Effect of Relative Humidity on the Rate of New Particle Formation for Different VOCs

Abstract

Atmospheric new particle formation (NPF) is an important source of aerosol particles and cloud condensation nuclei, which affect both climate and human health. In pristine environments, oxidation of biogenic volatile organic compounds (VOCs) is a major contributor to NPF. However, the impact of relative humidity (RH) on NPF from these precursors remains poorly understood. Herein, we report on NPF, as inferred from measurements of total particle number density with a particle diameter (d_p) > 7 nm, from three VOCs (sabinene, α-terpineol, and myrtenol) subjected to dark ozonolysis. From a series of comparative experiments under humid (60% RH) and dry (~0% RH) conditions and a variety of VOC mixing ratios (ξ_VOC, parts per billion by volume, ppb_v), we show varied behavior in NPF at elevated RH depending on the VOC and ξ_VOC. In general, RH-dependent enhancement of NPF at an ξ_VOC between <1 ppb_v and 20 ppb_v was observed for select VOCs. Our results suggest that gaseous water at particle genesis enhances NPF by promoting the formation of low-volatility organic compound gas-phase products (LVOCs). This is supported by measurements of the rate of NPF for α-pinene-derived SOA, where RH had a greater influence on the initial rate of NPF than did ξ_VOC and ξ_O3.

1. Introduction

Understanding the chemical and physical properties of atmospheric aerosols is critical to interpreting their direct and indirect impacts on climate change, such as by scattering and absorbing solar radiation and impacting cloud formation [1,2,3,4,5]. Organic aerosol (OA) accounts for up to 90% of Earth’s total aerosol mass budget [6,7,8], with secondary organic aerosol (SOA) making up 70–90% of OA fine particle mass [9,10]. Therefore, it is of interest to study organic new particle formation (NPF) and SOA mass formation (C_SOA), as these processes play a crucial role in the Earth’s atmosphere.

To date, a great deal of effort has been expended to identify sources of organic particles, as well as to better understand the impact of atmospheric parameters, such as temperature, that may influence NPF. In addition to advancing our fundamental knowledge of atmospheric chemical processes, quantitative data on the role of atmospheric parameters on NPF are of interest to improve the predictive accuracy of global climate models [11,12].

Atmospheric NPF is initiated by the formation of molecular clusters, often involving atmospheric ions, sulfuric acid, water, and highly oxidized organic molecules (HOMs) [11,13,14,15,16]; moreover, it has been proposed that the incorporation of HOMs can increase cluster survivability [17,18,19]. Upon cluster formation, a balance exists between losses of these clusters (for example, through processes such as evaporation, condensation, and coagulation) and growth to a critical size that can facilitate atmospheric particle formation via condensation of low-volatility gas-phase products [20]. Significant effort is being expended to understand cluster formation rates and particle growth factors to more accurately quantify and predict atmospheric NPF events [21,22] (and references therein). However, the ensuing discussion adopts a more practical view of NPF and subsequent particle growth as the emergence of new aerosol particles into the lower end of the measured particle size spectrum, followed by the growth of the particles [21,23].

It is generally accepted that the critical cluster size that favors survivability and growth is sub-3 nm [13,21]. Simplistically, once a critical cluster size is attained, the rate at which new particles are formed and grow to measurable sizes (J_ap) depends on the relative rates of production (P) of clusters and condensable species compared to vapor and cluster losses (L (i.e., sinks)) [23]). Hence, without fully understanding the underlying mechanisms of cluster formation and survivability, J_ap can be expressed as a competition between these two factors, as seen in Equation (1):

$${\mathrm{J}}_{\mathrm{a}\mathrm{p}}\propto \mathrm{P}-\mathrm{L}$$

(1)

By extension, any parameter that impacts either the P or the L process (or both) must, by necessity, also impact J_ap and NPF. For example, recent studies have shown a clear negative correlation between temperature and NPF, suggested to be due to the increased rate of production of stable clusters and lower equilibrium vapor pressures of condensable species [24,25,26].

Gaseous water is the third most abundant atmospheric gas; however, the mechanistic role it plays in organic NPF is not well understood. Recent reports on the effect of relative humidity (RH) on NPF have been contradictory, with a few studies reporting a positive correlation between NPF and increased humidity [27,28,29,30,31], while most report a negative correlation [29,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46]. Recent work from our laboratory demonstrated both enhancement and attenuation in NPF for the dark ozonolysis of α-pinene depending on the VOC mixing ratio (ξ_VOC, parts per billion by volume, ppb_v) [29].

Herein, we took an empirical approach toward observing the impact of RH on SOA and NPF behavior from the ozonolysis of a variety of volatile organic compound (VOC) precursors. Specifically, we report on the effect of RH on organic NPF for three VOCs (sabinene, α-terpineol, and myrtenol) that were subjected to dark ozonolysis at various ξ_VOCs and ozone mixing ratios (ξ_O3), and compare against previous studies that used α- and β-pinene [29] and cis-3-hexenyl acetate (CHA) [46]. Sabinene, α-terpineol, and myrtenol were selected for this work, as they are structural derivatives of α-pinene (RH has been shown to affect the NPF of α-pinene-derived SOA [29]), but they have varying solubilities. Of the three VOCs studied, sabinene and α-terpineol displayed enhancements in NPF under humid conditions (60% RH) relative to dry conditions (~0% RH), specifically at ξ_VOCs of less than a few ppb_v. However, myrtenol showed an attenuation in NPF under humid conditions at all ξ_VOCs. These results are in agreement with our recent observations from the dark ozonolysis of α- and β-pinene [29] and cis-3-hexenyl acetate (CHA) [46]. Our results suggest a variable correlation of NPF to increased RH at particle genesis, with a dependence on the VOC undergoing ozonolysis. In addition, the impact of ξ_VOC, ξ_O3, and RH on the J_ap was examined using α-pinene as a model for VOCs that have shown NPF enhancements. NPF rate data, coupled with the observation that the highly water-soluble α-terpineol showed the greatest enhancement in NPF under humid conditions, suggests gaseous water plays a role in NPF and SOA formation through changing chemical mechanisms and/or reaction rates.

2. Materials and Methods

2.1. Reagents

The following reagents were used for this work without further purification: α-pinene (98%, CAS: 7785-70-8, Alfa Aesar, Haverhill, MA, USA), β-pinene (99%, CAS: 19902-08-0, Sigma Aldrich, St. Louis, MO, USA), sabinene (75%, CAS: 3387-41-5, Sigma Aldrich, St. Louis, MO, USA), myrtenol (97.1%, CMX: 34687, Chem-Impex International Inc., Wood Dale, IL, USA), CHA (>97.0%, CAS: 3681-71-8, TCI, Portland, OR, USA), and α-terpineol (97+%, CAS: 98-55-5, Acros Organics, Waltham, MA, USA).

2.2. Instrumentation and Experimental Conditions

Particle number concentration (N, # cm⁻³), SOA mass (C_SOA, µg m⁻³), and particle geometric mean diameter (GMD, nm) were measured or derived continuously using a scanning mobility particle sizer (SMPS 3082, TSI Inc., Shoreview, MN, USA). The SMPS utilized a long DMA and a 3756 UCPC operating at 0.3 and 3 L/min for the aerosol and sheath flow rates, respectively, resulting in a particle mobility diameter range of 17–583 nm. Rates of particle formation (J_ap) were determined from measurements made with an electrical low-pressure impactor (ELPI+, Dekati Technologies Ltd., Kangasala, Finland) consisting of 15 impaction stages ranging from <6 nm to 15 µm. Ozone (O₃) was produced using a commercial generator (Ozone Technologies LLC, Model 1KNT, Jersey City, NJ, USA) by passing dry, particle-free air through a corona discharge; ozone concentration was continuously monitored using a commercial unit (Serinus 10, American Ecotech, Warren, RI, USA). RH and temperature inside the chamber were monitored using a Vaisala HUMICAP^® HMT130 dual sensor (Vaisala Inc., Vantaa, Finland). The resolution of the RH measurement was ±0.1%. All experiments were run in batch mode and performed under ambient temperature (22 ± 1 °C) and pressure (992 ± 5 mbar) in the University of Vermont Environmental Chamber (UVMEC, Figure 1), an 8 m³ Teflon chamber equipped with multiple reagent and sampling ports. The VOC injection port was run through a magnetically coupled fan to facilitate VOC/O₃ mixing. For all experiments (unless otherwise stated), ξ_O3 = 500 (±30) ppb_v and the RH was either dry (~0%) or humid (60%).

2.3. Data Analysis and Terminology

An enhancement factor (ΔN_max, Equation (2)) was used to quantify changes in measured NPF induced by RH:

$$\Delta {\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x}}=\frac{({\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},60\mathrm{R}\mathrm{H}}-{\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},0\mathrm{R}\mathrm{H}})}{{\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},0\mathrm{R}\mathrm{H}}}$$

(2)

here, N_max,60RH and N_max,0RH are the maximum particle number concentrations measured over the course of a humid and dry experiment, respectively. Both N_max,60RH and N_max,0RH were measured under nominally identical ξ_VOC and ξ_O3, as ΔN_max is a comparative measurement. Each ΔN_max was determined from the average N_max,60RH and N_max,0RH of two runs at each ξ_VOC; typical relative standard deviation for each metric was less than 10% [46].

The amplification factor (ΔE), used to describe the sensitivity of NPF to changes in each parameter, was determined using Equation (3):

$$\Delta \mathrm{E}=\frac{({\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},\mathrm{P}}-{\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},0})}{{\mathrm{N}}_{\mathrm{m}\mathrm{a}\mathrm{x},0}}$$

(3)

where N_max,P is the maximum particle number concentration measured at the indicated ξ_VOC, ξ_O3, or RH, and N_max,0 is the maximum particle number concentration measured at the initial point in each respective (ξ_VOC, ξ_O3, or RH) series. For example, if N_max = 1.0 × 10³, 6.0 × 10³, and 1.5 × 10⁴ particles cm⁻³ for a series of three ξ_VOCs (e.g., ξ_VOC = 10, 20, and 30 ppb_v) run at constant ξ_O3 and RH (500 ppb_v and <1%, respectively), then the corresponding amplification factor for each ξ_VOC is ΔE = 0, 5, and 14. Therefore, it is worth noting that ΔE = 0 at the initial point in each series.

2.4. Methodology

In this work, each VOC was subjected independently to dark ozonolysis under dry and humid conditions, from which ΔN_max was determined using Equation (2). For a typical experiment, the UVMEC was sealed while slightly over pressure (+2 mbar) to maintain a consistent initial chamber volume. The chamber fan was then turned on and O₃ was injected for a pre-determined time pulse (typically 100–120 s) to reach ξ_O3 = 500 ± 30 ppb_v. After an equilibration period of 10–15 min, the particle concentration was measured with the SMPS to ensure a clean background spectrum (<10 particles cm⁻³). An optimized method of VOC injection [46] was used to improve the reproducibility of NPF and SOA generation. Briefly, the liquid aliquot of VOC was injected into a heated three-neck bulb-flask under zero-flow conditions. After 45 s, airflow was diverted through the flask to rapidly transfer a bolus of evaporated VOC to the UVMEC via a calibrated split flow valve. To reproducibly attain the low mixing ratios used in this work, the output flow from the flask passed through a split valve that allowed only a small portion (~25%) of the vaporized VOC to enter the UVMEC. The volume of liquid VOC needed to attain the desired ξ_VOC within the UVMEC was calculated prior to VOC injection based on the measured split ratio for each experiment. Regardless of the split ratio, the flow to the UVMEC was kept constant at 2.000 +/− 0.015 L min⁻¹. Aerosol size distributions were monitored continuously for 1 h after injection of the VOC using the SMPS and ELPI+. Although we did not correct for wall losses explicitly, it was assumed that wall losses were nominally constant throughout all experiments.

3. Results and Discussion

3.1. Enhancement Factor of NPF (ΔN_max) for Sabinene, α-Terpineol, and Myrtenol as a Function of ξ_VOC

Of the three VOCs examined, sabinene and α-terpineol displayed enhancements in NPF (i.e., positive ΔN_max) under humid conditions with decreasing ξ_VOC; this is in good agreement with a recent study focused on α- and β-pinene dark ozonolysis [29]. Contrary to this behavior, myrtenol exhibited an attenuation in NPF under humid conditions at all ξ_VOCs studied (Figure 2c), analogous to the behavior of CHA in similar experiments [46]. Figure 2 shows the N_max produced for dry (tan, solid) and humid (green, hashed) experiments for the three VOCs as a function of ξ_VOC. Generally, as ξ_VOC decreased, the maximum absolute number of particles produced (regardless of RH) also decreased. However, for sabinene and α-terpineol, the particles produced under humid conditions comparatively increased relative to dry conditions as ξ_VOC decreased.

Table 1. ΔN_max for each VOC at varying ξ_VOCs. Dashes indicate that ΔN_max could not be calculated at that ξ_VOC because no NPF was observed under dry conditions. ΔN_max experiments were not conducted for sabinene and myrtenol at 20 ppb_v because it had already stabilized at lower ξ_VOC. Error in ξ_VOC estimated by propagating errors in air flow rate to chamber and syringe error. Errors in ΔN_max were estimate from the RSDs interpolated for the respective N_max,0RH based on a previous study assessing the repeatability of SOA experiments in the UVMEC [47].

ξVOC (ppbv)	Sabinene	α-Terpineol	Myrtenol
0.20 (±0.02)	2.88 (±0.25)	--	--
0.35 (±0.03)	0.47 (±0.04)	--	--
0.50 (±0.03)	−0.02 (±0.001)	--	−0.77 (±0.06)
1.00 (±0.03)	−0.02 (±0.001)	--	−0.64 (±0.02)
1.5 (±0.03)	--	9.77 (±0.86)	--
4.0 (±0.1)	−0.64 (±0.02)	2.35 (±0.07)	−0.66 (±0.02)
10.0 (±0.1)	−0.39 (±0.01)	0.04 (±0.001)	−0.85 (±0.03)
20.0 (±0.3)	--	−0.09 (±0.003)	--

summarizes the ΔN_max (determined from Equation (2)) for each VOC with respect to ξ_VOC (shown graphically in Figure 3), along with previously reported enhancement data for α-pinene, β-pinene, and CHA. It should be noted that for cases of ξ_VOC where no NPF was observed (for example, α-terpineol < 1 ppb_v), it was not possible to calculate ΔN_max, even if NPF was observed under humid conditions. Here, it becomes evident that the impact of RH on NPF from the ozonolysis of biogenic VOCs is variable and dependent on the molecular structure of the precursor. Although there was an attenuation in NPF and SOA formation at higher ξ_VOCs for all VOCs, at lower ξ_VOCs, only CHA and myrtenol OA continued to exhibit an attenuation in NPF and SOA mass in the presence of elevated RH at particle genesis. This latter observation is in accordance with other literature reports when working at higher ξ_VOCs (≥50 ppbv) [29,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46]. However, we demonstrate that even for those VOCs that suffer an attenuation in NPF and SOA mass formation under humid conditions at high ξ_VOCs, at lower, atmospherically relevant ξ_VOCs, there is an inversion in behavior, and an enhancement in NPF and SOA is measured in the presence of water at particle genesis. Across all VOCs studied here, the magnitude and sign of the RH-induced effect was dependent upon the specific VOC undergoing ozonolysis.

3.2. Addressing Solubility of VOCs

It is possible that the shutdown of NPF for some VOCs is due to their solubility. As discussed in a previous report [46], Teflon walls can develop a significant water film under humid conditions. This film could enhance vapor phase losses to the chamber walls of water-soluble VOCs, thereby causing a decrease in NPF. However, in accordance with the results from this previous work, the current results support the notion that the aqueous solubility of the individual VOCs does not influence the attenuation of SOA formation. As seen in Figure 4a, α-pinene, β-pinene, and sabinene, which are nearly insoluble in water (

Table 2. The VOCs addressed and their corresponding solubility. * Solubilities as reported in https://pubchem.ncbi.nlm.nih.gov/ (accessed on 23 February 2024).

VOC	Solubility in H2O (mg/L) *
Sabinene	2.5
α-Terpineol	7100
Myrtenol	427
α-Pinene	Insoluble
β-Pinene	Insoluble
CHA	900

), produced an enhancement in NPF under humid conditions at low ξ_VOCs. CHA and myrtenol are both soluble in water (Table 2), and the ozonolysis of these VOCs at high RH resulted in an attenuation of NPF at all ξ_VOCs. However, α-terpineol, which is more soluble than CHA and myrtenol (by an order of magnitude), showed the greatest enhancement in NPF under humid conditions of all the VOCs studied. This would indicate that the attenuation of NPF and SOA formation is not due to loss of product to the chamber walls under humid conditions (i.e., a solubility issue) but rather that the mechanism for which gaseous water affects the formation of SOA is unique to individual VOCs.

3.3. Effect of RH on the Rate of NPF for α-Pinene-Derived SOA

3.3.1. Effect of ξ_VOC, ξ_O3, and RH on ΔN_max

Three of the controllable variables that affect the formation of SOA at particle genesis in our batch-mode atmospheric chamber experiments were ξ_VOC, ξ_O3, and RH. It was determined that all three variables had an influence on ΔN_max (Figure 5). Under constant ξ_O3 (500 ppb_v), there existed a non-linear increase in ΔN_max as ξ_VOC decreased (Figure 5a). By increasing ξ_O3 while keeping a fixed ξ_VOC (10 ppb_v), ΔN_max generally increased (Figure 5b), as one would expect. Similarly, Figure 5c shows an increase in ΔN_max with increasing “humid” RH (ξ_VOC and ξ_O3 fixed at 10 and 500 ppb_v, respectively) compared to ~0% RH. Although the greatest enhancement of ΔN_max was influenced by a decrease in ξ_VOC, it is evident that all three chamber variables had an influence on ΔN_max at low ξ_VOCs. It is important to note that the relative importance of each parameter may depend on the absolute value of the parameter that remains fixed. For example, the ΔN_max corresponding to ξ_VOC may have differed if the fixed ξ_O3 = 10 ppb_v rather than 500 ppb_v. For the present study, each fixed parameter was chosen to permit comparison with other work in our laboratory [29,46,47] and in the literature.

3.3.2. Effect of ξ_VOC, ξ_O3, and RH on ΔE

The sensitivity of NPF from the dark ozonolysis of α-pinene to each chamber variable was quantified by way of the amplification factor (ΔE, comparative measure of the maximum particle number concentration produced at the given ξ_VOC/ξ_O3/RH relative to the initial point in each respective series). ΔE was derived from Equation (3) and assessed for each chamber variable (Figure 6). The y-axis is scaled to provide a direct, visual comparison of the enhancement between the three variables. Figure 6a shows that an increase in ξ_VOC with static ξ_O3 and RH (500 ppb_v and ~0%) led to an increase in ΔE. Similarly, Figure 6c shows that an increase in RH with static ξ_VOC and ξ_O3 (10 ppb_v and 500 ppb_v) also yielded a general increase in ΔE. However, somewhat surprisingly, this is not the case for an increase in ξ_O3 (Figure 6b), which yielded little to no change in ΔE (static ξ_VOC of 10 ppb_v and RH of ~0%), suggesting that regardless of the absolute value of ξ_O3, the system’s sensitivity to changes in the parameter remains constant.

3.3.3. Effect of ξ_VOC, ξ_O3, and RH on J_ap

To shed light on the effect of RH on NPF for the VOCs that exhibited an enhancement under humid conditions, a systematic study was conducted by which the effect of varying chamber conditions on the apparent rate of particle formation (J_ap) was examined. For this work, α-pinene-derived SOA was used as an example for the VOCs that exhibited an enhancement in NPF.

The general trends in normalized J_ap (Figure 7) match comparatively with the general trends for ΔE in each series (Figure 6), where increases in ξ_VOC and RH enhanced the apparent rate of particle formation but ξ_O3 did not. Although the absolute increase in J_ap with respect to ξ_VOC was greater than that with respect to RH (Figure 7a compared to Figure 7c), when plotting J_ap vs. ΔE (Figure 8), the slope of the log–log plot for RH-dependent J_ap was approximately 16 times greater than the ξ_VOC-dependent J_ap. This indicates that at low ξ_VOC of α-pinene, at particle genesis, changes in RH have a much greater impact on the apparent rate at which the OA particles form compared to the ξ_VOC and ξ_O3. This would indicate that RH-dependent rates of NPF are likely a major driving force of the eight-fold enhancement in NPF for α-pinene-derived SOA (seen here: Snyder et al. [29]).

In a study conducted by Chu et al. [48], the authors discuss the increased deviation in experimental aerosol formation from their predicted models corresponding to increased RH. The possible explanation posed is that the gaseous water would be taken up by particles, leading to greater surface area and subsequently growing the condensation sink for gas-phase products to partition onto. Although plausible at higher ξ_VOCs, where we also see an attenuation in NPF with elevated RH for all VOCs, that explanation does not fit our observations at low ξ_VOCs. At low ξ_VOCs, for certain VOCs, it appears as though the presence of gaseous water (in excess compared to the concentration of the VOC) led to a promotion of OH reactivity prior to ozonolysis of the VOC [49,50], which seemingly promotes low-volatility organic compound (LVOC) gas-phase product formation and subsequent nucleating species that yield NPF events at a greater rate than when ξ_VOCs are higher. This is supported by the observation (Figure 8) that ΔE showed a 16× greater dependence on RH than ξ_VOC.

Regarding pristine environments, pure organic NPF has also been observed [51,52,53,54,55,56], and the theoretical underpinning of organic cluster formation and nucleation has recently been reviewed [19]. Here, it is proposed that pure organic clusters are formed and perhaps stabilized by water molecules or dimers in direct relation to the vapor pressure of the oxidized products and their gas-phase concentrations. If the instantaneous gas-phase concentration of an oxidation product is greater than its equilibrium vapor pressure (i.e., the system is supersaturated in that product), then the probability of cluster formation and NPF increases (see, for example, [48,55,57,58,59]). This supposition is supported by recent laboratory studies that showed that decreases in system temperature (i.e., decreases in equilibrium vapor pressure) resulted in significant enhancements in NPF [24,25,26]. This enhancement was observed despite decreases in the formation of HOMs at lower temperatures [60].

Elevated RH is likely to increase L (Equation (1)) through processes such as enhanced coagulation scavenging of newly formed clusters and condensable gases [61], as well as increased coagulation and scavenging rates of ultrafine particles [42,62]. For example, water vapor uptake to particles could reduce organic (equilibrium) partial pressures according to Raoult’s law. Particle deliquescence would significantly dilute the organics in the particles, resulting in a reduction in organic partial pressures according to Raoult’s law [63,64,65,66]. In addition, an aqueous phase could attract water-soluble organics according to Henry’s law. Lastly, humidity on chamber walls may have an outsized impact on losses of ultrafine particles, which generally suffer greater wall loss rates than larger particles [67]. Therefore, as wall losses could increase at higher RH, the results reported herein could be interpreted as lower limits of the enhancements in NPF as a function of RH.

By all accounts, increases in particle liquid water (PLW) enhance the partitioning of semi- and low-volatility gas-phase products (S- and LVOCs) to the organic particulate (due to a lowering of the average molecular weight of particle constituents) [68,69,70,71,72]. Furthermore, PLW has been shown to increase particle diffusivity, enhancing condensation rates to organic particles [65,71,73,74,75]. Again, both of these effects would result in an increased sink (i.e., L, Equation (1)) at higher RH. The main L for newly formed particles in the atmosphere is coagulation onto an existing particle population [76], whereas in chamber experiments, the main L stems from particle losses to chamber walls, which are normally much more important than coagulation [77] because the existing particle population is usually absent or small. It cannot be discounted that elevated RH could lead to decreased vapor-phase wall losses, thereby increasing the concentration of LVOCs in the gas phase (C_LVOC considered analogous to the yield of LVOCs, φ_LVOC, used by Chu et al. [71]), which could favor NPF. There is only one case, however, of which the authors are aware where increased relative humidity resulted in very modest mitigation of gas-phase wall losses [61]. This singular case studied the loss of oxygenated vapors resulting from the reaction of OH with toluene, a very different chemical system from the current study.

Therefore, RH is likely playing a role by increasing P (Equation (1)) by any number of processes, such as modified oxidized product distributions, enhanced formation of nucleating species, enhanced rate of cluster formation, and/or enhanced survivability (i.e., stabilization) of clusters [31]. Regardless of the precise mechanism(s) by which NPF is impacted by the presence of water, therefore, it appears likely that an increase in water concentration must, by a gas-phase reaction, produce more LVOCs or the rates of production of LVOCs must be greater, effectively outcompeting vapor sinks such as condensation and partitioning in the presence of seed particles or, perhaps, coagulation and wall loss effects (both gas and particle) common in chamber experiments (see Charan et al. [78] for an excellent review on the topic).

4. Conclusions

Overall, the correlation between RH and organic NPF is variable, with a significant dependence on the specific VOC undergoing ozonolysis. In this work, three VOCs (sabinene, α-terpineol, and myrtenol) were subjected to dark ozonolysis under dry and humid conditions (~0 and 60% RH, respectively) at a variety of ξ_VOCs. When incorporating data from our previous work [29,46], four VOCs (α-pinene, β-pinene, sabinene, and α-terpineol) displayed enhancements in NPF under humid conditions as the ξ_VOC decreased; however, myrtenol and CHA showed an attenuation in NPF under humid conditions for all ξ_VOCs. For the VOCs that had NPF enhancements, gaseous water likely played a crucial role in the chemical formation of low-volatility gas-phase products at low ξ_VOCs. Therefore, these low-volatility products would have spawned new and more frequent NPF events compared to those at high ξ_VOCs and/or dry conditions. In addition, we note that the observed behavior is not solely due to a VOC’s solubility; this is supported by our observation where α-terpineol produced the greatest ΔN_max despite its solubility being an order of magnitude greater than that of CHA and myrtenol.

Furthermore, as a proof-of-concept study, α-pinene was used as a model to systematically probe the effect of each chamber variable (ξ_VOC, ξ_O3, and RH) on the observed enhancement in NPF. Here, it was determined that the ΔN_max and ΔE were dependent on all three chamber variables (ξ_VOC, ξ_O3, and RH), while J_ap was independent of ξ_O3. However, of the two dependences (ξ_VOC and RH), when compared to their respective amplification factors, the sensitivity of NPF was influenced more by RH than by ξ_VOC (approximately 16×). Therefore, for the VOCs that exhibit NPF enhancements at low ξ_VOCs, the presence of gaseous water at particle genesis during ozonolysis is shown to be the driving force behind the production of enhanced NPF events.

The role of humidity in atmospheric organic NPF is complex and dependent on the specific SOA precursor under consideration. Furthermore, while the current study focused primarily on two RH levels (~0% reflective of most laboratory studies of SOA and 60% reflective of the average global RH), the preliminary results from α-pinene clearly point to the need to study systematically these effects at intermediate RHs reflective of different environments. It may be possible that upon more extensive experimentation with a broader range of VOCs and across a secondary dimension of RH for each VOC, general parameters may be derived to facilitate incorporation of these effects into climate models to improve outcomes.