber	7000	Teflon	GC-FID, GC-MS, API-MS	296	0.97	Methyl nitrite	[180]
V	Chamber	480	Teflon	LP FTIR	295	0.97	H2O2	[182]
VI	Chamber	47	Steel	LP FTIR	295		Methyl nitrite	[179]
VII	Chamber	5800	Teflon	LP FTIR, GC-FID, GC-MS, API-MS/MS	298	1	Ethyl nitrite	[178]
VIII	Chamber	6500–7900	Teflon	GC-FID, GC-MS, API-MS	298	1	Methyl nitrite	[183]
IX	LISA CRAC EUPHORE			LP FTIR	298	1	HONO, n-propyl nitrite	[177]
X	Reactor	480	Duran	LF FIR, GC-PI	298	0.97	Methyl nitrite	[184]
XI	Bag	80	Teflon	GC-FID	298	1	H2O2	[186]
XII	Chamber	5000	Teflon	GC-FID	298	1	Isopropyl nitrite	[188]
XIII a	Chamber	28,000	Teflon	LIP	294–298		Cyclohexane	[187]
XIII b							Cyclohexane NOx

Priya and Senthilkumar [189] theoretically studied the reaction of MeSa with OH radicals using DFT methods with B3LYP, M06-2X [50], and MPW1K [51] functionals, and 6-311++G(d,p) basis set. The dominating hydrogen abstraction channel was the abstraction of OH hydrogen (Scheme 15). The dominating addition channel was the OH addition at para position (Scheme 16).

On the contrary, Seif et al. [190] theoretically found that the addition at the ipso position had the largest branching ratio and the addition at the para position, the smallest one. The rate constants k_i for the individual addition channels calculated at 298 K and 1 atm using RRKM theory at MN15-L/aug-cc-pVTZ level with Eckart tunneling were 4.65, 2.02, 1.01, 0.24, 3.27, and 0.57 10⁸ M⁻¹ s⁻¹, respectively for the positions from 1 to 6. The hydrogen abstraction mechanism appeared negligible because of the high Gibbs potential.

4.1.2. Reactions with NO₃

There are not many papers on the mechanisms of GLV reactions with NO₃. General principles can emerge from the works on reactions with other alkenes [168,170]. The reaction begins with the NO₃ addition to a double C=C bond which yields two isomeric alkyl radicals (Scheme 17). The following steps are like those in the GLV-OH mechanism. Fantechi et al. proposed a similar mechanism for the reaction of MBO with NO₃ radicals (Scheme 18) [182]. That mechanism includes an additional decomposition of alkoxy radicals, which leads to acetone. More decomposition steps for MBO-derived alkoxy radicals were proposed (Scheme 19) [191].

Products obtained from reactions of GLV with NO₃ are listed in

Table 9. Products observed in reactions of GLV with NO₃.

GLV	Product	Yield, % Molar	Expt TabLe 10	Ref.
1-penten-3-ol, (Z)-2-penten-1-ol	2-pentenyl nitrate	observed	I a	[192]
	Ethyl vinyl nitrate	observed
	2-penten-1-ol	observed	I b
(E)-2-hexenal	PAN analogue	100 ± 2.5	II	[193]
	CO	6.4 ± 4.2
	Formic acid	observed
MBO	Acetone	63	III	[191]
		68.7 ± 7.1	IV	[182]
	1-nitroxyacetaldehyde	67	III	[191]
	Formaldehyde	4	III	[191]
	Organic nitrates	12.5–13.5	IV	[182]
	Hydroxy-aldehyde nitrates, peroxynitrates, carbonyl nitrates, nitroxyalcohols, dinitrates	observed

, while

Table 10. Experiments used to determine the rate constants listed in Table 9.

Expt	Photoreactor			Detection	T	p	NO3 Source	Ref.
	Type	Vol. dm3	Material		K	atm
I a	Chamber	80	Teflon	GC-MS	298	1	N2O5	[192]
I b							NO2
II	Reactor	977	Pyrex	LP FTIR, UV	294	1	N2O5	[193]
III	Reactor	153	Glass	LP FTIR	297	1	N2O5	[191]
IV	Chamber	480	Teflon	LP FTIR	295	0.97	N2O5	[182]

briefly describes the corresponding experiments.

4.1.3. Reactions with O₃

The ozonolysis of alkenes generally begins with a primary ozonide formation by O₃ addition to a double C=C bond, which decomposes to Criegee biradicals and carbonyl products (Scheme 20) [194,195,196,197]. Those products can recombine to form a secondary ozonide.

Criegee biradicals stabilize by a range of reactions that depend on the R substituents and are presented below for individual GLV.

The basic mechanism of the ozonolysis of (Z)-3-hexenol includes the formation of propanol, 3-hydroxypropanal, and Criegee intermediates that stabilize by hydration and dehydration, leading to propanoic and 3-hydroxypropanoic acids (Scheme 21) [92,198].

Scheme 22 shows additional hydroperoxide product channels. They begin with Criegee stabilization by isomerization and include OH-driven abstraction steps, which may be plausible [198]. Further schemes present more isomerization channels of hexenylacetates.

Scheme 23 shows the basic mechanism for O₃ reaction with (Z)-3-hexenyl acetate [92,196,198,199,200]. The primary carbonyls formed are propanal and 3-oxypropyl acetate, while Criegee intermediates stabilize, leading to propanoic and 3-acetoxy propanoic acids.

Scheme 24 and Scheme 25 show the theoretically predicted stabilization pathways of Criegee intermediates, which afford several carbonyl products [196].

Scheme 26 shows the mechanism of the MBO ozonolysis. Smog chamber experiments confirmed some channels only at dry conditions [177,182].

Priya et al. [189] studied the ozonolysis of MeSa based on a simplified chemical mechanism. It contained the cycloaddition of ozone to the C₅-C₆ bond and abstraction of one oxygen by H₂ (Scheme 27). They managed to estimate the rate constants for those steps that were close to the experimental values.

Products observed in GLV reactions with ozone are collected in

Table 11. Products observed in reactions of GLV with ozone.

GLV	Product	Yield, Molar	Expt Table 12	Ref.
1-penten-3-ol	Formaldehyde	0.49 ± 0.02	I	[201]
		0.34 ± 0.04	IX	[199,200]
	2-hydroxybutanal	0.46 ± 0.03	I	[201]
		0.30 ± 0.05 a	IX	[199,200]
	Propanal	0.15 ± 0.02	I	[201]
		0.12 ± 0.01	IX	[199,200]
(Z)-2-penten-1-ol	Propanal	0.39 ± 0.03	I	[201]
		0.493 ± 0.075	V	[183]
		0.51 ± 0.02	IX	[199,200]
	3-hydroxypropanal	0.33 + 0.33/−0.16	V	[183]
	Glycolaldehyde	0.43 ± 0.04	I	[201]
	2-hydroxyacetaldehyde b	0.53 ± 0.04	IX	[199,200]
	Formaldehyde	0.07 ± 0.02	IX	[199,200]
	Acetaldehyde	0.08 ± 0.01	IX	[199,200]
	methylglyoxal	0.04 ± 0.01	IX	[199,200]
1-penten-3-one	Formaldehyde	0.37 ± 0.02	I	[201]
	2-oxobutanal	0.49 ± 0.03	I	[201]
	SOA	0.13–0.17	I	[201]
(Z)-3-hexen-1-ol	Propanal	Observed	II	[92]
		0.43 ± 0.02	VIII	[202]
		0.59	IX	[199,200]
	Propanoic acid	Observed	II	[92]
	2-propenal	Observed	II
	Acetaldehyde	Observed	II
	Ethane	0.069 ± 0.005	VIII	[202]
	OH	0.26	VI	[174]
		0.28 ± 0.06	VIII	[202]
(E)-2-hexenal	OH	0.62	VI	[174]
	Butanal	0.53	IX	[199,200]
		0.527 ± 0.055	IX	[203]
	Glyoxal	0.56		[199,200]
		0.559 ± 0.037	IX	[203]
	2-oxobutanal c	0.074 ± 0.006
	acetaldehyde	0.109 ± 0.020
	Propanal	0.067 ± 0.008
	cyclohexanone	0.032 ± 0.003
(Z)-3-hexenal	Propanal	0.35 ± 0.01	VIII	[202]
	Ethane	0.079 ± 0.004
	OH	0.32 ± 0.07
(Z)-3-hexenyl acetate	Propanal	Observed	II	[92]
		0.76 ± 0.04
	Propanoic acid	Observed	II	[92]
	2-propenal	Observed	II
	Acetic acid	Observed	II
	OH	0.16	VI	[174]
	acetaldehyde	0.05 ± 0.01	IX	[199,200]
	methylglyoxal	0.05 ± 0.01
	Acetone	0.04 ± 0.01
	glyoxal	0.003 ± 0.001
(E)-2-hexenyl acetate	butanal	0.47	IX	[199,200]
		0.473 ± 0.023	IX	[203]
	1-oxyethyl acetate	0.58	IX	[199,200]
		0.583 ± 0.141 c	IX	[203]
	Glyoxal	0.209 ± 0.004
	Propanal	0.102 ± 0.002
	2-oxobutanal c	0.091 ± 0.007
	Acetaldehyde	0.039 ± 0.003
	Cyclohexanone	0.090 ± 0.004
3-metyl-2-buten-3-ol	Acetone	0.083 ± 0.050	III	[182]
		Observed	IV	[178]
		0.67 ± 0.05	VII a	[177]
		0.23 ± 0.06	IX	[199,200]
	Formaldehyde	0.467 ± 0.055	III	[182]
		0.29 ± 0.03	IV	[178]
		0.44 ± 0.05	VII a	[177]
		0.55 ± 0.03	VII b	[177]
		0.36 ± 0.09	IX	[199,200]
	Formic acid	Observed	III	[182]
		0.14 ± 0.04	VII a	[177]
	Formic anhydride	0.16 ± 0.07
	2-hydroxy-2-methylpropanal	0.30 ± 0.060.47 (FTIR)	IV	[178]
		0.43 ± 0.12	VII a	[177]
		0.84 ± 0.08	VII b	[177]
		0.30 ± 0.02	IX	[199,200]
	Acetaldehyde	0.01 ± 0.01
	CO	0.343 ± 0.024	III	[182]
	CO2	0.232 ± 0.015	III
	OH	0.19	IV	[178]

^a measured as 2-oxobutanal; ^b measured as glyoxal; ^c tentative.

.

Table 12. Experiments used to determine the rate constants listed in Table 11.

Expt	Photoreactor			Detection	Temp.	p	Additions	Ref.
	Type	Vol. dm3	Material		K	atm
I	CRAC LISA	3910 977	FEP Pyrex	LP FTIR	293 b 295 c	1	CO a	[201]
II	Chamber	775	Teflon	GC-MS	296	1		[92]
III	Chamber	480	Teflon	LP FTIR	295	0.97		[182]
IV	Chamber	7000–8000, 5800	Teflon	API-MS/MS, GC-FID, GC-MS, GC-FTIR, LP_FTIR	298	0.97		[178]
V	Chamber	6500–7900	Teflon	GC-FID, GC-MS	298		Cyclohexane a	[183]
VI	Chamber	6700, 7900	Teflon	GC-FID	296	0.97	Cyclohexane a (E)-2-butene d	[174]
VII a	LISA CRAC	980 3910	Pyrex FEP	LP-FTIR GC-MS	298 e	1	CO a	[177]
VII b	CRAC	3910	FEP	LP-FTIR GC-MS	298 f	1	CO a	[177]
VIII	Chamber	6000		LP FTIR	298	1	1,3,5-trimethyl benzene a	[202]
IX	Chamber	3700–3900	Teflon	LC-UV	282–296	1	Cyclohexane a	[199,200]

^a OH scavenger; ^b RH < 0.4%; ^c RH < 1%; ^d reference reactant; ^e RH <0.2%; ^f RH ≈30%.

briefly describes the corresponding experiments. The formation of SOA particles during ozonolysis of GLV and their composition are discussed in Section 6.1.

4.1.4. Reactions with Cl

The gas-phase reaction of Cl atoms with GLV follows a mechanism like that for OH radicals, which includes the Cl addition to a double bond or hydrogen abstraction by Cl at a C atom with the formation of HCl [179,204,205]. Quantum-mechanical modeling showed that the addition is the dominant pathway while the hydrogen abstraction cannot be excluded [205,206]. A few studies have focused on the GLV-Cl reaction products (

Table 13. Products from gas-phase reactions of GLV with Cl radicals.

GLV	Product	Yield %	Expt Table 14	Ref.
1-penten-3-ol	Chloroacetaldehyde Propanal Acetaldehyde 1-penten-3-one	33 ± 1 39 ± 1 8 ± 3 <2	I	[205]
(Z)-2-penten-1-ol	2-chlorobutanal Propanal Acetaldehyde (Z)-2-pentenal	19 ± 1 27 ± 1 18 ± 2 36 ± 1	I	[205]
(E)-2-hexenyl acetate	butanal	not quantified	V	[186]
	formaldehyde
	propanal
2-methyl-3-buten-2-ol	Acetone	47 ± 5	II	[179]
		48 ± 4	III	[207]
		38.5 ± 20.6	IV	[204]
	chloroacetaldehyde	53 ± 5	II	[179]
		47 ± 5	III	[207]
	formyl chloride	<11	II	[179]
		6.8 ± 5.9	IV	[204]
	formaldehyde	6 ± 2	II	[179]
		7.2 ± 0.6	III	[207]
	glycolaldehyde	2.1 ± 0.2	III	[207]
	formic acid	Below the quantification limit	III	[207]
		1.8 ± 0.1	IV	[204]
	HCl	15.7 ± 13.5	IV	[204]
	CO	20 ± 9.3
	CO2	<15

and

Table 14. Experiments used to identify the products listed in Table 13.

Expt	Photoreactor			Detection	Temp.	p	Cl Source	Ref.
	Type	Vol. dm3	Material		K	Atm
I	Bag	400	Teflon	GC-FID, GC-MS	298	1	Trichloroacetyl chloride	[205]
II	Chamber	47	Steel	LP FTIR	295		Cl2	[179]
III	Chamber	140	Pyrex	FTIR	296 ± 2	0.92	Cl2	[207]
IV	Chamber	480	Teflon	FTIR	298 ± 2	0.97	Cl2	[204]
V	Bag	80	Teflon	GC-FID, GC-MS (SPME)	298	1	oxalyl chloride	[186]

).

Scheme 28 shows the Cl addition to 1-penten-3-ol at two possible positions accompanied by the formation of alkyl radicals that can either isomerize or react with O₂ molecules to form the peroxy radicals [205].

The peroxy radicals formed can isomerize or enter a self-reaction to decompose (Scheme 29). The products of those reactions can further decompose or react with molecular oxygen to give stable products and release HO₂ radicals (Scheme 30).

The hydrogen abstraction in 1-penten-3-ol occurs preferentially at C₃ position, followed by the formation of peroxy radicals that self-react to form the alkoxy radicals. The latter can also self-react to afford 1-penten-3-one with the release of H₂O₂ (Scheme 31) [205].

Scheme 32 shows the reaction pathways for the alkoxy radicals derived from the Cl radical addition and hydrogen abstraction reactions of (Z)-2-penten-1-ol [205]. In both cases, acetaldehyde is a secondary product.

Shashikala and Janardanan [208] studied the reaction of (E)-2-hexenal with Cl radicals using the density functional theory. The geometry optimization used the MP2/6-31G * level of theory (B3), and later single point energies of all MP2/6-31G * optimized stationary points were computed using BHANDHLYP/6–311 + G** (B2) level of theory. They found that the Cl radical was more reactive towards (E)-2-hexen-1-al than other atmospheric oxidants (OH, O₃, NO₃), both in the Cl addition and H abstraction channels.

The reaction of MBO with Cl was studied experimentally [179,207] and theoretically at the MP2(full)/6-311G(d,p) and CCSD(T)/6-311þG(d,p) level of theory [206]. In the absence of NO, the Cl addition appeared more favorable than the hydrogen abstraction. The addition at the C₄ position provided almost 50% of the products by the molar yield. The identification of glycolaldehyde and formic acid among the products revealed the formation of intermediate OH radicals [207]. Scheme 33 shows the plausible transformation of the alkoxy radicals produced from the initial reactions of MBO with Cl.

In NO’s presence, the peroxy radicals derived from the MBO reaction with Cl convert to alkoxy radicals (Scheme 34) [204]. Scheme 35 and Scheme 36 show further reactions of the alkoxy radicals.

Scheme 37 shows the reactions of the alkoxy radicals formed by the hydrogen abstraction from MBO [204].

Elizabeth Gaona-Colmán et al. [186] studied the Cl addition to (E)-2-hexenyl acetate. Scheme 38 shows the plausible transformation of the alkoxy radicals formed by that reaction.

Priya and Senthilkumar [209] theoretically studied the mechanism of Cl radical reactions with MeSa using density functional theory at B3LYP and M06-2X levels of theory with 6-311++G(d,p) basis set. They found that the reaction involved the hydrogen abstraction at the meta position throughout all courses. They concluded that the reaction of MeSa with a Cl radical via hydrogen abstraction from the aromatic ring followed by reaction with O₂ results in the formation of methyl 3-peroxy-2-hydroxy benzoate radical intermediate via a barrierless reaction (Scheme 39).

The authors evaluated reactions of methyl 3-peroxy-2-hydroxy benzoate radicals with HO₂, NO₂, and NO. The reaction with NO was the most favorable, with the lowest energy barrier close to 6.7 kJ mol⁻¹ (Scheme 40) [209].

4.1.5. Gas-Phase Photolysis

O’Connor et al. [161] analyzed the sunlight photolysis of (E)-2-hexenal, (Z)-3-hexenal and (E,E)-2,4-hexadienal in a Euphore smog chamber in dry purified air (0.05–1.0% RH and 286–294 K) using LP FTIR, GC-FID, and GC-MS with EI ionization. The samples were preconcentrated on glass beads at −160 °C, then rapidly desorbed onto the chromatographic column. The authors found that CO was the major product (34% yield) of the photolysis of (Z)-3-hexenal (which gave small amounts of products) according to the Norrish type 1 mechanism (Scheme 41). The (Z)-2-pentenyl radical form reacted with O₂ to produce the pentenyl peroxy radical, which, in turn, self-reacted to a pentenyl oxy radical. The pentenyl oxy radical reacted with oxygen to finally give (Z)-3-pentenal, which showed in the experiments. On the other hand, (E)-2-hexen-1-al was reversibly converted to (Z)-2-hexen-1-al.

Tang and Zhu [166] studied the formation of HCO radicals from the gas-phase photolysis of n-hexanal (and n-heptanal) using cavity ring-down spectroscopy. Scheme 42 shows thermodynamically feasible dissociation pathways after UV excitation of n-hexanal. The radical yields ranged from 0.1 to 0.08 for 2–8 mmHg partial pressures of hexenal. Furthermore, they used mass spectrometry to identify several other products: acetaldehyde CH₃CHO and butene C₄H₈ (yield 0.28–0.31); CO and pentane C₅H₁₂ (yield 0.20); C₅H₁₁, 2-methylcyclopentanol (yield 0.12); and 2-ethylcyclobutanol (yield 0.3).

4.2. Gas-Phase Kinetics

This section presents the rate constants for gas-phase reactions of GLV with OH and NO₃ radicals, O₃, and Cl atoms, and the photolysis rate constants. The rate constants were determined either by the relative or absolute approach. The relative method compares a GLV reaction with an oxidant to the oxidant reaction with another compound for which the rate constant is known. From that comparison, the method derives a relative rate constant for the GLV-oxidant reaction. The absolute method directly follows the decay of GLV and the oxidant to determine its absolute rate constant.

4.2.1. Gas-Phase Reactions with OH Radicals

Table 15. Relative and absolute (^a) rate constants for gas-phase reactions of GLV with OH at single temperatures.

GLV	k cm3 molecule−1 s−1	T K	P Atm	Expt Table 16	Ref.
Pentan-1-ol	(1.0 ± 0.1) × 10−11	298	1	I	[210]
	(1.08 ± 0.11) × 10−11 a	296	0.03–0.07	II	[211]
	(1.20 ± 0.16) × 10−11 a	298 ± 2	1	III	[212]
	(1.05 ± 0.13) × 10−11	ibid		IV
	(1.11 ± 0.11) × 10−11	298 ± 2	740 mmHg	V	[184]
	(1.23 ± 0.10) × 10−11	295 ± 2	1	VI	[213]
	(1.26 ± 0.07) × 10−11 (1.20 ± 0.06) × 10−11	298 ± 2	1	VII	[214]
1-penten-3-ol	(6.7 ± 0.9) × 10−11	298	1	VIII	[185]
(Z)-2-penten-1-ol	(1.06 ± 0.15) × 10−10
(E)-2-pentenal	(2.35 ± 0.21) × 10−11 a	298	0.132–0.526	IX	[85]
1-penten-3-one	3.6 × 10−11 b	298	1	X	[215]
Hexan-1-ol	(1.58 ± 0.35) × 10−11	296 ± 2	1	XI	[216]
(Z)-2-hexen-1-ol	(1.1 ± 0.4) × 10−10	296 ± 2		XII	[217]
	0.95 × 10−10 b	298
	(8.53 ± 1.36) × 10−11 e	298	1	XIII	[171]
(E)-2-hexen-1-ol	(1.0 ± 0.3) × 10−10	298	1	XIV	[176]
	(8.08 ± 1.33) × 10−11 e	298	1	XIII	[171]
(Z)-3-hexen-1-ol	(1.2 ± 0.2) × 10−10	298	1	XIV	[176]
	(9.57 ± 2.42) × 10−11 a	298		XV	[57]
	(1.08 ± 2.2) × 10−10	296 ± 2	0.974	XVI	[174]
	(1.01 ± 0.16) × 10−10 e	298	1	XIII	[171]
(E)-3-hexen-1-ol	(1.4 ± 0.3) × 10−10	298	1	XIV	[176]
	(1.14 ± 0.14) × 10−10	298 ± 2	1	XVII	[175]
	(0.8 ± 0.1) × 10−10	296 2	0.98 ± 0.01	XII	[217]
	1.28 × 10−10	298
	(9.10 ± 1.50) × 10−11 e	298	1	XIII	[171]
(E)-4-hexen-1-ol	(7.14 ± 1.20) × 10−11 e	298	1	XIII	[171]
(Z)-4-hexen-1-ol	(7.86 ± 1.30) × 10−11 e
n-hexanal	(2.60 ± 0.21) × 10−11 a	298	0.132–0.526	IX	[85]
	(2.86 ± 0.13) × 10−11	298 ± 2	1.00 ± 0.01	XVIII	[218]
	(3.17 ± 0.15) × 10−11	296 ± 2	1	XIX	[219]
	(2.71 ± 0.20) × 10−11	298 ± 2	1	XX	[220]
(E)-2-hexenal	(0.681 ± 0.049) × 10−11	296 ± 2	0.974	XVI	[174]
	(2.95 ± 0.45) × 10−11 a	298	0.132–0.526	IX	[85]
	(3.95 ± 0.17) × 10−11	298 ± 2	1	XXI	[221]
(Z)−3-hexenal	(6.9 ± 0.9) × 10−11	298	1	XXII	[160]
(Z)- 3-hexenyl formate	(4.61 ± 0.07) × 10−11	298	1	XXIII	[222]
	(4.24 ± 0.07) × 10−11 b
(E)-2-hexenylacetate	(6.88 ± 1.41) × 10−11	298 ± 1	750 mmHg	XXIV	[186]
(Z)- 3-hexenyl acetate	(1.21 ± 0.71) × 10−11	296 ± 2	0.974	XVI	[174]
(Z)-3-hepten-1-ol	(1.28 ± 0.23) × 10−10	298 ± 2	1	XVII	[175]
2-methyl-3-buten-2-ol	(6.9 ± 1.0) × 10−11	295 ± 1	0.921	XXV	[179]
	(5.67 ± 0.13) × 10−11	296 ± 2	0.974	XIX	[223]
	(3.9 ± 1.2) × 10−11	298 ± 2	0.974 ± 0.007	XXVI	[224]
	(6.6 ± 0.5) × 10−11	298 ± 2	1	XXVII	[225]
	(5.6 ± 0.6) × 10−11	298 ± 2	1	XXVIII	[177]
	(5.49 ± 0.44) × 10−11 a	300	5	XXIX	[226]
	(6.32 ± 0.27) × 10−11 a,d
	(6.61 ± 0.66) × 10−11 a,c
(Z)-3-octen-1-ol	(1.49 ± 0.35) × 10−10	298 ± 2	1	XVII	[175]
Nonanal	(3.6 ± 0.7) × 10−10	298	1	XXX	[188]
	(2.88 ± 0.20) × 10−11	298 ± 2	1	XX	[220]
Methyl salicylate	(3.20 ± 0.46) × 10−12	298	1	XXXI	[227]

^a absolute rate constant; ^b from SAR or LFER; ^c reaction with OD; ^d with 15% of O₂ added; ^e theoretical rate constants for the addition and hydrogen abstraction channels are given in Section 4.1.1 (Table 6).

contains the rate constants for gas-phase reactions of GLV with OH radicals determined experimentally at single temperatures,

Table 16. Experiments used to determine the rate constants listed in Table 15.

Expt	Photoreactor			Detection	OH Source	Add.	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I	Bag	140	Teflon	GC-FID	H2O2		Pentane 1,3-dioxolane Cyclohexane	[210]
II	Bulbs	5	Pyrex	RF	H2O		no	[211]
III	Cylinder	50	Teflon	GC-FID	H2O2 or CH3ONO		Cycloxexane	[212]
IV	Cell	1	Steel	LP UV			no
V	Cylinder	480	Duran	LP FTIR; GC-PI	CH3ONO	NO a	Cycloxexane	[184]
VI	Bag	100	Teflon	GC-FID	H2O2		Propane; hexane	[213]
VII	Bag	100	Tedlar	GC-MS	CH3ONO		Cyclohexane; p-xylene	[214]
VIII	Chamber	47	Steel	LP FTIR	CH3CH2ONO		propene	[185]
IX	Cell PLP	0.2	Pyrex	LIF	H2O2		no	[85]
X	SSR LFER							[215]
XI	Chamber	7600	Teflon	GC-FID	CH3ONO	NO a	Cyclohexane	[216]
XII	Bag	200	Teflon	GC-FID	H2O2		Allyl ether Cycloxexane 1-methyl-cyclohexane	[217]
XIII	Bag	150	Teflon	GC-FID	H2O2		Cyclohexane 2-methyl-3-buten-1-ol	[171]
XIV	Bag	80	Teflon	GC-FID, SPME	H2O2		Methyl methacrylate (E)-2-buten-1-ol	[176]
XV	Cell PLP	0.2	Pyrex	UV, LIF	H2O2		no	[57]
XVI	Chamber	6700 7900	Teflon	GC-FID	CH3ONO	NO a	(E)-2-butene	[174]
XVII	Chamber	1080	Quartz	LP FTIR	H2O2		(E)-2-butene isobutene	[175]
XVIII	Reactor	250	Steel	LP FTIR	CH3ONO Other nitrites		Propene 1-butene	[218]
XIX	Chamber	7000	Teflon	GC-FID	CH3ONO	NO a	1,3,5-trimethylbenzene Methyl vinyl ketone	[219,223]
XX	Cylinder	108O	Pyrex	LP FTIR	CH3ONO		(E)-2-butene; Methylvinyl ketone	[220]
XXI	Bag	100	Tedlar	GC-MS	CH3ONO		2-methyl-2-butene, β-pinene	[221]
XXII	Chamber	6000	PFA	LP FTIR	CH3ONO		1,3,5-trimethylbenzene	[160]
XXIII	Bag	400	Teflon	GC-FID	H2O2		Cyclohexane, n-octane, 1-butene	[222]
XXIV	Bag	80	Teflon	GC-FID	H2O2		(E)-3-hexen-1-ol 2-buten-1-ol	[186]
XXV	Chamber	47	Steel	LP FTIR	CH3ONO		Ethylene, propylene	[179]
XXVI	Chamber	480	Teflon	LP FTIR	H2O2		Isoprene, propene	[224]
XXVII	Chamber	6000	Teflon	LP FTIR	CH3ONO	NO a	di-n-butyl ether, propene	[225]
XXVIII	LISA CRAC EUPORE	980, 3910 2 × 105	Pyrex FEP Teflon	LP FTIR	HONO H2O2	NOx	Isoprene	[177]
XXIX	Discharge-flow reactor	0.02	Pyrex	LIF	F + H2O, H + NO2		no	[226]
XXX	Chamber	5000	Teflon	GC-FID	Isopropyl nitrite	NOx	Octane	[188]
XXXI	ICARE-CHRS chamber	7300	Teflon	HR PTR-TOF-MS	CH3ONO		Methyl ethyl ketone; Toluene; Di-n-butyl ether	[227]

^a suppresses the O₃ formation.

briefly describe the experiments carried out to determine the constants.

Table 17. Absolute (^a) rate constants for gas-phase reactions of GLV with OH radicals—Arrhenius parameters and values at selected temperature T.

GLV	A	EA/R	kT	T	T Range	P	Expt	Ref.
	cm3 Molecule−1 s−1	K	cm3 Molecule−1 s−1	K	K	Atm	Table 18
Pentan-1-ol	(6.7 ± 3.8) × 10−12 a	−(132 ± 176)	(1.04 ± 0.59) × 10−11 a	298	273–373	100 mmHg	I	[210]
1-penten-3-ol	(6.8 ± 0.7) × 10−12 a	−(690 ± 20)	(7.12 ± 0.73) × 10−11 a	297	243–404	20–100 mmHg	II	[228]
	(7.7 ± 1.6) × 10−12 a	−(606 ± 60) a	(5.65 ± 0.76) × 10−11 a	298	263–353		III	[57]
(E)-2-penten-1-ol	(6.8 ± 0.8) × 10−12 a	−(680 ± 20)	(6.76 ± 0.70) × 10−11 a
1-penten-3-one	(4.4 ± 2.8) × 10−12 a	−(507 ± 180) a	(2.36 ± 0.47) × 10−11 a
(E)-2-pentenal	(7.9 ± 1.2) × 10−12 a	−(510 ± 20)	(4.3 ± 0.6) × 10−11 a	297	244–374	0.03–0.197	IV	[173]
(E)-2-hexen-1-ol	(5.4 ± 0.6) × 10−12 a	−(690 ± 20)	(6.15 ± 0.75) × 10−11 a	298	263–353		III	[57]
(Z)-3-hexen-1-ol	(1.3 ± 0.1) × 10−11 a	−(580 ± 10)	(1.06 ±0.12) × 10−10 a	297	243–404	20–100 mmHg	II	[228]
hexanal	(4.2 ± 0.8) × 10−12 a	−(565 ± 65)	(2.78 ± 0.50) × 10−11 a	298	263–353	0.066	IV	[159]
(E)-2-hexenal	(7.5 ± 1.1) × 10−12 a	−(520 ± 30)	(4.4 ± 0.5) × 10−11 a	297	244–374	0.03–0.197	IV	[173]
	(9.8 ± 2.4) × 10−12 a	−(455 ± 80)	(4.68 ± 0.50) × 10−11 a	298	263–353	0.066	IV	[159]
2-methyl-3- buten-2-ol	(8.2 ± 1.2) × 10−12 a 8.43 × 10−12 b	−(610 ± 50) −619.4 b	(5.4 ± 0.4) × 10−11 a (6.4 ± 0.6) × 10−11 b	299 298	254–410 254–360	1.3	IV	[165]
Methyl salicylate	5.0962 × 10−11 a,c,d	917	1.9595 × 10−12 a,c,d	298	278–298	1	V	[190]
	1.54 × 10−12 a,d,e	5053.9	6.64 × 10−20 a,d,e	298	278–350	1	VI	[189]

^a absolute rate constants; ^b values for reactions with OD radicals; ^c converted from the original units; ^d calculated theoretically; ^e initial OH addition, the paper contains Arrhenius equations for other steps of the reaction mechanism.

shows the Arrhenius parameters for reactions studied over temperature ranges,

Table 18. Experiments used to determine the rate constants listed in Table 17.

Expt	Photoreactor			Detection	OH Source	Ref.
	Type	Vol. dm3	Material
I	Cell, LFP		Pyrex	LIF	H2O2	[210]
II	Cell, LFP	0.15	Pyrex	LIF	H2O2	[173,228]
III	Cell PLP	0.2	Pyrex	UV, LIF	H2O2	[57,159]
IV	Cell, PLP	0.15	Pyrex	LIF	HONO, DONO	[165]
V	RKKM theory at MN15-L/aug-cc-pVTZ level with Eckart tunneling correction					[190]
VI	DFT theory using B3LYP, M06-2X, and MPW1K functionals with 6-311++G(d,p) basis set.					[189]

briefly describe the experiments carried out to determine the constants. The OH radicals were generated using several methods: photolysis of H₂O₂ at 248, 252, or 254 nm; photolysis of H₂O at 165 nm; photolysis of methyl nitrite CH₃ONO at 300 nm (Equation (15)); photolysis of ethyl nitrite CH₃CH₂ONO; photolysis of HNO₃ at 248 nm; photolysis of HONO at 355 nm; reaction of F atoms with H₂O or H atoms with NO₂.

Most of the reactions have negative activation energies, as their rate decreases with increasing temperature. The exception is methyl salicylate, which is the only aromatic compound studied. In some cases, evaluated theoretically, the additional channel of a GLV-OH reaction had a positive activation energy while the hydrogen abstraction channel had a negative one [189].

Gibilisco et al. [176] correlated the rate constants for reactions of OH radicals with unsaturated alcohols obtained experimentally ((E)-2-hexen-1-ol, (E)-3-hexen-1-ol, and (Z)-3-hexen-1-ol, Table 15 and from the literature with the energy of the highest occupied molecular orbital (HOMO) of those alcohols calculated with the Gaussian package:

ln k_OH (cm³ molecule⁻¹ s⁻¹) = -(1.3 ± 0.1) E_HOMO − (10.3 ± 1.3)

(10)

Based on their results and the literature data, Gibilisco et al. [175] correlated the rate constants for reactions of GLV with OH radicals against those for reactions with Cl atoms:

log k_OH = (0.29 ± 0.04) log k_Cl − 10.8

(11)

Peirone et al. [217] showed that the SAR relation [229] based on group-reactivity factors provided an estimation close to the experimental rate constants (Table 15).

Basandorj et al. [226] showed that the gas-phase reactions of OH and OD radicals with 2-methyl-3-buten-2-ol were faster by 15% in the presence of O₂. At temperatures higher than 335 K, that rate constant of those reactions depended on the total pressure and was analyzed for the dependence on the concentration of an inert gas component. The ab initio theoretical calculation supported the analysis. The second-order rate constant for OH radicals decreased with pressure and temperature according to Equation (12):

$$k^{II}=\left(\frac{k_0\left(T\right)\left[M\right]}{1+\frac{k_0\left(T\right)\left[M\right]}{k_{\infty }\left(T\right)}}\right)F_c^{\left(\frac{1}{1+[\log (k_0\left(T\right)\left[M\right]/k_{\infty \left(T\right)})]{}^2}\right)}$$

(12)

where k₀ is the thermolecular rate constant at the low-pressure limit, k_∞ is the rate constant at the high-pressure limit, F_c is the collisional broadening factor, and M is the inert gas. For k_∞ = 8.3 × 10⁻¹² exp(610/T) cm³ molecule⁻¹ s⁻¹ at 100 mmHg [165] and F_c = 0.6, the temperature dependence of k₀ was evaluated from the experimental data:

$$k_0=\left(2.5 \pm 7.4\right)\times {10}^{-32}\exp \left[\frac{4250\pm 1150}{T}\right]{ \mathrm{cm}}^6{\mathrm{molecule}}^{-2}{\mathrm{s}}^{-1}$$

(13)

The kinetic Equation (13) was consistent with a Lindemann-Hinshelwood mechanism:

OH + MBO → HO − MBO*

(14a)

OH + MBO → HO − MBO*

(14b)

HO–MBO* + M → HO − MBO + M*

(14c)

DFT calculations showed that the radical formed by the addition of OH to the internal carbon was more stable than that formed by the addition to the terminal carbon. Additionally, the authors provided geometries, energies, and vibrational frequencies of all structures considered.

Formation of OH radicals by photodissociation of methyl nitrite often used to generate OH radicals in laboratory experiments follows the mechanism below (e.g., [225]):

CH₃ONO + hν → CH₃O + NO

(15a)

CH₃O + O₂ → CH₂O + HO₂

(15b)

HO₂ + NO → OH + NO₂

(15c)

Methyl nitrite can be synthesized by the dropwise addition of 50% sulfuric acid to a saturated solution of NaNO₂ in methanol.

4.2.2. Gas-Phase Reactions with NO₃ Radicals

Table 19. Relative and absolute (^a) rate constants for gas-phase reactions of GLV with NO₃ at single temperatures.

GLV	k cm3 molecule−1 s−1	T K	P Atm	Expt. Table 20	Ref.
1-penten-3-ol	(1.39 ± 0.19) × 10−14 a	298 ± 3	1.00 ± 0.03	I a	[192]
(Z)-2-penten-1-ol	(1.53 ± 0.23) × 10−13 a
	(3.5 ± 1.9) × 10−13			I b
	(3.11 ± 0.11) × 10−13	295 ± 2	1	II	[230]
(E)-2-pentenal	(1.93 ± 0.40) × 10−14
1-penten-3-one	3.39 × 10−14 f	298	1	III	[215]
(Z)-4-hexen-1-ol	(2.93 ± 0.58) × 10−13 a	298 ± 3		I a	[231]
(Z)-3-hexen-1-ol	(2.67 ± 0.42) × 10−13 a
	(2.72 ± 0.83) × 10−13	296 ± 2	0.974	IV	[174]
(E)-3-hexen-1-ol	(4.43 ± 0.91) × 10−13 a	298 ± 3		I a	[231]
	(5.2 ± 1.8) × 10−13 g			I b
(Z)-2-hexen-1-ol	(1.56 ± 0.24) × 10−13 a			I a
	(4.05 ± 0.45) × 10−13 d (3.57 ± 0.62) × 10−13 e	295 ± 2	1	II	[230]
(E)-2-hexen-1-ol	(1.30 ± 0.24) × 10−13 a	298 ± 3		I a	[231]
hexanal	(1.14 ± 0.14) × 10−14	298 ± 2	1.00 ± 0.01	V	[218]
	(1.73 ± 0.18) × 10−14	298 ± 2	1	V	[232]
	(4.75 ± 0.62) × 10−14 b (1.07 ± 0.12) × 10−14 c	296 ± 2	1	VI	[219]
	(1.7 ± 0.1) × 10−14	297 ± 2	1	VII a	[233]
(Z)-2-hexenal	(1.36 ± 0.29) × 10−14	295 ± 2	1	II	[230]
(E)-2-hexenal	(1.21 ± 0.10) × 10−14	296 ± 2	0.974	IV	[174]
	(4.7 ± 1.5) × 10−15 a	294 ± 3	1	VIII	[193]
(Z)-3-hexenylacetate	(2.46 ± 0.75) × 10−13	296 ± 2	0.974	IV	[174]
2-methyl-3-buten-2-ol	(2.1 ± 0.3) × 10−14 a	294	1	IXa	[234]
	(1.6 ± 0.6) × 10−14			IX b
	(0.86 ± 0.29) × 10−14	298 ± 2	0.974 ± 0.007	X	[224]
Nonanal	(1.8 ± 0. 2) × 10−14 (2.2 ± 0. 3) × 10−14	297 ± 2	1	VII b VII c	[233]
Methyl salicylate	(4.19 ± 0.92) × 10−15	298	1	XI	[227]

^a absolute rate constant; ^b methacrolein reference; ^c 1-butene reference, ^d (E)-2-butene reference, ^e cyclopentene reference; ^f estimated from SAR or LFER; ^g overestimated (see text).

contains the rate constants for gas-phase reactions of GLV with OH radicals determined experimentally at single temperatures,

Table 20. Experiments used to determine the rate constants listed in Table 19.

Expt	Photoreactor			Detection	NO3 Source	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I a	Discharge flow tube			Off-axis continuous-wave CEAS	F + HONO	no	[192]
I b	Bag	56	Teflon	GC-FID	N2O5	(E)-2-butene, 1-butene
II	Flow-tube	0.005	Glass	CI MS	N2O5	(E)-2-butene, Cyclopentane, 1,3-butadiene	[230]
III	SRR, LFER						[215]
IV	Chamber	6700 7900	Teflon	GC-FID	N2O5	(E)-2-butene, propene	[174]
V	Reactor	250	Steel	LP FTIR	N2O5	Propene 1-butene	[218,232]
VI	Chanber	6500	Teflon	GC-FID	N2O5	Methacrolein, 1-butene	[219]
VII a	Bag	100	PVF	(a) FTIR	N2O5	(a) propene	[233]
VII b				(b,c) GC (SPME)		(b) butanal
VII c						(c) 1-butene
VIII	Reactor	977	Pyrex	FTIR, UV	N2O5	no	[193]
IX a	Flow tube FFD			LP FTIR	F + HNO3	no	[234]
IX b	Reactor	153			N2O5	propene
X	Chamber	480	Teflon	LP FTIR	N2O5	propene	[224]
XI	ICARE-CHRS chamber	7300	Teflon	HR PTR-TOF-MS	NO2 + O3	acetaldehyde	[227]

briefly describe the experiments in which the constants were determined.

Table 21. Absolute rate constants for gas-phase reactions of GLV with NO₃ radicals—Arrhenius parameters and values at temperature T.

GLV	A	EA/R	kT	T	T Range	P	Expt	Ref.
	Molecules cm−1 s−1	K	Molecules cm−1 s−1	K	K	mmHg	Table 22
(E) -2-pentenal	(5.40 ± 0.30) × 10−12	1540 ± 200	(2.88 ± 0.29) × 10−14	298	298–433	1	I	[235]
(E) -2-hexenal	(1.2 ± 0.3) × 10−12	926 ± 85	(5.49 ± 0.95) × 10−14	298	298–433	1		[235]
2-methyl-3-butene-2-ol	4.6 × 10−14	400	(1.2 ± 0.09) × 10−14	298	267–400	1.2–8.8	II	[236]

shows the Arrhenius parameters for reactions studied over temperature ranges,

Table 22. Experiments used to determine the rate constants listed in Table 21.

Exp	Photoreactor			Detection	OH Source	Add.	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I	Flow tube FFD			LIF	F + HNO3		no	[235]
II	Flow tube			LP VIS	N2O5		no	[236]
III
IV

briefly describe the experiments in which the constants were determined. Methods used to generate the NO₃ radicals include thermal decomposition of N₂O₅, the reaction of F atoms with HNO₃, and the reaction of NO₂ with O₃.

Pfrang et al. [192,231] showed that relative rate constants for the gas-phase reactions of NO₃ with (Z)-2-hexen-1-ol, (E)-3-hexen-1-ol, (Z)-2-penten-1-ol, and 1-penten-3-ol at 298 ± 3 K obtained using N₂O₅ as a source of NO₃ were higher than the absolute rate constants obtained using off-axis continuous-wave cavity-enhanced absorption spectroscopy (CEAS) using the reaction of F atoms with HNO₃ as a source of NO₃. That was caused by slow reactions of N₂O₅ with GLV ((5.0 ± 2.8) × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for (Z)-2-penten-1-ol; (9.1 ± 5.8) × 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹ for 1-penten-3-ol; and (3.1 ± 2.3) × 10⁻¹⁸ cm³ molecule⁻¹ s⁻¹ for (E)-3-hexen-1-ol).

4.2.3. Gas-Phase Reactions with O₃

Table 23. Relative and absolute (^a) rate constants for gas-phase reactions of GLV with O₃ at single temperatures.

GLV	k cm3 molecule−1 s−1	T K	P atm	Expt Table 24	References
pentan-1-ol	(2.9 ± 0.2) × 10−17	298	1		[210]
1-penten-3-ol	(1.64 ± 0.15) × 10−17 a	298	1	I	[201]
	(1.79 ± 0.18) × 10−17 a	289 ± 1	1	II	[237]
(Z)-2-penten-1-ol	(11.5 ± 0.66) × 10−17 a	298	1	I	[201]
	(1.69 ± 0.25) × 10−16 a	288 ± 1	1	II	[237]
1-penten-3-one	(1.17 ± 0.15) × 10−17 a	298	1	I	[201]
(Z)-2-hexen-1-ol	(7.44 ± 1.03) × 10−17 a	298	1	III a	[238]
	10.4 × 10−17	298		III b
(E)-2-hexen-1-ol	(5.98 ± 0.73) × 10−17	298	1	IV	[239]
	(1.66 ± 0.22) × 10−16 a	298	1	III a	[238]
	2.86 × 10−16	298		III b
(Z)-3-hexen-1-ol	6.7 × 10−17	296	1	V	[92]
	6.4 × 10−17
	(1.05 ± 0.95) × 10−16 a	258		II	[237,240]
	(6.4 ± 1.7) × 10−17	296 ± 2	0.974	VI	[174]
	(6.04 ± 0.07) × 10−17	298	1	IV	[239]
	(5.47 ± 0.71) × 10−17 a	298	1	III a	[238]
	4.40 × 10−17	298		III b
(E)-3-hexen-1-ol	(5.83 ± 0.86) × 10−17	298	1	IV	[239]
	(6.19 ± 0.72) × 10−17 a	298	1	III a	[238]
	5.8 × 10−17			III b
(Z)-4-hexen-1-ol	(7.09 ± 0.91) × 10−17 a	298	1	III a	[238]
	8.38 × 10−17			III b
(E)-4-hexen-1-ol	(1.05 ± 0.14) × 10−16 a			III a
	0.88 × 10−16			III b
(E)-2-hexenal	(1.28 ± 0.28) × 10−18	287.3 ± 1.4	1	II	[203]
	(3.0 ± 2.1) × 10−18	296 ± 2	0.974	VI	[174]
(Z)-3-hexenal	(3.45 ± 0.30) × 10−17	298	1	VII	[160]
(Z)-3-hexenyl formate	(4.06 ± 0.66) × 10−17 a	298	1	VIII a	[241]
	6.60 × 10−17 a,b			VIII b
(E)-2-hexenyl acetate	(2.18 ± 0.28) × 10−17	288.4 ± 0.4	1	II	[203]
(Z)-3-hexenyl acetate	3.6 × 10−17; 5.4 × 10−17	296	1	V	[92]
	(5.4 ± 1.4) × 10−17	296 ± 2	0.974	VI	[174]
	(5.77 ± 0.70) × 10−17 a	298	1	VIII a	[241]
	6.84 × 10−17 a,b			VIII b
(Z)-3-hexenyl propionate	(7.62 ± 0.88) × 10−17 a	298	1	VIII a	[241]
	1.084 × 10−16 a,b			VIII b
(Z)-3-hexenyl butyrate	(1.234 ± 0.159) × 10−16 a			VIII a
	1.789 × 10−16 a,b			VIII b
2-methyl-3-buten-2-ol	(1.0 ± 0.03) × 10−17 a	291 ± 1		II	[237]
	(8.3 ± 1.0) × 10−18 a	293 ± 2	1	IX	[242]
	(8.6 ± 2.9) × 10−18	298 ± 2	0.974 ± 0.007	X	[224]
	(8.6 ± 1.0) × 10−18 a	298 ± 2	1	XI	[177]
1-octen-3-ol	(5.00 ± 0.58) × 10−24 a	298	1	XII	[48]
Methyl salicylate	~4 × 10−21 c	298 ± 5	1	XIII	[164]
	(3.33 ± 2.01) × 10−19	298	1	XIV	[227]

^a absolute rate constant; ^b calculated theoretically; ^c rough estimate due to low conversion of the reference reactant toluene.

contains the rate constants for gas-phase reactions of GLV with O₃ determined experimentally at single temperatures,

Table 24. Experiments used to determine the rate constants listed in Table 23.

Expt	Photoreactor			Detection	Addition	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I	Cork LISA	3910 977	Teflon Pyrex	LP FTIR, O3 analyzer		no	[201]
II	Chamber	3500–3700	Teflon	UV photometer	Cyclohexane a	no	[203,237,240]
III a				O3 analyzer	Cyclohexane a	no	[238]
III b	DFT method at the BH&HLYP/6-31+G(d,p) level of theory
IV	Chamber	480	Pyrex	LP FTIR	No b	1,4-cyclohexadiene, isoprene	[239]
V	Chamber	775	Teflon	GC-MS	Fluorobenzene c	no	[92]
VI	Chamber	6700 7900	Teflon	GC-FID	Cyclohexane a	(E)-2-butene, propene, 2-methyl-2-butene	[174]
VII	Chamber	6000	PFA	LP FTIR	CO a	1,3,5-trimethylbenzene	[160]
VIII a	Flow tube	0.012		O3 analyzer		no	[241]
VIII b	DFT calculations with the BHandHLYP functional and the 6-311+G(d,p) basis sets at the BHandHLYP/6-311+G(d,p) level of theory.
IX	Reactor	977	Pyrex	LP FTIR	CO a	no	[242]
X	Chamber	480	Teflon	LP FTIR	Propane a	Propene, isobutene	[224]
XI	LISA CRAC	980 3910	Pyrex FEP	LP FTIR GC-MS	CO a	no	[177]
XII	Chamber	775 8000	Teflon	NIR LDI MS, O3 analyzer		no	[48]
XIII	Chamber	50	Teflon	GC-FID		Toluene	[164]
XIV	ICARE-CHRS chamber	7300	Teflon	HR PTR-TOF-MS		3,3,3-trifluoropropene	[227]

^a OH scavenger; ^b see comment in the text; ^c internal standard.

describe the experiments carried to determine the constants.

Table 25. Relative rate constants for gas-phase reactions of GLV with O₃₋ Arrhenius parameters and values at selected temperature T (obtained by relative methods if not marked absolute).

GLV	A	EA/R	kT	T	T Range	P	Expt	Ref.
	Molecules cm−1 s−1	K	Molecules cm−1 s−1	K	K	Atm	Table 26
1-penten-3-ol	(1.82 ± 2.08) × 10−16	730 ± 348	(1.61 ± 0.21) × 10−17	298 ± 2	273–333	1	I a	[243]
			(1.75 ± 0.25) × 10−17	298 ± 2			I b
			0.20 × 10−17	298			I c
(Z)-2-penten-1-ol	(2.32 ± 1.94) × 10−15	902 ± 265	(11.90 ± 1.40) × 10−17	298 ± 2	273–333	1	I a	[243]
			(9.07 ± 1.29) × 10−17	298 ± 2			I b
			1.50 × 10−17	298			I c
(E)-2-penten-1-al	(1.38 ± 0.73) × 10−16 a	1406 ± 163	(1.24 ± 0.06) × 10−18 a	298	233–373	0.8–1	II	[244]
(E)-3-hexen-1-ol	(1.74 ± 1.65) × 10−15	1020 ± 300	(5.97 ± 0.99) × 10−17	298 ± 2	273–333	1	I a	[243]
			(6.50 ± 0.95) × 10−17	298 ± 2			I b
			1.48 × 10−17	298			I c
(E)-2-hexenal	(1.79 ± 0.54) × 10−16 a	1457 ± 90	(1.37 ± 0.03) × 10−18 a	298	233–373	0.8–1	II	[244]
(Z)-2-hexenyl acetate	(2.02 ± 0.65) × 10−15 c	1306 ± 44 c	2.50 × 10−17 b	298	200–370	0.996	III	[196]
(E)-2-hexenyl acetate	(7.21 ± 0.93) × 10−15 c	1856 ± 33 c	1.39 × 10−17 b
(Z)-3-hexenyl acetate	(2.83 ± 0.39) × 10−15 c	993 ± 35 c	9.84 × 10−17 b
(E)-3-hexenyl acetate	(1.68 ± 0.23) × 10−14 c	925 ± 34 c	7.37 × 10−16 b
Methyl salicylate	2.97 × 10−12 b,d	6185.6	2.9 × 10−21 d	298	278–350	1	IV	[189]

^a absolute rate constant; ^b practically did not depend on pressure in the range of 0.01–10,000 mmHg; ^c evaluated based on the reference data; ^d formation of primary ozonide.

shows the Arrhenius parameters for reactions studied over temperature ranges,

Table 26. Experiments used to determine the rate constants listed in Table 25.

Exp	Photoreactor			Detection	Add.	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I a	Flow tube	63	Pyrex	LP FTIR	Cyclohexane a	2,3-dimethyl-1,3- butadiene	[243]
I b				GC-MS (SPME)		1-heptene
I c	DFFT method with M06–2X functional and 6-311 þ G(d,p) triple split valence basis set
II	Flow tube	63	Pyrex	LP FTIR	Cyclohexane a	no	[244]
III	DFT and modified RRKM						[196]
IV	DFT using B3LYP, M06-2X and MPW1K functionals with 6-311++G(d,p) basis set.						[189]

^a OH scavenger.

briefly describe the experiments carried to determine the constants. In each experiment, external devices generated ozone.

Gibilisco et al. [239] studied the gas-phase reactions of O₃ with (Z)-3-hexen-1-ol, (E)-3-hexen-1-ol, and (E)-2-hexen-1-ol without any OH scavengers added because they assumed that OH radicals formed during ozonolysis affect both the major and the reference reactants and OH influence is self-canceling and marginal. That assumption is not exact because the amounts of reactants consumed by OH radicals add to the quantities consumed by O₃ and thus are not self-canceling:

$$\frac{{\Delta }_{R1, total}-{\Delta }_{R1, OH}}{{\Delta }_{R2, total}-{\Delta }_{R2, OH}}\ne \frac{{\Delta }_{R1, ozone}}{{\Delta }_{R2, ozone}}$$

(16)

Several authors showed that the rate constants calculated with DFT methods agreed well with the experimental constants [238,241]. Besides, Zhang et al. [241] found that the theoretical and empirical rate constants for gas-phase reactions of O₃ with (Z)-3-hexenyl esters increased with the length of the ester group.

4.2.4. Gas-Phase Reactions with Cl Radicals

Table 27. Relative rate constants for gas-phase reactions of GLV with Cl at single temperatures.

GLV	K cm3 Molecule−1 s−1	T K	P Atm	Expt Table 28	Ref.
Pentan-1-ol	(2.9 ± 0.2) × 10−10	298	1	I	[210]
	(2.51 ± 0.13) × 10−10	298 ± 2	1	II	[212]
	(2.57 ± 0.25) × 10−10	295 ± 2	1	III	[213]
1-penten-3-ol	(2.96 ± 1.22) × 10−10	258 ± 1	1	IV	[205]
	(2.37 ± 0.38) × 10−10	262 ± 1	1		[205]
	(2.40 ± 0.54) × 10−10	273 ± 1	1		[205]
	(2.35 ± 0.31) × 10−10	298 ± 1	1		[205]
	(2.78 ± 0.30) × 10−10	313 ± 1	1		[205]
	(2.57 ± 0.28) × 10−10	333 ± 1	1		[205]
(Z)-2-penten-1-ol	(2.99 ± 0.53) × 10−10	296 ± 2	1	V	[245]
	(3.00 ± 0.49) × 10−10	298 ± 1	1	IV	[205]
	(2.66 ± 0.47) × 10−10	313 ± 1	1		[205]
	(3.26 ± 0.48) × 10−10	333 ± 1	1		[205]
(E)-2-pentenal	(1.31 ± 0.19) × 10−10	298	1	VI	[246]
1-penten-3-one	(2.91 ± 1.10) × 10−10	298	1	VII	[247]
	(1.9 ± 0.4) × 10−10	297–400	1	VIII	[248]
(E)-2-hexen-1-ol	(3.41 ± 0.65) × 10−10	296 ± 2	1	V	[245]
	(3.49 ± 0.82) × 10−10	298 ± 3	1	IX	[249]
(Z)-3-hexen-1-ol	(3.15 ± 0.58) × 10−10	296 ± 2	1	V	[245]
	(2.94 ± 0.72) × 10−10	298 ± 3	1	IX	[249]
(E)-3-hexen-1-ol	(3.05 ± 0.59) × 10−10	296 ± 2	1	V	[245]
	(3.42 ± 0.79) × 10−10	298 ± 3	1	IX	[249]
(Z)-3-hepten-1-ol	(3.80 ± 0.86) × 10−10	298 3	1		[249]
Hexanal	(2.88 ± 0.37) × 10−10	298	1	VI	[246]
	(3.23 ± 0.15) × 10−10	298 ± 2	1	X	[220]
(E)-2-hexenal	(1.92 ± 0.22) × 10−10	298	1	VI	[246]
(Z)-3-hexenyl formate	(2.45 ± 0.15) × 10−10	298	1	IV	[222]
	4.94 × 10−10 a	298	1		[222]
(E)-2-hexenylacetate	(3.10 ± 1.13) × 10−10	298 ± 1	750 mmHg	XI	[186]
2-methyl-3-buten-2-ol	(4.7 ± 1.0) × 10−10	298 ± 2	0.974 ± 0.007	XII	[224]
	(3.3 ± 0.4) × 10−10	295 ± 1	0.921	XIII	[179]
1-octen-3-ol	(4.03 ± 0.77) × 10−10	296 ± 2	1	V	[245]
(Z)-3-octen-1-ol	(4.13 ± 0.68) × 10−10	298 ± 3	1	IX	[249]
Nonanal	(4.82 ± 0.20) × 10−10	298 ± 2	1	X	[220]
methyl salicylate	(2.8 ± 0.44) × 10−12	298 ± 5	1	XIV	[164]
	(1.65 ± 0.3) × 10−12	298	1	XV	[227]

^a from SAR or LFER.

contains the rate constants for gas-phase reactions of GLV with Cl radicals determined experimentally at single temperatures,

Table 28. Experiments used to determine the rate constants listed in Table 27.

Expt	Photoreactor			Detection	Cl Source	Add.	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I	Bag	140	Teflon	GC-FID	Cl2		Pentane 1,3-dioxolane Cyclohexane	[210]
II	Cylinder	50	Teflon	GC-FID	Cl2, COCl2		Cycloxexane	[212]
III	Bag	100	Teflon	GC-FID	Cl2		Propane; hexane	[213]
IV	Chamber	400	Teflon	GC-FID	CCl3COCl		Octane, propene, cyclohexane	[205,222]
V	Chamber	600	Teflon	Ion flow tupe MS	Cl2		Tetrahydrofuran, propan-1-ol, octane	[245]
VI	Chamber	200	Teflon	GC-FID	Cl2		Ethene; propene; 1-butene	[246]
VII	Bag	80	Teflon	GC-FID	ClC(O)C(O)Cl		Chloroethene 1,1-dichloroethene acrylonitrile	[247]
VIII	Bulb	0.5	Pyrex	GC-FID; GC-MS; FTIR	Cl2		Ethane	[248]
IX	Chamber	480	Pyrex	LP FTIR	Cl2		1-butene, isobutene	[249]
X	Cylinder	108O	Pyrex	LP FTIR	Cl2		(E)-2-butene; Methy lvinyl ketone	[220]
XI	Bag	80	Teflon	GC-FID	ClC(O)C(O)Cl		(n-butyl methacrylate n-butyl acrylate	[186]
XII	Chamber	480	Teflon	LP-FTIR	Cl2		Ethene	[224]
XIII	Chamber	47	Steel	LP-FTIR	Cl2		Ethane, cyclohexane	[179]
XIV	Chamber	50	Teflon	GC-FID	Cl2		Acetone	[164]
XV	ICARE-CNRS chamber	7300	Teflon	HR PTR-TOF-MS	Cl2		Acetone	[227]

briefly describe the experiments carried out to determine the constants.

Table 29. Relative and absolute (^a) rate constants for gas-phase reactions of GLV with Cl atoms—Arrhenius parameters and values at selected temperature T.

GLV	A	EA/R	kT	T	T Range	P	Expt	Ref.
	cm3 Molecule−1 s−1	K	cm3 Molecule−1 s−1	K	K	Atm	Table 30
hexanal	(7.91 ± 0.66) × 10−11 a	−(349 ± 51)	(2.56 ± 0.24) × 10−11 a	298	265–381	1	I	[250]
2-methyl-3-butene-2-ol	(2.83 ± 2.50) × 10−14	−(2670 ± 249)	(2.13 ± 0.19) × 10−10	298	256–298	1	II	[251]
methyl salicylate	1.2703 × 10−8 a,b,c	1438.4	1.01 × 10−10 a,b,c,d	298	278–350	1	III	[209]

^a absolute rate constants; ^b determined theoretically; ^c calculated here from the reference data; ^d 100 times higher than experimental values from Table 23 [209].

shows the Arrhenius parameters for reactions studied over temperature ranges,

Table 30. Experiments used to determine the rate constants listed in Table 29.

Expt	Photoreactor			Detection	Cl Source	Add.	Reference Reactants	Ref.
	Type	Vol. dm3	Material
I	Cell (PLP)	0.2	Pyrex	RF	Cl2	No	No	[250]
II	Chamber	400	Teflon	GC-FID	CCl3COCl		Cycloxexane, methanol, propene, 1-butene	[251]
III	DFT methods at B3LYP and M06-2X levels of theory with 6-311++G(d,p) basis set							[209]

briefly describe the experiments carried out to determine the constants.

Gibilisco et al. [249] correlated the rate constants for gas-phase reactions of Cl with various unsaturated VOC against the energy of the highest occupied molecular orbital (E_HOMO) calculated with the Gaussian package (Equation (12)).

$$\ln k \left({\mathrm{cm}}^3{\mathrm{molecule}}^{-1}{\mathrm{s}}^{-1}\right)=-\left(0.3\pm 0.1\right)E_{HOMO}-\left(19.0\pm 0.4\right)$$

(17)

4.2.5. Gas-Phase Photolysis

Table 31. GLV photolysis rate constants j, absolute, and relative to the photolysis rate constant of NO₂, determined at given effective quantum yields Φ and zenith angles θ.

GLV	j s−1	j/j(NO2)	Φ	θ °	T K	Expt Table 32	Ref.
1-penten-3-ol	(1.61–2.36) × 10−6 a		1		298	I	[57]
(E)-2-pentenal	3.06 × 10−4		1	20	298	II	[162]
	2.87 × 10−4			30
	2.58 × 10−4			40
	2.18 × 10−4			50
	1.63 × 10−4			60
	9.73 × 10−5			70
1-penten-3-one	(0.36−1.39) × 10−5 a		1		298	I	[57]
n-hexanal	1.7 × 10−5	0.2 × 10−2	0.28		286–294	III	[161]
	1.14 × 10−5		0.28		298	I	[159]
	(1.81 ± 0.10) × 10−5		0.31 ± 0.02	17	298	IV	[220]
(Z)-3-hexenal	2.1 × 10−5	0.4 × 10−2	0.34		286–294	III	[161]
	(2.61 ± 0.08) × 10−5	(4.7 ± 0.4) × 10−3	0.25 ± 0.06		298	V	[160]
(E)-2-hexenal	1.0 × 10−4	1.8 × 10−2	0.36		286–294	III	[161]
	3.80 × 10−4		1	20	298	II	[162]
	4.05 × 10−4			30
	3.80 × 10−4			40
	3.42 × 10−4			50
	2.89 × 10−4			60
	2.17 × 10−4			70
(Z)-3-hexen-1-ol	(1.61−2.36) × 10−5 a		1		298	I	[57]
Methyl salicylate	(2.82 ± 0.26) × 10−5 b Insignificant c				298	VI	[227]

^a upper limits, 0–10 km above the Earth’s surface; ^b at 254 nm; ^c at 365 nm and > 300 nm.

shows the experimental rates of GLV photolysis, absolute or relative to the NO₂ photolysis rate. All constants were based on the measured UV spectra of GLV (Section 3.2).

Table 32. Experiments used to determine the rate constants listed in Table 31.

Expt	Photoreactor			Detection	Additions	Ref.
	Type	Vol. dm3	Material
I	Calculation conditions: clear-sky, July noon, Ciudad Real (Spain). The actinic flux was based on the TUV model a.					[57,159]
II	The actinic flux was based on the TUV model a.					[162]
III	EUPHORE	2 × 105	PTFE	LP-FTIR, GC-FID, GC-MS		[161]
IV	EUPHORE	2 × 105	PTFE	LP-FTIR		[220]
V	Chamber	6000	PFA	LP-FTIR	CO b, isoprene c	[160]
VI	ICARE-CNRS	7300	FEP	HR PTR-TOF-MS		[227]

^a Tropospheric Ultraviolet Model [252]; ^b OH scavenger; ^c OH tracer.

provides more details.

Photolysis was an insignificant degradation path in the troposphere for 1-penten-3-ol and (Z)-3-hexen -1-ol [57], (Z)-3-hexen-1-al [161], MBO [165], and MeSa [227]. Photodegradation of (Z)-3-hexenal was faster than its reaction with O₃ (1 × 10¹² molecule cm⁻³) and slower than its reaction with OH (2 × 10⁶ molecule cm⁻³). Substantial photodegradation occurred for (E)-2-pentenal and (E)-2-hexenal [162] as well as for 1-penten-3-one and hexanal [57].

4.2.6. SAR and LFPR Methods

We did not intend to review the methods of estimating the rate constants based on structure-activity or linear free energy relationships. The interested readers are referred to the original publications on the correlations for gas-phase reactions of organic compounds with OH radicals [222,229,253,254,255], with OH, O₃, and NO₃ radicals [215,255,256,257,258,259], with Cl radicals [215,222,245,260], and with O₃ [255,261]. Besides, SAR correlations were proposed for some aqueous-phase reactions [262,263,264].

5. Aqueous-Phase Kinetics

There is little data published on the aqueous-phase reactions of GLV, so we present all findings in one section. Reaction mechanisms are discussed in Section 6.3, together with multiphase experiments. Rate constants determined are collected in

Table 33. Relative rate constants for aqueous-phase reactions of GLV with OH radicals—Arrhenius parameters and values at 298 K and pH = 5.4, and yields of SOA products.

GLV	A	EA	T Range	pH	k298 K a	SOA	Ref.
	1011 M−1 s−1	kJ mol−1	K		108 M−1 s−1	%
1-penten-3-ol	10.4 ± 0.3	13 ± 2	278–318	7	6.3 ± 0.1	-	[163]
1-penten-3-one					7.2 b	-	[215]
(Z)-2-hexen-1-ol	2.6± 0.1	9 ± 2	273–318	7	6.7 ± 0.3	-	[163]
(Z)-3-hexen-1-ol	8.1 ± 0.9	12 ± 0.3	278–298	5.4	5.1 ± 0.8	52	[22]
(E)-2-hexenal	5.2 ± 0.1	12 ± 1	273–318	7	4.8 ± 0.3	-	[163]
(Z)-3-hexenyl acetate	190 ± 40	17 ± 2	278–298	5.4	8.7 ± 1.1	8	[22]
methyl salicylate	58 ± 40	14 ± 1	278–298	5.4	7.8 ± 0.5	87
methyl jasmonate	58 ± 9	15 ± 2	278–298	5.4	6.8 ± 0.8	67
2-methyl-3-butene-2-ol	23 ± 5	13 ± 2	278–298	5.4	7.5 ± 1.4	20

^a experimental values; ^b estimated from SAR or LFER.

,

Table 34. Relative rate constants for aqueous-phase reactions of GLV with SO₄⁻ and NO₃ radicals at pH = 7—Arrhenius parameters (278–318 K) and values at 298 K [163].

GLV	X	A	EA	k298 K
		109 M−1 s−1	kJ mol−1	108 M−1 s−1
1-penten-3-ol	SO4−	7.9 ± 0.1	5 ± 1	9.4 ± 1.0
	NO3	150 ± 10	17 ± 2	1.5 ± 0.2
(Z)-2-hexen-1-ol	SO4−	110 ± 10	10 ± 2	28.3 ± 3
	NO3	38 ± 1	9 ± 1	8.4 ± 2.3
(E)-2-hexenal	SO4−	2.9 ± 0.1	4 ± 1	4,8 ± 0.2
	NO3	31 ± 1	17 ± 2	0.28 ± 0.07

,

Table 35. Relative rate constants for aqueous-phase reactions of GLV with singlet molecular oxygen ¹O₂ * and ³C * triplet state (³DMB * and ³MAP *) at pH = 5.4, and yields of some SOA products [265].

GLV	1O2 *			3DMB *			3MAP *
	A	EA	k298 K	k		SOA	k
	M−1 s−1	kJ mol−1	106 M−1 s−1	106 M−1 s−1		%	106 M−1 s−1
				pH = 2.1 298 K	pH = 5.1 278–298 K		pH = 5.1
(Z)-3-hexen-1-ol	6.1 × 1020	82 ± 7.4	2.5 ± 0.3	0.33 ± 0.04	0.24 ± 0.1	-	1.2 ± 0.8
(Z)-3-hexenyl acetate	2.2 × 1015	50 ± 7.2	3.9 ± 0.8	14 ± 2	14 ± 7	38	7.3 ± 2
methyl salicylate	-	-	≤ 0.1	29 ± 2	12 ± 4	80	8.0 ± 0.8
methyl jasmonate	3.6 × 1023	96 ± 4.8	6.0 ± 0.7	3.6 ± 0.5	4.2 ± 3	84	1.2 ± 0.5
2-methyl-3-butene-2-ol	6.7 × 109	22 ± 1.7	7.5 ± 1.4	0.28 ± 0.1	0.13 ± 0.07	-	0.55 ± 0.2

.

Richards-Henderson et al. [22] studied the aqueous-phase oxidation of (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, MeSa, MeJa, and MBO induced by OH radicals formed by the photodissociation of H₂O₂. Control experiments without H₂O₂ showed that the photodegradation of reactants did not occur. The total mass yield of SOA products was determined gravimetrically after blowing the post-reaction solutions to dryness with N₂. Unreacted GLV in dry residues was determined chromatographically after the reconstitution of samples with water. Rate constants for reactions were determined using a competitive kinetics technique against sodium benzoate (Table 33).

Sarang et al. [163] determined the relative rate constants for the aqueous-phase reactions of 1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexenal with OH, NO₃, and SO₄⁻ radicals (Table 33 and Table 34). They used the Laser Flash Photolysis-Laser Long Path Absorption (LFP-LLPA) technique over the 278–318 K temperature range. The rate constants weakly increased with temperature. The impact of diffusion of reactants on the rate constants was: 30–60% (reactions with OH radicals); 5–23% (reactions with SO₄⁻ radicals); and 0.2–8% (reactions with NO₃ radicals).

Richards-Henderson et al. [265] determined the relative rate constants for reactions of several GLV with ³C * triplet state and ¹O₂ * singlet molecular oxygen generated by irradiation of organic chromophores—3,4-dimethoxybenzaldehyde (DMB) or methoxyacetophenone (MAP) and Rose Bengal, resp.—with simulated sunlight. They used a competitive kinetics method against the reference reactants phenol or syringol (for ³C *) and tyrosine or furfuryl alcohol (for ¹O₂ *). The results are in Table 33. The yields of SOA products were determined using the procedure applied in the experiments with OH radicals [22] (see above). All ¹O₂ * reactions and reactions of ³DMB * with hexenol and methylbutenol were too slow to produce measurable amounts of SOA.

Mael et al. [266] synthesized intermediate products of the oxidation of 1-metyl-3-buten-1-ol—(3,3-dimethyloxiran-2-yl)methanol (2-methyl-3-buten-1-ol 2,3-epoxide), 2-(oxiran-2-yl)propan-2-ol (2-methyl-3-buten-1-ol 3,4-epoxide), and 3-methylbutane-1,2,3-triol), and (2,3-dihydroxyiso-pentanol, DHIP)—to study their aqueous-phase reactions with sulfates and nitrates as well as hydrolysis thereof. Products obtained from reactions of both epoxides in 1 M solutions of sulfate included DHIP and its sulfuric acid esters (two isomers). Products of reactions of 3,4 epoxide in 1 M nitrate solutions included two isomers of nitric acid esters of DHIP (Section 6.3, Scheme 44).

In addition, those authors determined the rate constants for the following reactions: (i) acid (H⁺) catalyzed hydrolysis of 2,3-epoxide (4.075 ± 0.004) × 10⁻¹ M⁻¹ s⁻¹; (ii) acid (H⁺) catalyzed hydrolysis of 3,4-epoxide (4.36 ± 0.13) × 10⁻³ M⁻¹ s⁻¹; (iii) neutral hydrolysis of tertiary sulfuric acid ester from 2,3-epoxide (4.42 ± 0.20) × 10⁻⁶ s⁻¹; (iv) neutral hydrolysis of tertiary nitric acid ester from 3,4-epoxide (2.10 ± 0.14) × 10⁻³ s⁻¹; (v) neutral hydrolysis of primary sulfuric acid ester from 3,4-epoxide < 2 × 10⁻⁷ s⁻¹; (vi) neutral hydrolysis of primary nitric acid ester from 3,4-epoxide < 2 × 10⁻⁷ s⁻¹; and D₂SO₄ esterification of DHIP (1.292 ± 0.047) × 10⁻⁴ s⁻¹, probably at room temperature.

Noziere et al. [132] found that a chemical reaction accompanied the uptake of MBO in aqueous solutions of sulfuric acid (Section 3.1). The first-order rate constant for that reaction increased with the concentration of the acid:

$$n\left(\mathrm{k}\right)=\left(0.19\pm 0.02\right)\mathrm{W}-\left(14.6\pm 1.3\right) {\mathrm{s}}^{-1}$$

(18)

where W = 40–62% is the weight concentration of sulfuric acid. The reaction product was assigned to the mass spectrometer signal m/z 87 recorded in solutions. The proposed chemical mechanism included a pinacol rearrangement of protonated MBO leading to 3-methyl butanone (Section 6.3, Scheme 50). Liu et al. [267] used a rotated wetted wall reactor coupled with TOF MS with single-photon ionization to study uptake and aqueous-phase reactions of 2-methyl-3-buten-2-ol with H₂O₂ (0.1–1.0% wt) in aqueous solutions of sulfuric acid (40–60% wt) at 293 K and 5–108 mm Hg total pressure. MBO was monitored as the MBO⁺-CH₃ ion cluster (m/z 71). The uptake coefficient was defined as the probability of MBO loss (from the gas phase) per collision with the aqueous surface (Equation (19)).

$$\mathsf{\gamma }=\frac{4\mathrm{kV}}{\mathsf{\omega }\mathrm{A}}$$

(19)

where ω (m s⁻¹) is the mean molecular speed of MBO, V (cm³) is the volume of the reaction zone, A (cm²) is the surface area of solution, and k (s⁻¹) is the first-order rate constant for the removal of MBO from the gas phase defined by Equation (20).

$$ln\left(\frac{\mathrm{S}}{{\mathrm{S}}_0}\right)=-\frac{\mathrm{kL}}{{\mathrm{v}}_{\mathrm{av}}}$$

(20)

where S and S₀ are MS signals of MBO with and without loss from the gas phase, L (cm) is the contact distance between the gas and the solution, and v_av (cm s⁻¹) is the average gas flow velocity. In the absence of H₂O₂, the uptake of MBO was reversible at 40% acid concentration, irreversible (reactive) at 60% acid, and mixed at 50% acid. The addition of H₂O₂ at concentrations higher than 1% substantially increased the reactive character of the uptake. The uptake coefficient increased with the concentration of both sulfuric acid and H₂O₂. Equations (21a)–(21d) approximated the original data.

$$ln\left(\gamma \right)=0.1591W-18.059,R^2=0.9909, at \left[{\mathrm{H}}_2{\mathrm{O}}_2\right]=0$$

(21a)

$$ln\left(\gamma \right)=0.167W-17.656,R^2=0.9891,at \left[{\mathrm{H}}_2{\mathrm{O}}_2\right]=0.1 \% \mathrm{wt}$$

(21b)

$$ln\left(\gamma \right)=0.2159W-19.04,R^2=0.9633,at \left[{\mathrm{H}}_2{\mathrm{O}}_2\right]=0.5 \% \mathrm{wt}$$

(21c)

$$ln\left(\gamma \right)=0.2188W-18.468,R^2=0.9935,at \left[{\mathrm{H}}_2{\mathrm{O}}_2\right]=1.0 \% \mathrm{wt}$$

(21d)

where W (% wt) is the weight concentration of H₂SO₄.

The gas-phase products accompanying the MBO uptake were determined online using MS and GC approaches and offline using FTIR. Isoprene was the only gaseous product during MBO uptake in sulfuric acid solutions. The products identified during the uptake in the mixed solutions of H₂O₂ and H₂SO₄ were acetaldehyde, acetone, and a product observed online that decomposed into isoprene when analyzed offline. Scheme 44 (Section 6.3) includes the mechanism proposed by the authors.

Ren et al. [268] studied the aqueous-phase reactions of MBO with sulfate radical-anions generated from K₂S₂O₈ either by the photodissociation at 254 nm or by thermal dissociation at room temperature. Due to essentially different rates of dissociation, the photo experiments lasted for 1 hour while the thermal experiments—up to 150 days. The same products were identified in both types of experiments, albeit in different proportions (Section 6.1, Tables 39 and 40 (organosulfates)). The reaction mechanism is discussed in Section 6.3.

Liyana-Arachchi et al. [269] analyzed the behavior of MBO molecules and OH radicals at an air-water interface using a molecular dynamics approach. They found that encounters of the species at the interface were sufficiently numerous to indicate a possibility of a chemical reaction.

Heath et al. [270] determined the pseudo-first-order rate constants for the reaction of MeJa with OH radicals in irradiated aqueous solution (bulk) and thin aqueous films. The OH radicals were generated by the photolysis of H₂O₂. The extent of the reaction was followed by measuring the concentration of MeJa with HPLC-UV/DAD. The rate of reaction in the films was more significant than in bulk and increased with decreasing film thickness (

Table 36. Pseudo-first order rate constant for the aqueous-phase reaction of MeJa with OH radicals in bulk solutions and aqueous films.

Film Thickness, µm	∞ (Bulk)	193.1	77.2	38.6
k1st, 10−4 min−1	2.83 ± 0.02	9.62 ± 0.43	11.0 ± 0.5	12.7 ± 0.6

). The readers may doubt whether the illumination of the bulk solution and thin films was equally effective.

Hansel et al. [21,271] applied HPLC-ESI-MS techniques to identify several intermediate and final low-volatile products formed in the aqueous-phase oxidation of MeJa and MeSa by OH obtained by the photodissociation of H₂O₂ and proposed plausible formation mechanisms (Section 6.3, Schemes 47 and 48, resp.).

In summary, the rate constants for aqueous-phase reactions of GLV with OH radicals were high (~10⁹ M⁻¹ s⁻¹) and close to the diffusional limit (Table 33). Rate constants for reactions with SO₄⁻ were a little lower but still close to the diffusional limit (Table 32). Reactions with NO₃ were still slower, with rate constants of the order 10⁸ M⁻¹ s⁻¹. Reactions with singlet molecular oxygen and excited triple-state carbon appeared the slowest, with rate constants ranging between 10⁵ and 10⁷ M⁻¹ s⁻¹. All reactions of GLV with radicals weakly accelerated with temperature had energy of activation between 4 and 20 kJ mol⁻¹.

6. Multiphase and Heterogeneous Transformation

6.1. Smog-Chamber Studies

Products of GLV processing in smog chambers are listed in:

Table 37. Products identified in smog chamber oxidation of (Z)-3-hexen-1-ol by OH and O₃.

Name	Oxidant	MW	Formula	Structure	Phase	Ref.
acetaldehyde	O3	44	C2H4O		gas	[92]
2-propenal		56	C3H4O
propanal		58	C3H6O
acetic acid		60	C2H4O2
2-propenoic acid		72	C3H4O2
3-hydroxypropanal		74	C3H6O2		PM	[198]
propionic acid		74	C3H6O2		Gas, PM	[92,198]
2-hydroxyacetic acid		76	C2H4O3		PM	[198]
3-hydroxy-2- oxopropanal		88	C3H4O3
2,3-di-hydroxypropanal		90	C3H6O3
2-hydro-peroxypropanal		90	C3H6O3
3-hydroxy-2- oxopropanoic acid		104	C3H4O4
2-hydroperoxy-3- hydroxypropanal		106	C3H6O4
2-ethyl-1,3-dioxan-4-ol	OH, O3	132	C6H12O3		PM	[65,198]
3-(2-hydroxy ethoxy) propanoic acid		134	C5H10O4			[65]
2-(2-hydroxyethyl)- 1,3-dioxan-4-ol		148	C6H12O4			[65,198]
2-(1,3-dioxin-2- yl) ethylformate	O3	158	C7H10O4		PM	[198]
3-(3–hydroxy propanoyloxy) propanoic acid	OH, O3	162	C6H10O5		PM	[65]
2-(4-hydroxy-1,3- dioxan-2-yl)ethyl formate	O3	176	C7H12O5		PM	[198]
2-(1,3-dioxin-2-yl) ethyl propionate		186	C9H14O4
1-((2-ethyl-1,3-dioxan- 4-yl)oxy)propan-1-ol		190	C9H18O4
2-(2-((3-hydroxyprop- 1-en-1-yl)oxy)ethyl)- 1,3-dioxan-4-ol		204	C9H16O5
2-(4-hydroxy-1,3- dioxan-2-yl)ethyl propionate		204	C9H16O5
3-(3-(2- hydroxyethoxy) propanoyloxy) propanoic acid	OH, O3	206	C8H14O6		PM	[65]
3-(formyloxy)-3-(2- oxoethoxy) propyl propionate	O3	218	C9H14O6		PM	[198]
3-(3-(2-hydroxy ethoxy)-3- oxopropanoyloxy) propanoic acid	OH, O3	220	C8H12O7		PM	[65]
3-(3-(2-hydroxyethoxy)-3-hydroxypropanoyloxy) propanoic acid		222	C8H14O7
1-(2-(4-hydroxyl-1,3-dioxan-2-yl)-ethoxy)-propane-1,3-diol		222	C9H18O6
1,3-dihydrox-3- (2-hydroxyethoxy) propyl- 3- hydroxy propanoate		224	C8H16O7
3-( 3-(3-hydroxy propanoyloxy) Propanoyloxy propanoic acid		234	C9H14O7
3-(carboxyoxy)-3-(2- oxoethoxy) propyl propionate	O3	234	C9H14O7		PM	[198]
1-(1-((2-ethyl-1,3- dioxan-4-yl)oxy)propoxy) propan-1-ol		248	C12H24O5
3-(3-(3-(3-hydroxy propanoyloxy) propanoyloxy) propanoyloxypropanoic acid	OH, O3	306	C12H18O9		PM	[65]

(photooxidation and ozonolysis of (Z)-3-hexen-1-ol),

Table 38. Products identified in smog chamber ozonolysis of (Z)-3-hexenyl acetate.

Name	MW	Formula	Structure	Phase	Ref.
2-propenal	56	C3H4O		Gas	[92]
Propanal	58	C3H6O
Acetic acid	60	C2H4O2
2-propenoic acid	72	C3H4O2
Propionic acid	74	C3H6O2		Gas, PM	[92,198]
2-hydroperoxy propanal	90	C3H6O3		PM	[198]
3-oxo-propyl acetate	116	C5H8O3		PM	[272]
2-acetoxyacetic acid	118	C4H6O4		PM	[198]
2,3-dioxopropyl acetate	130	C5H6O4
2-hydroxy-3-oxopropyl acetate	132	C5H8O4
3-acetoxy-propanoic acid	132	C5H8O4		PM	[198,272]
3-acetoxy-2-oxopropanoic acid	146	C5H6O5		PM	[198]
2-hydroperoxy-3- oxopropyl acetate	146	C5H8O5
3-acetoxypropane peroxoic acid	148	C5H8O5		PM	[272]
3,4-dioxohexyl acetate	172	C8H12O4		PM	[198]
2-hydroxyethyl 3- acetoxypropanoate	176	C7H12O5		PM	[272]
2-(3-oxopropyl)ethyl 3- acetoxy propanoate	232	C10H16O6		PM	[272]
3-acetoxypropanoyl 3- acetoxypropanoate	246	C10H14O7		PM	[198]
2-(2-(3-acetoxy propanoyloxy)ethoxy) propanoic acid	248	C10H16O7		PM	[272]
5-acetoxy-3-oxopentyl-3-acetoxypropanoate	274	C12H18O7
2-(3- acetoxypropanoyloxy)ethyl 3-acetoxypropanoate	290	C12H18O8
[3-[2-(3- acetoxypropoxy)ethoxy]-3-oxo-propyl] 3-acetoxypropanoate	348	C15H24O9

(ozonolysis of (Z)-3-hexenyl acetate),

Table 39. Products identified in SOA from smog-chamber and aqueous-phase reactions of 2-methyl-3-buten-2-ol and ambient SOA samples (except organosulfates listed in Table 40).

MW	Name	Formula	Structure	Phase	Ref.	Ambient Aerosol
30	formaldehyde	CH2O		Gas	[273,274]
32	methanol	CH4O		Aqu	[268] b
44	acetaldehyde	C2H4O		Gas	[273]
46	formic acid	CH2O2		Aqu	[268] b
48	formaldehyde hydrated (methanediol)	CH4O2
58	acetone	C3H6O		Gas, aqu	[268,273,274] b
58	glyoxal	C2H2O2		Gas	[187,273]
60	glycolaldehyde	C2H4O2
61	acetic acid	C2H4O2		Aqu	[268] b
72	methylglyoxal	C3H4O2		Gas	[273]
74	glyoxylic acid	C2H2O3
76	glycolic acid	C2H4O3		Aqu	[268] b
78	glycolaldehyde hydrated	C2H6O3
86	1,3-butanedione	C4H6O2		Gas	[273]
86	2-oxopropanedial	C3H2O3
88	2-hydroxy-2-methylpropanal	C4H8O2		Gas	[187,273]
88	2-hydroxypropaneial	C3H4O3		Gas	[273]
100	2,3-dioxobutanal	C4H4O3
114	2,3-dioxobutane-1,4-dial	C4H2O4
116	2-oxovaleric acid	C5H8O3
118	1,3-dihydroxy-3-methyl- butan-2-one	C5H10O3		Gas, PM
118	2,3-dihydroxy-3- methylbutanal	C5H10O3		Gas
120	2,3-dihydroxyisopentanol	C5H12O3		PM, aqu	[266] a, [268] b, [273]	PM2.5 [273,275]
132	2,3-dihydroxy-2-methylbutane dialdehyde	C5H8O4		Gas, PM	[273]
134	2,3-dihydroxy-3-methyl- butanoic acid	C5H10O4		PM
134	2-hydroxy-2- methylpropenedioic acid	C4H6O5
136	2-methylerythritol	C5H12O4				PM2.5 [273,275]
136	2-methylthreitol	C5H12O4				PM2.5 [273,275]
164	2,3-dihydroxy-2- methylsuccinic acid c	C5H8O6				PM2.5 [276]

^a aqueous phase reactions of methylbutenol epoxides (Section 5); ^b aqueous-phase addition of sulfate radical anions (Section 5); ^c and isomers.

(photooxidation of 2-methyl-3-buten-2-ol and ambient samples),

Table 40. Organosulfates identified in smog chamber and aqueous-phase experiments with GLV, eventually confirmed in samples of ambient aerosols.

	Product			Parent Compound	Formation Process	Ref.	Ambient Aerosol
MW	Name	Formula	Structure
140	Glycolaldehyde sulfate	C2H4O5S		MBO	SO4− addition	[268] b	PM2.5 [277] PM10 [278]
154	Hydroxyacetone sulfate	C3H6O5S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM1 [280,281], PM2.5 [277,279,282,283,284,285], PM10 [278]
156	Glycolic acid sulfate	C2H4O6S		MBO	SO4− addition	[268] b	PM1 [280,281,286], PM2.5 [277,282,283,284,285,287,288]
158	Hydrated glycol sulfate	C2H6O6S		MBO	SO4− addition	[268] b
170	Lactic acid sulfate	C3H6O6S		(E)-2-pentenal	Ozonolysis	[289]	PM1 [280,286], PM2.5 [277,282,283,284,287,289,290,291,292]
170	1-sulfooxy-2- hydroxybutane	C4H10O5S		(E)-2-pentenal	Ozonolysis	[289]	PM2.5 [289]
170	2-Sulfoxy-3- hydroxy-propanal	C3H6O6S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM2.5 [279]
186	3-sulfoxy-2- hydroxy-propanoic acid	C3H6O7S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM2.5 [279]
198	3-hydroxy-3- methyl-butan-2-one sulfate	C5H10O6S		MBO	SO4− addition	[268] b
198	4-sulfoxy-1- hydroxy-3- m3thyl-butan-2-one	C5H10O6S		MBO	SO4− addition	[268] b	PM2.5 [277], PM2.5 [288]
200	2,3-dihydroxy-3-methyl-butane sulfate	C5H12O6S		MBO	OH photolysis SO4− addition	[266] a, [268] b,[293]	PM1 [280,281] ?, PM2.5 [277,283,293], [288] ?
210	(Z)-5-sulfoxy- hex-3-enoic acid	C6H10O6S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM1 [280,281] ?, PM2.5 [279], PM10 [278] ?
212	6-(sulfoxy) hexanoic acid	C6H12O6S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM1 [280,281] ?, PM2.5 [279]
214	3-sulfoxy-2- hydroxypentanoic acid	C5H10O7S		(E)-2-pentenal (E)-2-pentenoic acid	Ozonolysis, SO4− (aqu)	[289]	PM1 [280] ?, PM2.5 [289]
214	2-sulfoxy-3- hydroxypentanoic acid	C5H10O7S		(E)-2-pentenal	Ozonolysis	[289]	PM1, [280,281] ?, PM2.5 [289]
226	(E)-5-sulfoxy-4- hydroxy-hex-2-enoic acid	C6H10O7S		(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM2.5 [279]
226		C6H10O7S		(Z)-3-hexenal	Ozonolysis	[294]	PM2.5 [294]
230	3-sulfooxy-2,4- dihydroxypentanoic acid	C5H10O8S		(E)-2-pentenal, (Z)-3-hexenal (Z)-2-hexenal	Ozonolysis ozonolysis	[289]	PM2.5 [289]
270		C9H18O7S	-	(Z)-3-hexen-1-ol	OH photolysis, ozonolysis	[279]	PM2.5 [279]

^a aqueous phase reactions of methylbutenol epoxides (Section 5); ^b aqueous-phase addition of sulfate radical anions (Section 5); ^? unresolved structure.

(organosulfates from various experiments and ambient samples). One should remember that compounds identified in aerosol samples may have multiple precursors. The experiments are briefly described below.

Barbosa et al. [279] studied the formation of SOA from the hydroxyl radical (OH)-initiated oxidation and ozonolysis of (Z)-3-hexen-1-ol. They used either non-acidified or acidified sulfate seed aerosols under different relative humidity conditions (12–95% and 5–52%, resp.) and 291–300 K temperatures. The OH radicals were generated by the photolysis of isopropyl nitrite. SOA yields in OH–GLV experiments varied from 13 to 34.2 μg m⁻³ (0.8–2%) for acidic seeds and from 15.2 to 32.1 μg m⁻³ (0.9–2%) for nonacidic seeds, with weak maxima at intermediate RH (~30%). The dependence is biased by the variation of other parameters, notably the NO and NO_x concentrations. SOA yields in O₃–GLV experiments varied from 5 to 52 μg m⁻³ (0.5–5.1%) for acidic seeds and from 5 to 30 μg m⁻³ (0.5–2.9%) for nonacidic seeds, and were significantly higher at low RH (0.7~5% at RH = 4–5% vs. 0.5–1% at RH = 44–51%). They discovered the formation of several organosulfates (Table 40) and confirmed their presence in PM_2.5 field samples (Section 6.2).

Hamilton et al. [65] studied the SOA formation during ozonolysis or OH initiated oxidation of (Z)-3-hexen-1-ol and (Z)-3-hexenylacetate in the European Photoreactor chamber in Valencia. They used isoprene as a reference compound. The photolysis of HONO generated the OH radicals. The authors used the LC-ITMS approach to identify the products contained in SOA particles collected on quartz-fiber filters [272]. Ozonolysis of (Z)-3-hexenyl acetate produced 3-acetoxypropanal, 3-acetoxypropanoic acid, 3-acetoxypropane peroxic acid, and a range of oligomers with ester and ether linkages (Table 38). Almost all oligomers contained a terminal acetate group, which prevented further oligomeric growth. Ozonolysis of (Z)-3-hexen-1-ol produced a vast number of oligomers originating from 3-hydroxypropanal and 3-hydroxypropanoic acid (Table 37), according to the mechanism in (Section 6.3, Scheme 51).

Aerosol mass yields in the OH–GLV experiments were 43.2 μg m⁻³ (3.1%) for (Z)-3-hexen-1-ol, 9.7 μg m⁻³ (0.93%) for (Z)-3-hexenylacetate, and 18.5 μg m⁻³ (1.2%) for isoprene (initial concentrations 449, 498, and 511 ppbv, resp.). Aerosol mass yields from ozonolysis of GLV were 854 μg m⁻³ (9.6%) for (Z)-3-hexen-1-ol and 800 μg m⁻³ (8.5%) for (Z)-3-hexenylacetate.

O’Dwyer et al. [201] observed SOA formation during ozonolysis of 1-penten-3-ol, (Z)-2-penten-1-ol and 1-penten-3-one at 298 K and atmospheric pressure in a 3910 dm³ smog chamber (see Section 4.1.3). They used dry conditions and no seed aerosol. A burst of new particles, 30–50 nm in diameter, was observed immediately after the ozone introduction (~180 s). The particles’ number decreased in time, but their size increased to 130–150 nm at ~1.2 hours. The aerosol mass yields were 0.136–0.166 for 1-penten-3-ol, 0.156–0.164 for (Z)-2-penten-1-ol, and 0.028–0.034 for 1-penten-3-one.

Hamilton et al. [272] studied the composition of SOA obtained from the ozonolysis of (Z)-3-hexenylacetate in the EUPHORE chamber at 292 K (average). FTIR followed the decay of hydrocarbons. The formation and growth of SOA was followed by a scanning mobility particle sizer, condensation particle counter, and differential mobility analyzer. Samples of SOA were collected on quartz fiber filters using a high-volume pump. The filters were extracted by sonication with water, methanol, tetrahydrofuran, or acetonitrile. The extracts were evaporated to dryness and reconstituted with a water/methanol solvent (50:50 vol) for LC-MS/MS analysis with Li⁺ cationization. The major products identified are listed in Table 38. Fifteen other oligomers were distinguished, all with 3-acetoxypropanoic acid monomer units.

Jain et al. [198] observed the formation of SOA from ozonolysis of (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, or effluents from grass clippings (Festuca, Lolium, and Poa) in a 775 l Teflon chamber at 298 ± K. Ozone was generated externally with a corona discharge apparatus. The initial molar ratio of O₃ and GLV was 1:1. They measured the particle number and mass size distribution using a scanning mobility particle sizer. In grass clipping experiments, SOA formed immediately after ozone injection into the chamber. The mass loading of SOA reached 16 μg m⁻³ from (Z)-3-hexenyl acetate and 12 μg m⁻³ from (Z)-3-hexen-1-ol. The addition of propionaldehyde enhanced the SOA formation indicating it was a precursor of condensing products (Section 6.3, Scheme 51). They analyzed the particle composition using near-infrared laser desorption/ionization mass spectrometry. Compounds observed in SOA from grass clipping experiments included first-generation products (m/z 70–150) and later products (m/z 150–300).

Fischer et al. [48] determined the (1.03 ± 0.07%) SOA mass yields from gas-phase ozonolysis of 1-octen-3-ol in two Teflon chambers (see Section 4.2.3) equipped with scanning mobility particle sizers and electrical low power impactors. Ozone and 1-octen-3-ol mixing ratios were 200 ppb. They also observed SOA formation from ozonolysis of volatiles emitted in situ from cut sugarcane. The sugarcane volatiles included toluene, 3-hexenal, 2-hexenal, (Z)-3-hexenyl acetate, (Z)-3-hexen-1-ol, and 1-octen-3-ol. The gas-phase ozonolysis products included methyl ester of butanoic acid, heptanal, octanal, nonanal, and decanal, all identified against the NIST spectral library. The maximum SOA concentration observed was 1.6 ± 0.2 μg m⁻³. Several particle-phase products were identified using NIR-LDI-AMS (Section 6.3, Scheme 53). The SOA particles had a non-liquid character shown by the bounce-factor determinations.

Faiola et al. [295] investigated the SOA formation during dark ozonolysis and OH-induced photooxidation of Pinus sylvestris emissions. They compared the emissions from healthy plants and plants infested by large pine aphids (Cinara pinea Mord.). The initial concentration of MeSa was one of the parameters measured. SOA yield from ozonolysis ranged from 6.3 to 14.6% and did not depend on the infestation. SOA yield from the OH photooxidation of the emissions was: 10.5–23.2% for healthy plants and 17.8–26.8% for the aphid-infested plants.

Waza [296] studied the ozonolysis of (E)-2-hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexenylacetate in a rectangular 9.8 m³ Teflon-film chamber at ambient temperature and pressure and 50% RH. He observed the formation and growth of SOA particles. The SOA yields were smaller than those observed for α-pinene in comparable experiments.

Joutsensaari et al. [297] demonstrated the formation and growth of SOA particles from ozonolysis of emissions of white cabbage cultivars sprayed with MeSa in an illuminated 2.6 m³ growth chamber. The emissions contained terpenes, sesquiterpenes, (Z)-3-hexen-1-ol, (E)-2-hexenal, and (Z)-3-hexenyl acetate.

Shalamzari et al. [294] studied the growth of SOA on seed aerosol during the ozonolysis of (Z)-3-hexenal in a 14.5 m³ steel chamber with Teflon-coated walls. Relative humidity and acidity of seed aerosols varied. The OH radicals were generated from the NO_x-mediated photooxidation of organic precursors. SOA samples were collected on Teflon-impregnated glass fiber filters. The methanol extracts of the filters were obtained using sonication, preconcentrated with a rotary evaporator, filtered, and evaporated to dryness with nitrogen stream. The residue was reconstituted with methanol, dried again, reconstituted with methanol/water solvent (1:1 col), and analyzed with UPLC-MS using reversed-phases or ion-pairing techniques. A significant MW 226 organosulfate was identified as a SOA-bound component from the ozonolysis of (Z)-3-hexenal (Table 40).

In a similar study, Shalamzari et al. [289] studied SOA from ozonolysis of (E)-2-pentenal, (Z)-3-hexenal, and (Z)-2-hexenal. They identified the following organosulfates, which could be related to (E)-2-pentenal: 3-sulfooxy-2,4-dihydroxy pentanoic acid (MW 230); 2-sulfooxy-3-hydroxy pentanoic acid and 3-sulfooxy-2-hydroxy pentanoic acid (MW214); lactic acid sulfate (MW 170); and 1-sulfooxy-2-hydroxy butane (MW 170) (Table 40). Those organosulfates were also produced during ozonolysis of (Z)-3-hexenal and (Z)-2-hexenal. 3-sulfoxy-2-hydroxy pentanoic acid was also formed during aqueous-phase sulfation of (E)-2-pentenoic acid with sulfate radical anions. The formation mechanisms of those products were suggested.

Harvey et al. [92] observed the formation of SOA from ozonolysis of (Z)-3-hexenol, (Z)-3-hexenylacetate, their mixtures, and emissions from turf grass (Festuca, Lolium, and Poa). They used a 775 l Teflon chamber at 296 K. Using a scanning mobility particle sizer, they measured total aerosol mass, aerosol particle number, and mass size distribution. The recorded SOA yields were 0.26 ± 0.01 and 0.24 ± 0.08%, respectively. For each GLV, the products identified included: propanoic acid, propenoic acid, acetic acid, propanal, and 2-propenal (from each GLV). Acetaldehyde was produced only from hexenyl acetate.

Carrasco et al. [177] studied the SOA formation during the gas-phase reactions of MBO with O₃ and with OH radicals in the presence of NO. Experiments with ozone were carried out in the CRAC chamber, while experiments with OH–in the 204 m³ EUPHORE outdoor chamber with natural sunlight illumination (Section 4.2, Table 16 and Table 24). The chambers had multireflection optical systems coupled to FTIR spectrometers for monitoring the concentrations of gaseous reactants and products and the analyzers for monitoring O₃, NO_x, and NO_y. Scanning mobility analyzers monitored the number, concentration, and size distribution of SOA particles. No significant yields of SOA occured in the reactions with OH radicals. Ozonolysis produced 9–25 μg m⁻³ aerosol mass at wet conditions (RH = 20–30%) and 74 μg m⁻³ at dry conditions equivalent to 0.3–1.5% and 1.8% yields, resp.

Chan et al. [187] studied SOA formation during photooxidation of 2-methyl-3-buten-1-ol initiated by OH radicals at 293–298 K, 4–66% RH, and high NO_x or low NO_x conditions in 28 m³ Caltech dual indoor chambers. The OH radicals were generated by the HONO photooxidation at high NO_x and H₂O₂ photooxidation at low NO_x. GC-FID, chemical ionization MS, and laser-induced phosphorescence were used to determine the concentration of 2-methyl-3-buten-1-ol, gas phase products, and glyoxal, resp. The mass yield of SOA was 0.001–0.0014 in the low NOx experiments and lower than 0.001 in the high NO_x experiments.

Jaoui et al. [273] used a 14.5 m³ steel chamber with Teflon coated walls to study the photooxidation of 2-methyl-3-buten-2-ol initiated by OH radicals in the presence and absence of NO_x and SO₂. They generated OH radicals by the in situ photolysis of H₂O₂ or methyl nitrite and added ammonium sulfate seed aerosol to promote the condensation of low volatile products. In some experiments, they added SO₂ as well. The reactants and gas-phase carbonyl products were determined using GC-FID and GC-MS. The aerosol particles were collected on quartz filters for further analysis of composition and EC/OC contents. The samples were extracted from filters, derivatized with BSTFA, and analyzed using GC-MS approaches. Size distribution, volume, and total number density of aerosol particles were measured during experiments with a scanning mobility particle sizer. Besides, samples of ambient PM_2.5 aerosol were collected on quartz filters in the Research Triangle Park, NC USA, extracted, derivatized, and analyzed for composition. Conversion of MBO in all experiments was 70%, the yield of SOA was 0.7% (one experiment), and the yield of secondary organic carbon (SOC) ranged from 0.2 to 0.9%. The gas products identified included formaldehyde, acetaldehyde, glyoxal, glycolaldehyde, methylglyoxal, glyoxylic acid, 2,3butanedione, 2-oxopropane-1-3-dial, 2-hydroxy-2-methylpropanal, 2-hydroxypropane-1,3-dial, 2,3-dioxobutanal, 2,3-dioxobutane-1,4-dial, 3-hydroxy-2-oxo-isopentanol, 2,3-dihydroxy-3-methylbutanal, 2,3-dihydroxy-2-methylbutane dialdehyde, and 2-oxovaleric acid. Table 39 shows products identified in the particle phase.

Zhang et al. 2012 [293] studied the photooxidation of 2-methyl-3-buten-2-ol initiated by OH radicals in the presence of low NO in a 274 m³ outdoor smog chamber with natural sunlight. The OH radicals were generated from H₂O₂. Neutral or acidic seed aerosol was prepared from ammonium sulfate and sulfuric acid. Aerosol was sampled on glass-fiber filters. The samples were extracted with methanol and analyzed using LC-MS techniques. The SOA yield increased with the acidity of seed aerosol up to 1%. The main product identified–MW 200 organosulfate–was also determined in ambient SOA (Section 6.2). The same product was identified in aerosol samples from smog-chamber experiments with the uptake of (3,3-dimethyloxiran-2-yl) methanol (a 2-metyl-3-buten-1-ol epoxide) on seed aerosols carried out to confirm the epoxide pathways of SOA formation [298].

Novelli et al. [274] studied the photooxidation of MBO by O₃ in the presence of NO in the 270 m³ outdoor chamber SAPHIR. They followed the evolution of OH and HO₂ radicals and showed it was consistent with box modeling based on the MCM 3.3.1 mechanism. The hydrogen-shift pathways suggested by [299] were insignificant due to relatively low reaction rates. The major products observed in the chamber were acetone and formaldehyde.

6.2. Ambient Aerosols

Many compounds linked to the atmospheric transformation of GLV were firmly identified in ambient aerosol samples. In this section, we present the results from campaigns that intentionally regarded the reactions involving GLV.

Barbosa et al. [279], in a study supporting their chamber experiments, discovered several organosulfates in PM_2.5 samples collected on prebaked quartz filters in the Atlantic Forest area (open tropical rainforest type) of the Botanical Garden in Rio de Janeiro in 2016 (Table 40). They identified organosulfates also in the companion smog-chamber experiments.

Ambient aerosol samples collected in K-puszta in summer 2003 contained 2,3-dihydroxypentanoic acid, which could originate from 2-pentenal [300], a product of (Z)-3-hexenal [161] photolysis.

Shalamzari et al. [294] analyzed PM_2.5 samples collected in K-puszta from May to June 2006 (BIOSOL campaign). They used high-volume dichotomous PM_2.5 samplers with quartz-fiber filters and sampled separately during days and nights. Punches from the filters were extracted with methanol, preconcentrated, filtered, and evaporated to dryness. The residues were reconstituted in methanol, divided in portions, and evaporated to dryness. Before the UPLC-MS analysis, the residues were reconstituted in methanol/water (1:4 vol) solvent. A characteristic MW 226 organosulfate H₂C=C-CH(OH)-CH(OSO₃H)-CH₂-C(O)OH was identified in the samples, which could form by ozonolysis of (Z)-3-hexenal as shown in companion chamber experiments (Section 6.1, Table 40).

Shalamzari et al. [289] continued the analysis of K-puszta aerosol samples. They identified the following organosulfates linked to GLV in the companion chamber studies (Section 6.1, Table 40): 3-sulfoxy-2,4-dihydroxypentanoic acid (MW 230), 2-sulfoxy-3-hydroxypentanoic acid (MW 214), lactic acid sulfate (MW 170), 1-sulfooxy-2-hydroxybutane (MW 170).

Zhang et al. [293] collected ambient PM_2.5 aerosol samples using high-volume samplers in Manitou Forest Observatory in the Rocky Mountains in 2011. The samples were extracted from filters with methanol and analyzed using LC-MS techniques. The MW 200 organosulfate was identified, which also formed from MBO in companion smog-chamber experiments (Section 6.1, Table 40). The compound was suggested as a plausible marker of MBO SOA.

6.3. Aqueous and Multiphase Mechanisms

GLV and products of their gas-phase reactions (Section 4.1) can partition into aqueous solutions or particle aqueous-phases. They can hydrolyze or react with other dissolved species by various mechanisms reviewed in this section.

GLV epoxides form by the oxidation of GLV in the gas phase. Scheme 43 shows the formation of MBO epoxides, including the 1,5-hydrogen shift and elimination of HO₂ radical [266,298].

In the aqueous phase, GLV epoxides can hydrolyze or react with acids or acidic anions like sulfate or nitrate as reported for MBO (Scheme 44) [266,267], (Z)-3-hexenal [294], and (E)-2-pentenal [289]. Hydrolysis is acid-catalyzed [267].

GLV-derived hydroperoxides form during the ozonolysis or photooxidation of GLV in the gas phase [279]. After partitioning to the aqueous phase, they can react with acids (Scheme 45).

Aqueous-phase reactions of GLV with radicals and radical-anions can proceed by the addition of a radical to a C=C double bond, electron transfer, or hydrogen abstraction. The addition mechanisms lead to many products and are most interesting. The mechanism involving sulfate radical-anions and oxygen was proposed for isoprene [301,302], methyl vinyl ketone [303], (Z)-2-pentenoic acid [289], and recently for olefins, including MBO [268]. Scheme 46 shows the reaction mechanism for MBO based on the latter citation. The reaction starts with the addition of SO₄⁻ to the double C=C bond in MBO and the formation of carbon-centered alkyl sulfate radical (two isomers). Oxygen molecule adds to a radical center to form the peroxyalkyl sulfate radical, which can enter a self-reaction [181] and turn into the sulfate butanediol and hydroxybutanone sulfate products. In an alternative path, the peroxyradical loses one oxygen atom to some available reducing agent and turns into the alkoxy sulfate radical. The alkoxy sulfate radical can undergo electron transfer from a suitable donor and turn into the sulfate butanediol product or can undergo hydrogen abstraction by an oxygen molecule and turn into the hydroxybutanone product. Besides, the alkoxy sulfate radical can participate in several fragmentation processes to give smaller molecules like glycolic sulfates, acetic acid, formic acid, and methanol.

Hansel et al. [21,271] proposed the chemical mechanisms of aqueous-phase transformation of MeJa and MeSa. For MeJa, the mechanism started with the addition of OH radical to a double C=C bond producing four isomeric hydroxyalkyl radicals (Scheme 47). Then, an O₂ molecule is added to each isomer to give four isomers of a hydroperoxide radical. Each hydroperoxide isomer could either react with other peroxy radicals present to give alcohols, carbonyls, and organic peroxides, or react with HO₂ radicals and lose O₂ to produce hydroyperoxides. MeSa could react with OH radicals by two distinct paths. They start with the OH addition to the aromatic ring and the phenolic hydrogen abstraction by OH (Scheme 48). In the first path, 1,2- and 1,4-dihydroxycyclohexadienyl radicals formed, which reacted with molecular oxygen to give the peroxyl radicals. The peroxyl radical decomposed by elimination of an HO₂ radical to give dihydroxybenzene. The latter could similarly react with OH and O₂ to afford trihydroxybenzene products. Dihydroxycyclohexadienyl radicals could also lose an H₂O molecule and form an H-adduct radical, converting to phenoxyl radical. The two radicals can recombine to form dimers. In the hydrogen abstraction path, the phenoxyl radical forms directly from MeSa. Trihydroxybenzene products can from peroxy radicals via an alternative pathway (Scheme 49).

An interesting concept was the pinacol rearrangement of protonated MBO, which led to carbonyl derivatives (Scheme 50) [132].

The ozonolysis of GLV has been studied in multiphase chamber experiments but not in the aqueous phase alone [48,92,198,279]. Hydroperoxides formed in the gas phase possibly partitioned to the particle aqueous-phase, and participated in acid-catalyzed sulfation (Scheme 45) [279]. 3-Hydroxypropanal, produced by the decomposition of primary (Z)-3-hexen-1-ol ozonide, reactively partitioned to the particle phase. Therein, it plausibly entered several oligomerization reactions to give higher molecular weight products (Scheme 51) [65,198]. A similar mechanism is valid for the particle-phase oligomerization of 3-oxopropyl acetate and propanal (Scheme 52) [92]. Both compounds originated from the gas-phase ozonolysis of (Z)-3-hexenyl acetate. The formation of several particle-phase products from 1-octen-3-ol through Criegee intermediates was proposed based on AMS analyses of SOA (Scheme 53) [48].

7. Atmospheric Impact of GLV

Holopainen et al. [72] provided a very recent concept review on the role of plant volatiles in the atmosphere-biosphere relations Like F.W. Went in his visionary approach from the 1960s [304,305], they elaborated on a framework linking plant emission with atmospheric processes and formation of soil components. However, that paper presented a general idea and did not go into a detailed treatment.

7.1. Atmospheric Lifetimes of GLV

An atmospheric lifetime of a given compound C, which decays in a gas-phase bimolecular reaction with an atmospheric oxidant X, is estimated based on the assumption that the rate of C decay follows Equation (22):

$$\frac{d \left[C\right]}{dt}=-k\left[X\right]\left[C\right]$$

(22)

We assume that the concentration of X is constant, so Equation (22) becomes the explicitly integrable pseudo-first-order equation:

$$ln\left(\frac{\left[C\right]}{{\left[C\right]}_0}\right)=-kt$$

(23)

For the simplicity of comparison, we assume that the atmospheric lifetime is the time ratio [C]/[C]₀ that will be equal to e. After rearrangement, we obtain:

$$t_{atmos}=\frac{1}{k\left[X\right]}$$

(24)

By analogy, the atmospheric lifetime of a compound due to the gas-phase photolysis is:

$$t_{atmos}=\frac{1}{j}$$

(25)

where j is the rate constant of the photolytic decay of this compound.

The atmospheric lifetimes of GLV reported in the literature or calculated from reported rate constants are collected in

Table 41. Atmospheric lifetimes of GLV at 298 K due to gas-phase reactions with OH (2 × 10⁶ molecules cm⁻³), NO₃, (5 × 10⁸ molecules cm⁻³), O₃ (7 × 10¹¹ molecules cm⁻³), and Cl (1 × 10⁴ molecules cm⁻³).

GLV	OH		NO3		O3		Cl		Photolysis
	τ h	Ref.	τ h	Ref.	τ h	Ref.	τ h	Ref.	τ h	Zenith Angle, °	Ref.
Pentan-1-ol	24 b	[184]
1-penten-3-ol	2.1	[185]	40	[192] a	23.5	[201]	118	[205] a	174–120 c		[57]
	2.83	[57] b			22.3	[237] a
	1.95	[228] a			24.3	[243] b
(Z)-2-penten-1-ol	1.3	[185]	3.6	[192] a	3.7	[201]	93	[245]
					2.5	[237] a	92.6	[205] a
					2.1	[243] b
(E)-2-penten-1-ol	2.05	[57] a
(E)-2-pentenal	5.9	[85] a	28.8	[230] a	309	[244] b	212	[246] b	0.73	26	[162]
	3.23	[173] a
1-penten-3-one	1.18	[57] b	16.4	[215] a	29.2	[201]			77–20 ac		[57]
hexanal	4.4	[85] a	32	[232] a			97	[246] b	24.4–13.4 ac		[159]
	5.0	[159] a
(Z)-2-hexen-1-ol	1.25	[217] b	3.6	[231]	5.3	[238]
	4.27	[171] b	1.5	[230]
(E)-2-hexen-1-ol	1.4	[176]	4.3	[231]	6.7	[175]	81	[245]
	4.04	[171] b			2.4	[238]	81	[249]
	2.26	[57] a
(Z)-3-hexen-1-ol	1.3	[174,228] a	2.1	[231]	3.8	[237,240] a	91	[245]	17.4–11.8 ac		[57]
	1.0	[176]	2.0	[174]	2.8	[243] b	95	[249]
	4.78	[57] b			6.6	[175]
	5.05	[171] b			6.2	[174]
					7.2	[238]
(E)-3-hexen-1-ol	1.2	[176]	1.3	[231]	6.9	[175]	86	[245]
	1.25	[175]			6.4	[238]	81	[249]
	1.75	[217] b			6.6	[243] a
	4.55	[171] b
(Z)-4-hexen-1-ol	3.92	[171] b	1.9	[231]	5.6	[238]
(E)-4-hexen-1-ol	3.57	[171] b			3.8	[238]
(Z)-2-hexenal			40.8	[230] a
(E)-2-hexenal	3.12	[174] b	81.2	[174]	192	[174]	145	[246] b	0.48	26	[162]
	2.97	[159] a	41	[230]	274	[244] b			2.7		[161]
			10	[235]
			125	[193]
(Z)-3-hexenal					11.5	[160] a			13.3		[161]
									5.6	30	[160]
(Z)-3-hepten-1-ol	1.09	[175]					73	[249]
1-octen-3-ol							69	[245]
(Z)-3-octen-1-ol	0.94	[175]					67	[249]
2-methyl-3-buten-2-ol	3.56	[224] a	26.5	[234] b	39.9	[237] a	130	[251] a	>5 yrs		[165]
	2.01	[179] a	46	[236]	47.8	[242] a	59.1	[224] a
	2.17	[165] a			46.1	[224] a	84.2	[179] a
	2.5	[177]
(E)-2-hexenylacetate	2	[186]					9.0	[186]
(Z)-3-hexenyl formate	3	[222] b			9.8	[241]	113.5	[222]
(Z)-3-hexenyl acetate	1.76	[174] b	2.3	[174]	7.3	[174]
					6.9	[241]
					4.0	[196] a
(Z)-3-hexenyl propionate					5.2	[241]
(Z)-3-hexenyl butyrate					3.2	[241]
Methyl salicylate	70.88	[190]			99192 11.3 yrs	[164] b	15378 1.64 yr	[164] b
					15.6 yrs	[189] a	275 b	[209]

^a calculated using the rate constant from the reference; ^b recalculated from the reference values to another concentration; ^c at 0–10 km above Earth’s surface.

.

Sarang et al. [163] analyzed the atmospheric significance of the aqueous-phase reactions of 1-penten-3-ol, (Z)-2-hexen-1-ol, and (E)-2-hexen-1-al with OH, NO₃, and SO₄⁻ radicals. The aqueous-phase reactions reduced the atmospheric lifetimes of GLV at sufficiently high liquid water contents (LWC). The radical concentrations in the aqueous phase predicted by CAPRAM modeling [306] were: [OH] = 3.5 × 10⁻¹⁵–3 × 10⁻¹² M, [NO₃] = 1.9 × 10⁻¹⁵–8.6 × 10⁻¹⁴ M, [SO₄⁻] = 2.3 × 10⁻¹⁵–3.6 × 10⁻¹³ M. LWC ranged from 10⁻¹² to 10⁻⁴ m³ m⁻³ (aerosol to storm). Henry’s equilibria governed the partitioning of reactants between the phases. Fluxes of 1-penten-3-ol and (Z)-2-hexen-1-ol removed by the gas-phase and aqueous-phase reactions were comparable only in urban and remote clouds with high liquid water content. (E)-2-hexenal was removed faster by gas-phase processing in all clouds. Generally, the aqueous-phase processes did not influence the GLV lifetimes in systems with LWC lower than 10⁻⁶ m³ m⁻³ that is most clouds and all aerosol systems. In systems with high LWC, like storms, the lifetime of 1-penten-3-ol decreased markedly due to the aqueous-phase reactions with OH radicals. Similarly, the lifetimes of all three GLV studied decreased due to the reactions with NO₃. Aqueous-phase reactions of GLV with SO₄⁻ radicals could decrease the GLV lifetimes from years to hours at high LWC and radical concentrations. The authors estimated that the removal of GLV from the atmosphere by the aqueous-phase reaction with SO₄⁻ was faster than the removal by combined gas and aqueous phase reactions with OH or NO₃ in clouds and rains of high LWC, provided those radicals were in sufficient proportion to OH and NO₃ radicals. For 1-penten-3-ol the proportions were: [OH]/[SO₄⁻] ≤ 0.16, and [NO₃]/[SO₄⁻] < 6.5; for (Z)-2-hexen-1-ol–[OH]/[SO₄⁻] ≤ 0.40, and [NO₃]/[SO₄⁻] < 3; and for (E)-2-hexen-1-al–[OH]/[SO₄⁻] ≤ 0.11, and [NO₃]/[SO₄⁻] < 1.

7.2. SOA Potential of GLV

There is strong evidence that GLV contribute to the formation of ambient SOA (Section 6). Several smog-chamber experiments (Section 6.1) demonstrated the SOA formation from individual GLV, GLV mixtures, and plant emissions. To date, several compounds identified in ambient aerosol samples are known to form by reactions of GLV with atmospheric oxidants (Section 6.2).

Hamilton [65] estimated that the global SOA production from GLV (namely (Z)-3-hexen-1-ol and (Z)-3-hexenyl acetate) was 1–5 TgC yr⁻¹. The calculation based on SOA mass yields from the OH photooxidation of isoprene and each GLV in the smog chamber experiments (1.2%, 3.1%, and 0.93%, resp., Section 6.1) scaled up to 3%, 7.5%, and 2.25%. The scaled GLV yields were totaled and applied to the yearly emission of a group of bVOC, including hexenal, hexenol, hexenyl acetate, and six non-GLV compounds estimated as 10–50 TgC yr⁻¹ [45]. However, it is not clear whether that range regarded the whole group or each of its members. Thus, the estimated production of 1–5 TgC yr⁻¹ of SOA from GLV is somewhat imprecise and uncertain. Furthermore, the chamber yields reflected the masses of SOA carbon obtained from the parent compounds. They cannot convert any amount of a parent compound to the amount of derived SOA if the molecular composition of that compound is unknown.

Better estimates of global annual emissions are available for (Z)-3-hexenal (4.9 Tg yr⁻¹), (Z)-3-hexenol (2.9 Tg yr⁻¹), and MBO (2.2 Tg yr⁻¹)—claimed to be the most abundant GLV (Section 1, [50]). Yields of SOA from OH-initiated processing of (Z)-3-hexen-1-ol in smog chambers ranged from 2% [279] to the scaled-up value of 7.5% [65]. The corresponding global annual formation of SOA from (Z)-3-hexen-1-ol ranges from 0.058 to 0.218 Tg yr⁻¹. The maximum SOA yield from the smog-chamber ozonolysis of (Z)-3-hexenol was 5.2% [279], which gives 0.151 Tg yr⁻¹ of SOA globally. Smog-chamber yields of SOA from 2-methyl-3-buten-2-ol were 0.7% for OH reactions and 0.3–1.8% for ozonolysis. Respective global annual production of SOA is 0.015 Tg yr⁻¹ and 0.007–0.040 Tg yr⁻¹, respectively. We can assume that the chamber SOA yields from the oxidation of (Z)-3-hexanal are the same as for (Z)-3-hexen-1-ol. Then, the atmospheric SOA yields from(Z)-3-hexanal are 0.098–0.368 Tg yr⁻¹ from the OH reactions and 0.255 Tg yr⁻¹ from the ozonolysis. Thus, the estimated overall SOA yields from (Z)-3-hexen-1-ol, MBO, and (Z)-3-hexenal would be 0.58–1.05 Tg yr⁻¹.

Besides overall global estimates, one can calculate SOA emission from individual sources. The annual SOA production from C₆ GLV alcohols and aldehydes emitted by cut grass in Sydney and Melbourne was 0.1–0.2 Gg yr⁻¹, based on the emission estimates [19] (Section 2.3) and assumed 5% SOA yield. The annual global production of 940 μg m⁻² yr⁻¹ SOA from grass mowing [92] multiplied by 55.5 × 10⁶ km² of the total area of world grassland [307] translates into 52 Gg yr⁻¹ of SOA.

Switchgrass, grown to produce biofuels, produces 0.7 kg ha⁻¹ yr⁻¹ of 1-penten-3-ol, hexenols, and hexanals during growth and harvest (based on [88], Table 2). If the SOA yield from GLV is assumed at 5% and the total cultivation area is 9.5 × 10⁶ km² (data for 16 major countries in 2013, [308]), the total SOA input is 33 Gg yr⁻¹. That number can increase because the available marginal land suitable for switchgrass cultivation amounts to 22.3 × 10⁶ km² [309].

Estimating SOA production from GLV based on SOA yields from smog-chamber experiments and GLV emission is somewhat approximate. The formation and growth of aerosol in the atmosphere depends on many factors that differ between the chamber and atmospheric conditions like the concentration of reactants, time of reactions, competition by other reactants, and influence of meteorological phenomena.

The global yield of SOA should increase due to the aqueous-phase processing of GLV. Based on the calculated values of the Henry’s constant, Hansel et al. [21] concluded that the intermediate and final products of MeJa and MeSa reactions with OH radicals were better soluble in water than the parent compounds and were plausible candidates for components of aqueous SOA.

Richards-Henderson et al. [22] estimated that for typical atmospheric conditions, the reactions of (Z)-3-hexen-1-ol, (Z)-3-hexenyl acetate, MeSa, MeJa, and MBO with OH radicals in the aqueous phase produced SOA, albeit about 15 times less than the gas phase reactions. The primary reason was low solubility of GLV in water. One can expect this ratio to change if the GLV solubility increases due to factors such as ionic strength or the presence of other organic compounds.

Richards-Henderson et al. [265] estimated the aqueous-phase reactions of singlet oxygen ¹O₂ * with (Z)-3-hexen-1-ol, cis-3-hexenyl acetate, MeSa, MeJa, and MBO were relatively insignificant atmospherically. In contrast, the reactions of ³DMB * with (Z)-3-hexenyl acetate, MeSa, and MeJa produced a more aqueous SOA. They showed that the aqueous-phase reactions of MeJa with OH and ³DMB * are the major conversion paths creating SOA, while other GLV convert to SOA mostly in the gas-phase reactions with OH and O₃.

Hansel et al. [21] found that MeJa and MeSa oxidation products in the aqueous phase were much less volatile than the parent compounds, so they were likely to remain in the condensed phases as SOA components.

8. Conclusions

This review proves the substantial interest in Green Leaf Volatiles chemistry among atmospheric scientists. GLV should remain mainly in the atmospheric gas phase because of limited solubility in water. Their aqueous-phase reactions may be significant in atmospheric systems of high liquid water contents. GLV processing at the interfaces, both air-water, and air-particle, is practically a terra incognita. A few papers indicate it may play a role [156,157,269].

The SOA formation from GLV has been proved and warrants further research. Although the previous estimates of SOA production were overestimated (1–5 TgC yr⁻¹ [65]), the values estimated in this work are still substantial and range from 0.58 to 1.05 Tg yr⁻¹ from (Z)-hexen-1-ol, (Z)-3-hexenal and MBO jointly, 33 Gg yr⁻¹ from switch grass cultivation for biofuels, or 52 Gg yr⁻¹ from grass mowing. All calculations utilize SOA yields determined in smog-chamber experiments, which consequently deserve research attention.

The GLV transformation in the atmosphere may be minor globally, compared to major players like isoprene or monoterpenes (mostly α-pinene). The main reason is far smaller GLV emission fluxes. However, the GLV influence on the air quality may be very significant under local scenarios, including high emissions of GLV, like harvesting, switchgrass cultivation, or lawn mowing. It may increase if fumigation of plants with GLV is widely introduced to agriculture, horticulture, and forestry.

Further research should focus on better estimation of SOA yields in smog-chamber experiments, including the influence of relative humidity and acidity of particles, better introduction of GLV to air quality models, at least for local scenarios, and better estimation of GLV emissions for the latter purpose. Aqueous-phase studies should pay more attention to interfacial processes and product characterization. More experimental data on solubility and air-water partitioning of GLV would certainly be welcomed.

The health effects of GLV and GLV atmospheric transformation products have not been studied and warrant further research via abiotic and biotic experiments. The results may be significant to social groups exposed to high GLV emissions like farmers and residents of houses with grass yards.