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Article

Molecular Characterization of Organic Aerosol in Summer Suburban Shanghai Under High Humidity

by
Xiancheng Tang
1,†,
Junfang Mao
1,†,
Dongmei Cai
1,
Zhiwei Zhang
1,
Haixin Nong
1,
Ling Li
1,2,* and
Jianmin Chen
1,2,*
1
Department of Environmental Science & Engineering, Fudan University, Shanghai 200433, China
2
Institute of Eco-Chongming (IEC), 20 Cuiniao Rd., Chongming, Shanghai 202162, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Atmosphere 2025, 16(6), 659; https://doi.org/10.3390/atmos16060659
Submission received: 24 April 2025 / Revised: 22 May 2025 / Accepted: 26 May 2025 / Published: 30 May 2025
(This article belongs to the Section Aerosols)
Browse Figures
Versions Notes

Abstract

:
In this study, the chemical compositions of PM2.5 (particulate matter < 2.5 μm) and the molecular compositions of methanol-soluble organic carbon (MSOC) in suburban Shanghai during summer were measured to investigate the molecular characteristics of organic aerosol (OA) under high humidity. Diurnal variation analysis reveals the influence of relative humidity (RH) on secondary organic aerosol (SOA) components. Organosulfates (OSs), particularly nitrooxy-OSs, exhibit a positive correlation with increasing humidity rather than atmospheric oxidants in this high-humidity site. This suggests that high RH can promote the formation of OSs, possibly through enhancing particle surface area and volume, and creating a favorable environment for aqueous-phase or heterogeneous reactions in the particle phase. A considerable proportion of CHOS compounds may be derived from anthropogenic aliphatic hydrocarbon derivatives. These compounds exhibit slightly elevated daytime concentrations due to increased emissions of long-chain aliphatics from sources such as diesel combustion, as well as photochemically enhanced oxidation to OSs. In contrast, CHONS compounds increased at night, driven by high-humidity liquid-phase oxidation. Terpenoid derivatives accounted for 13.4% of MSOC and contributed over 40% to nighttime CHONS. These findings highlight humidity’s important role in driving daytime and nighttime processing of anthropogenic and biogenic precursors to form SOA, even under low SO2 and NOx conditions.

1. Introduction

Organic aerosols (OAs), constituting 20–90% of ambient PM2.5 (particulate matter < 2.5 μm) [1,2], play crucial roles in global climate regulation and carbon cycling [3]. Based on their formation pathways, OAs are broadly categorized into primary organic aerosols (POA) emitted directly from biogenic and anthropogenic sources, and secondary organic aerosols (SOA) formed through atmospheric oxidation of volatile organic compounds (VOCs) in the presence of oxidants (e.g., OH/NO3 radicals, O3) [4]. The complex chemical composition of OAs encompasses diverse organic species including hydrocarbons, alcohols, aldehydes, carboxylic acids, organosulfates, organonitrates, and organic amines [5]. Among these, humic-like substances, imidazoles, nitroaromatic compounds (NACs), and high-molecular-weight polycyclic aromatic hydrocarbon derivatives have been identified as key brown carbon (BrC) chromophores influencing global radiative balance [6,7]. However, current analytical techniques have only characterized 10–30% of OAs due to their heterogeneous sources and intricate atmospheric processing [6], highlighting the critical need for a molecular-level characterization of OA composition and transformation mechanisms.
SOAs, formed predominantly through VOC oxidation, represent a major component of atmospheric particulate matter [8,9]. Organosulfates (OSs), the most abundant organic sulfur compounds in aerosols, significantly contribute to organic sulfur mass and participate in key SOA formation pathways [10,11]. Recent studies have identified nitrooxy-organosulfates (nitrooxy-OSs, containing both -ONO2 and -OSO3 functional groups) as crucial contributors to SOA formation through multiphase reactions [12,13]. These species primarily form via heterogeneous interactions between acidic sulfate particles and organic precursors from biogenic/anthropogenic sources [13,14,15,16]. Studies have demonstrated that elevated relative humidity (RH) significantly enhanced particulate OS formation due to the increasing of acidic sulfate-catalyzed aqueous-phase formation reactions [17].
Emerging evidence suggests distinct formation pathways for different OSs. Long-chain aliphatic OSs, characterized by low oxidation states and unsaturation degrees, likely originate from vehicle emissions or combustion-derived long-chain alkanes, which constitute up to 90% of anthropogenic emissions in urban areas [18]. Laboratory studies by Riva et al. [19] demonstrated the photochemical linkage between n-alkane oxidation and aliphatic OS formation. In contrast, biogenic VOC (BVOC)-derived OSs exhibit short carbon chains, high oxygenation levels, and double-bond equivalent (DBE) values comparable to their precursor terpenoids (isoprene, monoterpenes, and sesquiterpenes) [20]. These compounds have been demonstrated to be able to form through nitrate-radical mediated nighttime oxidation pathways [14,21,22], some exhibiting distinct peaking at night [23]. Furthermore, aromatic-like OSs with low H/C ratios, particularly polyaromatic variants, are predominantly associated with incomplete combustion processes (e.g., fireworks, coal combustion, industrial smelting) [24]. Current estimates suggest that alkane- and aromatic-derived OSs constitute major unrecognized OSs precursors [20,25].
Field observations remain essential for validating laboratory-derived SOA formation mechanisms. While numerous studies have emphasized the critical role of atmospheric oxidants and sulfate availability in OS formation [13,26], significant knowledge gaps persist regarding the relative contributions of biogenic versus anthropogenic sources to OSs. Particularly scarce are systematic investigations of the combined effects of summer-specific meteorological factors (high humidity, intense solar radiation) on SOA formation mechanisms and their diurnal variations. As a megacity in China’s Yangtze River Delta (YRD) region, Shanghai presents a unique atmospheric environment where localized high-intensity VOC emissions synergize with favorable meteorological conditions to drive substantial SOA production. In this study, we conducted a 12-day field campaign during high-humidity summer 2023 at Dianshan Lake (Qingpu District, Shanghai, China), collecting daytime and nighttime PM2.5 samples for comprehensive molecular characterization using ultra-high-performance liquid chromatography (UHPLC) coupled with Orbitrap mass spectrometry (MS) (UHPLC–Orbitrap MS). Therefore, the purpose of this study is to elucidate molecular characteristics of SOA under high-humidity conditions, and to distinguish biogenic versus anthropogenic influences on aerosol composition in a typical megacity environment.

2. Data Sources and Methods

2.1. Sample Collection

The sampling site for this study was located on the rooftop of the Scientific Observation and Research Station at the shore of Dianshan Lake in Zhujiajiao Town, Qingpu District (31.5° N, 120.5° E), at an elevation of approximately 10 m. The surrounding area lacks major industrial pollution sources; the Huqingping Highway is situated less than one kilometer from the site, and the Huyu Expressway is approximately two kilometers away. The observation period spanned from 31 July to 11 August 2023.
PM2.5 particles were collected onto pretreated quartz fiber filters (20 × 25 cm2) using a high-volume air sampler (XT-1025, Shanghai Xintuo, Shanghai, China) equipped with an inertial impactor-type cutting operated at a constant flow rate of 1.13 m3/min. Prior to sampling, the filters were pre-combusted at 500 °C for 6 h. After sampling, the filters were stored in a freezer at −20 °C. To investigate the diurnal variation in PM2.5, daytime sampling was conducted from 08:00 to 19:30, and nighttime sampling from 20:00 to 07:30 the following day, with each cycle lasting 11.5 h. Field blank samples were also collected for quality control, which were collected using identical sampling procedures without opening the sampler, and subsequently stored and analyzed using the same procedures as other samples.

2.2. Sample Analysis

A multi-wavelength thermal/optical carbon analyzer (DRI Model 2015, Magee Scientific, Berkeley, CA, USA) was used to quantify the organic carbon (OC) and elemental carbon (EC) contents in PM2.5 samples. Water-soluble inorganic ions in the PM2.5 samples were analyzed quantitatively using ion chromatography (940 Professional IC Vario, Metrohm, Herisau, Switzerland) following an extraction procedure involving filter excision (47 mm diameter) with a sampling cutter, fragmentation, ultrasonication with 30 mL deionized water for 30 min, and filtration through a 0.22 μm aqueous syringe filter.
The overall molecular composition of the collected PM2.5 was characterized using UHPLC (Dionex UltiMate 3000, Thermo Scientific, Bremen, Germany) coupled with a diode array detector (DAD) and a high-resolution Orbitrap MS (Thermo Scientific, Bremen, Germany) equipped with an electrospray ionization (ESI) source. A 12 cm2 portion was obtained from each PM2.5 quartz fiber filter sample and subjected to ultrasonic extraction in 3 mL of methanol, performed three times for 30 min each. The combined extracts were filtered through a 0.45 μm polytetrafluoroethylene (PTFE) syringe filter. The filtrate was evaporated under a gentle stream of nitrogen and reconstituted in 60 μL of methanol containing 1 ppm of deuterated decyl sulfate, which serves as an internal standard in mass spectrometry.

2.3. Data Processing

The obtained mass spectrometry data were analyzed using the MZmine-2.30 software to obtain the m/z ratios, retention times, and peak areas of organic compounds. The molecular formulas identified are expressed as CcHhOoNnSs, where c is the number of carbon atoms in the range of 1–50, h is the number of hydrogen atoms in the range of 2–100, o is the number of oxygen atoms in the range of 0–40, n is the number of nitrogen atoms in the range of 0–5, and s is the number of sulfur atoms in the range of 0–2 [27]. To eliminate the chemically meaningless molecular formulas, formulas were further constrained by setting H/C, O/C, N/C, and S/C in the ranges of 0.3–3, 0–3, 0–0.5, and 0–0.2, respectively [28,29].
The double-bond equivalent (DBE) provides a preliminary estimation of the number of unsaturated structural features in a compound, such as double bonds, triple bonds, and rings. It is calculated using the following equation:
DBE = c − h/2 + n/2 + 1
The aromaticity equivalent (Xc) is an effective metric for identifying aromatic compounds containing long alkyl chains as well as polycyclic aromatic hydrocarbons [30]. In this study, Xc was calculated using the following formula:
Xc = [3 × (DBE − (p × o + q × s)) − 2]/(DEB − (p × o + q × s))
p and q are the proportional coefficients for oxygen and sulfur atoms, respectively, and were both set to 0.5 for compounds detected in ESI, in this study [31]. For compounds containing an odd number of oxygen and sulfur atoms, the value of (p × o + q × s) was rounded to the nearest integer.

3. Results and Discussion

3.1. General Molecular Characterization and BrC Absorption

Figure 1 illustrates the temporal variations in chemical composition, RH, temperature, PM2.5, and gaseous pollutants mass concentrations observed at Dianshan Lake, a suburban site in Shanghai. During the summer observation, the mean daytime temperature and RH measured 31.7 °C and 68.9%, respectively, while nighttime values registered 27.9 °C and 85.6%, exhibiting typical high-humidity characteristics, especially at nighttime. Throughout most of the period, PM2.5 levels remained below China’s National Ambient Air Quality Standard (Grade I, ≤35 μg·m−3), with OC dominating the chemical composition. Intermittent mild pollution episodes occurred, characterized by significantly elevated OC concentrations. The average OC contributions to PM2.5 mass reached 41.5% and 27.4% during daytime and nighttime, respectively. The average concentrations of sulfate at daytime and nighttime were 61.6 μg m−3 and 48.3 μg m−3, respectively. Nitrate concentrations at nighttime (38.5 μg m−3) were significantly higher than those at daytime (27.8 μg m−3). As shown in Figure 1c, the average concentration of SO2 during the observation period was 2.7 μg m−3, occasionally exceeding 4 μg m−3. The average NO2 concentration was 14.3 μg m−3, with nighttime concentrations (20.8 μg m−3) 3–5 times higher than daytime levels (7.6 μg m−3), which may be associated with the lower nocturnal boundary layer. Notably, O3 concentrations were relatively high (average 77.2 μg m−3) and showed a regular fluctuation pattern, increasing at daytime (average 106.2 μg m−3) and decreasing at nighttime (average 48.2 μg m−3). The observed higher daytime PM2.5 concentrations compared to nighttime may be attributed to combined effects of increased fresh OAs emissions and photochemical processing during daytime.
By using UHPLC–Orbitrap MS in negative mode (ESI-), 2160–6312 organic compounds were detected in samples, which can be categorized into two elemental groups: oxygen-free compounds (CHX, encompassing CH, CHN, CHS, and CHNS) and oxygen-containing compounds (CHO, CHON, CHOS, and CHONS). As depicted in Figure 2a,b, oxygen-containing compounds dominated the molecular composition, with CHO species constituting the largest fraction (54.8% daytime, 49.8% nighttime), followed by CHOS (19.6% daytime, 14.6% nighttime), CHON (16.1% daytime, 17.3% nighttime), CHONS (8.3% daytime, 16.8% nighttime), and CHX (1.4% daytime, 1.5% nighttime). Daytime organic compound abundance was 1.58× higher than nighttime, reflecting diurnally divergent atmospheric processing.
Figure 2c,d illustrate the relationships between relative abundances of CHOS and CHONS in PM2.5 with RH and oxidizing gases (O3 + NO2). These OSs, particularly nitrooxy-OSs, exhibit a positive correlation with increasing humidity, while showing weak or even negative correlations with atmospheric oxidizing capacity, which is reflected by the concentration of oxidizing gases in this study. This suggests that high humidity at the observation site can promote the formation of OSs. Wang et al. [17] also observed this phenomenon in comparative studies of Sichuan and Beijing, finding that high RH conditions in Sichuan facilitated OSs formation, with atmospheric oxidants trending inversely with RH. It was speculated that higher RH increases particle surface area and volume, liquefies particles, and provides a favorable medium for aqueous-phase or heterogeneous reactions in the particle phase, thereby promoting the generation of organic sulfates.
Coupling with DAD enabled precise identification of key BrC chromophores (Figure S1). At 365 nm, CHON compounds, particularly nitroaromatics (e.g., C6H5NO3, C6H5NO4, C7H7NO3), were the primary contributors to BrC absorption (12.2–16% of total absorption), followed by CHONS (e.g., C10H17O7NS, 5.8–10.8%). Nitrophenols exhibit stronger light absorption in daytime samples than in nighttime samples, likely due to their enhanced photochemical formation during the day [32]. CHONS made greater contributions during pollution episodes, with minimal diurnal variation. C10H17O7NS, which accounts for a large proportion of light absorption in the samples, may be derived from monoterpenes. It has been identified in aerosol observations in multiple regions, and its formation is likely dominated by heterogeneous oxidation under high NOx environments [33].

3.2. Diurnal Variations Characterization of CHO, CHON, CHOS, and CHONS Compounds

CHO compounds, abundant in methanol-soluble organic carbons (MSOCs) of aerosols, accounted for 37–60% of total detected species, with higher daytime abundances (54.8% vs. 49.8% at night). Compared to winter studies [31], summer CHO compounds featured a significant increase in aliphatic species (Xc < 2.500, 72.3% of CHO), primarily derived from biogenic precursors, vehicle exhaust, and aromatic precursor oxidation [34]. Monocyclic (2.500 ≤ Xc < 2.714, 18.4%) and naphthalene-series (2.714 ≤ Xc < 2.800, 8%) aromatic compounds suggested industrial emissions and fossil fuel combustion contributions [35] (Figure 3).
CHON species (12–29% of total abundance, 16.1% day vs. 17.3% night) displayed complex molecular formulas on the Van Krevelen diagram, with 74% having O/N ≥ 3, indicative of nitrate ester (–ONO2) group [34,36]. Compared to winter studies [31], summer abundances were lower, likely due to thermal instability and photolytic decomposition under high solar radiation. For example, nitrated aromatic compounds (NACs) can degrade into HONO, OH, and NO2. Detected abundant nitrophenols and nitrocatechols (e.g., C6H5NO3, C7H7NO3, C6H5NO4), associated with biomass burning and BrC, coexisted with multi-oxygenated CHON formulas possibly containing acidic (–COOH) and basic (–NH2) groups, resembling amino acid derivatives [37,38]. Highly unsaturated compounds (low H/C, O/C) likely originated from combustion-derived nitrogen-containing polycyclic aromatic compounds.
Atmosphere 16 00659 g003
Figure 3. (a–d) Van Krevelen (VK) diagrams [39] for CHO, CHON, CHOS, and CHONS compounds. (e,f) DBE versus number of carbon atoms for CHOS and CHONS compounds. The sizes of the symbols are proportional to the cube root of the abundance of a compound.
Figure 3. (a–d) Van Krevelen (VK) diagrams [39] for CHO, CHON, CHOS, and CHONS compounds. (e,f) DBE versus number of carbon atoms for CHOS and CHONS compounds. The sizes of the symbols are proportional to the cube root of the abundance of a compound.
Atmosphere 16 00659 g003
CHOS and CHONS compounds also accounted for a large proportion (13–53% of total abundance, 27.8% day vs. 31.4% night), commonly recognized as OSs and nitrooxy-OSs, respectively [40]. CHOS with C > 8, 3 < O < 7, and low DBE (0/1) included abundant aliphatic OSs (e.g., C12–16H2m+2O4S and C10–16H2mO5S, the subscript “m” denotes the number of carbon atoms), with higher daytime abundances (25.5% vs. 22.9% at night) of total CHOS (Figure 3). Their large carbon numbers, low unsaturation, and low oxidation degree suggest precursors are likely long-chain alkanes [41]. Studies show incomplete diesel combustion generates nanoparticles rich in long-chain alkanes and cycloalkanes [42]. The sampling site, located 900 m from a traffic corridor with frequent diesel truck traffic during the study, implies diesel-derived long-chain alkanes likely serve as key precursors for atmospheric long-chain aliphatic OSs. During the daytime, increased traffic activity may lead to elevated pollutant emissions. Yang et al. has demonstrated that daytime photoxidation reactions synergizing UV, O3, and SO42− can promote the formation of aliphatic organosulfates from long-chain alkanes [12]. This may help explain the higher daytime abundances of these compounds. In addition, monocyclic and polycyclic aromatic CHOS species accounted for only a small fraction, suggesting limited influence from incomplete combustion sources, such as residential solid fuel burning [42].
In CHONS, 54.8% of molecular formulas meet the oxygen stoichiometric criterion (O ≥ 4S + 3N), suggesting the presence of -OSO3 and -ONO2 functional groups, characteristic of nitrooxy-OSs. As shown in Figure 3, these compounds exhibit significantly higher nighttime abundances (14.2% vs. 6.2% at daytime), with the most abundant substance being C10H17O7NS (nighttime abundance was 4.83 times that of daytime). This substance is identified as a typical oxidation product of monoterpenes (e.g., α-pinene, β-pinene, α-terpinene, terpinene) under high NOx conditions [42,43]. Additionally, C15H25O7NS, with high abundance, likely originates from sesquiterpene oxidation [43]. In this study, the average concentrations of NOx (16.7 μ g m−3) and SO2 (2.7 μ g m−3) is not significantly high, indicating that the increase in humidity at nighttime has a promoting effect on the formation of these derivatives, which is consistent with our description in Section 3.1.

3.3. Contribution of Terpene Derivatives to SOA

Figure 4 illustrates the diurnal abundances and proportions of terpenoid (isoprene, monoterpene, and sesquiterpene) derivatives in CHO, CHON, CHOS, and CHONS compounds. Among these compound classes, CHONS exhibits the highest abundance proportion of terpenoid derivatives, followed by CHOS, whereas CHON shows the lowest proportion of such derivatives.
Isoprene derivatives account for about 5% of CHOS compounds during both day and night, and 4.9% (day) and 2.5% (night) in CHONS compounds. Compared with prior quantitative studies, where Wang et al. [44] observed 36.3% of quantified OSs in Shanghai, Hettiyadura et al. [26] found 12.6% of PM2.5 OC in Atlanta; the results of the abundance calculations in this study show lower proportions, which may be attributed to the low response of the instrument equipped with an ESI source to isoprene derivatives in negative mode.
Monoterpene/sesquiterpene derivatives account for 14.4% (day) and 12.3% (night) in CHOS compounds, and 14.7% (day) and 38% (night) in CHONS compounds. Their higher daytime abundance in CHOS is due to daytime photochemistry-promoting formation, whereas the nocturnal increase in CHONS aligns with the literature-reported nocturnal accumulation pattern of nitrooxy-OSs derived from monoterpenes, driven by enhanced liquid-phase oxidation under high-humidity conditions [12,17,44]. Compared with the findings of Yang et al. [12], who reported monoterpene-derived organosulfates accounting for 19% of quantified OSs in Shanghai, and Anusha et al. [16], who observed these compounds as comprising only 0.5% of OC in Atlanta, this study finds that monoterpene-derived compounds account for 34.4% of nighttime CHONS species, indicating a potentially significant regional contribution, especially at nighttime.
At this observation site, terpenoid derivatives constituted 12.5% (day) and 14.7% (night) abundance of total MSOC, markedly higher than the 2–5% reported in four Chinese cities (Beijing, Shanghai, Guangzhou, Hong Kong) by Wang et al. [45]. This discrepancy is likely attributed to the suburban Shanghai location, characterized by abundant vegetation and high humidity at nighttime during the observation period. Strong sunlight and high temperature at daytime promoted high plant emissions, while high humidity conditions at nighttime promoted liquid-phase reaction of terpenes. These factors promote the formation of terpene derivatives, resulting in a high proportion of terpene derivatives in the total OA.

4. Conclusions

This study characterized the chemical composition of PM2.5 and MSOC in suburban Shanghai during summer, focusing on the molecular features of OA under high-humidity conditions. Diurnal variation analysis indicates that RH plays an important role in SOA formation, revealing that OSs, especially nitrooxy-OSs, correlated strongly with increasing humidity, surpassing the influence of atmospheric oxidants. This suggests that high RH promotes OS formation by enhancing particle surface area and volume, and facilitating aqueous-phase or heterogeneous reactions within the particle phase.
Compositional analysis further revealed distinct sources and diurnal characteristics for CHOS and CHONS compounds. A considerable proportion of CHOS likely derived from anthropogenic aliphatic hydrocarbon derivatives (e.g., diesel combustion emissions), exhibited modest daytime concentration increases, driven by increased emissions of long-chain aliphatic compounds coupled with their photochemical oxidation into OS precursors, which subsequently form through heterogeneous uptake onto acidic aerosol particles. In contrast, CHONS compounds show nighttime increasing, attributed to liquid-phase oxidation under high humidity. Their formation was strongly linked to biogenic terpenoid derivatives—daytime high temperatures boosted plant terpene emissions, while nighttime high humidity promoted terpene-driven liquid-phase reactions, both amplifying terpene-derived OA.
These findings underscore the critical role of humidity in SOA generation, even under low SO2 and NOx conditions, demonstrating how high humidity drives divergent daytime and nighttime chemistry for anthropogenic and biogenic precursors. The study highlights humidity as a key factor influencing aerosol composition by affecting both reaction mechanisms and precursor contributions, with implications for improving the understanding of SOA formation in humid environment.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/atmos16060659/s1, Figure S1 The formulas of brown carbon (BrC) and their light absorption percentages at 365 nm (left: daytime, right: nighttime); Figure S2 DBE versus number of carbon atoms for CHO and CHON compounds; Table S1. Detected terpenoid-derived using UPLC-Orbitrap MS.

Author Contributions

X.T.: writing—original draft, investigation, methodology, data curation, formal analysis. J.M.: data curation, writing—original draft preparation. D.C.: data curation, methodology. Z.Z.: formal analysis. H.N.: formal analysis. L.L.: conceptualization, data curation, writing-reviewing, and editing. J.C.: supervision, resources. X.T. and J.M. contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Key Research and Development Program of China (2022YFC3701100).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Atmosphere 16 00659 g001
Figure 1. Time series of (a) chemical composition (left: daytime, right: nighttime) and PM2.5 concentrations, (b) RH, temperature, and (c) gaseous pollutants concentrations at Dianshan Lake. The pie charts in (a) show the average chemical composition at daytime and nighttime.
Figure 1. Time series of (a) chemical composition (left: daytime, right: nighttime) and PM2.5 concentrations, (b) RH, temperature, and (c) gaseous pollutants concentrations at Dianshan Lake. The pie charts in (a) show the average chemical composition at daytime and nighttime.
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Figure 2. (a) Relative abundance of CHO, CHON, CHOS, CHONS, and CHX and their proportions in total abundances (left: daytime, right: nighttime), with time series of PM2.5. Note that the cumulative relative abundance for the August 11 daytime sample (the highest of all the samples) was set arbitrarily to 100%. The pie charts show the average organic compounds composition at daytime and nighttime. (b) Abundance proportion of detected organic compounds (left: daytime, right: nighttime). (c) The relationships between relative abundances of CHOS and CHONS in PM2.5 with RH. (d) The relationships between relative abundances of CHOS and CHONS in PM2.5 and oxidizing gases (O3 + NO2).
Figure 2. (a) Relative abundance of CHO, CHON, CHOS, CHONS, and CHX and their proportions in total abundances (left: daytime, right: nighttime), with time series of PM2.5. Note that the cumulative relative abundance for the August 11 daytime sample (the highest of all the samples) was set arbitrarily to 100%. The pie charts show the average organic compounds composition at daytime and nighttime. (b) Abundance proportion of detected organic compounds (left: daytime, right: nighttime). (c) The relationships between relative abundances of CHOS and CHONS in PM2.5 with RH. (d) The relationships between relative abundances of CHOS and CHONS in PM2.5 and oxidizing gases (O3 + NO2).
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Figure 4. Relative abundances and proportional contributions of isoprene-derived, monoterpene/sesquiterpene-derived, aromatic, and others within CHO, CHON, CHOS, and CHONS compounds.
Figure 4. Relative abundances and proportional contributions of isoprene-derived, monoterpene/sesquiterpene-derived, aromatic, and others within CHO, CHON, CHOS, and CHONS compounds.
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Tang, X.; Mao, J.; Cai, D.; Zhang, Z.; Nong, H.; Li, L.; Chen, J. Molecular Characterization of Organic Aerosol in Summer Suburban Shanghai Under High Humidity. Atmosphere 2025, 16, 659. https://doi.org/10.3390/atmos16060659

AMA Style

Tang X, Mao J, Cai D, Zhang Z, Nong H, Li L, Chen J. Molecular Characterization of Organic Aerosol in Summer Suburban Shanghai Under High Humidity. Atmosphere. 2025; 16(6):659. https://doi.org/10.3390/atmos16060659

Chicago/Turabian Style

Tang, Xiancheng, Junfang Mao, Dongmei Cai, Zhiwei Zhang, Haixin Nong, Ling Li, and Jianmin Chen. 2025. "Molecular Characterization of Organic Aerosol in Summer Suburban Shanghai Under High Humidity" Atmosphere 16, no. 6: 659. https://doi.org/10.3390/atmos16060659

APA Style

Tang, X., Mao, J., Cai, D., Zhang, Z., Nong, H., Li, L., & Chen, J. (2025). Molecular Characterization of Organic Aerosol in Summer Suburban Shanghai Under High Humidity. Atmosphere, 16(6), 659. https://doi.org/10.3390/atmos16060659

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