2. State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, China;
3. Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512, USA;
4. California Air Resources Board, El Monte, CA 91731, USA;
5. School of Environment, Harbin Institute of Technology, Harbin 150001, China;
6. National Satellite Meteorological Center, China Meteorological Administration, Beijing 100081, China;
7. Lin’an Regional Atmosphere Background Station, Lin’an 311307, China
Organic compounds are abundant in ambient particle, most of which are secondary organic aerosol (SOA) in nature, formed from the oxidation of volatile organic compounds (VOCs) through reactions with O3, •OH, and •NO3 radicals and nucleation processes or condensation onto pre-existing particles (Kanakidou et al., 2005; Wong et al., 2015). SOA is continually aging and experiencing further chemical processing in the ambient environment, yet their formation and transformation mechanisms in the atmosphere remain not fully characterized (Heald and Spracklen, 2009; Wong et al., 2015). Because SOA production in the atmosphere is very complex, there are large gaps in the current understanding of SOA (Ervens et al., 2011). Moreover, current models often have large uncertainties in predicting the mass concentration of ambient SOA (Cheng et al., 2015). In addition to emissions, meteorological parameters, i.e., relative humidity (RH), temperature, and solar radiation, also have important influences on ambient SOA concentration and production (Sun et al., 2014; Cheng et al., 2015).
RH is an important factor influencing the mechanisms of SOA formation and related chemical and physical properties (Hallquist et al., 2009; Sun et al., 2013). Water is ubiquitous in the atmosphere, which can be easily taken up by soluble chemical components in ambient aerosols. Liquid water accounts for nearly 30% by volume in biogenic SOA under 85% RH (Varutbangkul et al., 2006). Therefore, chemical reactions or physical processes of aging organic aerosol (OA) involve water as a reactant or solvent, and the composition of OA is affected by RH in the atmosphere (Ervens and Volkamer, 2010; Zhang et al., 2012). Recently, the effects of RH on SOA formation have been investigated through both chamber studies (Zhao et al., 2006; Zhang et al., 2011; Zhou et al., 2011) and field observations (Sun et al., 2013; Cheng et al., 2015; Zheng et al., 2015; Wang et al., 2016). Most of the lab studies indicated that heterogeneous reactions under high RH might play important roles in SOA formation, including various precursors, e.g., aromatic compounds (Zhou et al., 2011), ethylene (Jia and Xu, 2016), β-pinene (Boyd et al., 2015), isoprene (Nguyen et al., 2011; Zhang et al., 2011), etc. Based on aromatic systems, Zhou et al. (2011) suggested that aromatic SOA yields under humid conditions can be a factor of 2 to 5 higher than that under dry conditions. Water uptake of glyoxal and methylglyoxal onto pre-existing aerosols contributing to larger SOA mass at higher RH was also demonstrated by Ervens and Volkamer (2010).
Furthermore, plenty of field studies also observed higher SOA yields under humid conditions in ambient air (Sun et al., 2013; Cheng et al., 2015). Ge et al. (2012) found that SOA formation and the oxidation degree of OA were enhanced by aqueous-phase processing during foggy days. Similarly, enhanced SOA concentrations under high RH conditions were observed by Kaul et al. (2011) as well. Based on online non-refractory submicron aerosol (NR-PM1) observation, the mass concentrations of most aerosol species, including oxygenated organic aerosol (OOA) and coal combustion OA (CCOA) increase linearly as a function of RH during winter in Beijing (Sun et al., 2013). Aqueous-phase production of SOA is an important pathway for SOA formation (Lim et al., 2010; Ervens et al., 2011, 2014). Moreover, different RH conditions have different impacts on organic oxidation product formation (Nguyen et al., 2011; Zhang et al., 2011). A significant change in isoprene SOA composition between humid (90% RH) and dry (< 2% RH) conditions was observed byNguyen et al. (2011). They found that isoprene SOA formed under high RH conditions contains a significantly shorter chain length of oligomer esters (5–7 monomer residues), while that under dry conditions is dominated by long oligomer esters (8–10 monomer residues). However, based on field measurements using an online high-resolution soot-particle aerosol mass spectrometer (SP-AMS) during humid (RH > 60%) late winter conditions in Nanjing,Wu et al. (2018) recently found that
In fact, as ambient conditions are much more variable than the controlled chamber conditions, elucidating SOA and its influencing factors is more complex and difficult. Based on the field observations, the aqueous-phase chemistry on different categories of SOA may be dominated by different mechanisms in the ambient environment. Thus, the knowledge of SOA formation and transformation mechanisms under different RH conditions in the ambient atmosphere is rather limited. In this work, we examine the RH dependence of aerosol chemistry, intending to illuminate the impacts of high RH on SOA abundance in the ambient atmosphere. A three-month measurement campaign of aerosol particle composition was conducted during summer 2015 at a regional background location in East China. The ambient daily average RH throughout the study was generally high, ranging from 57% to 99%, and the overall campaign average RH was high at 84%, providing an ideal opportunity to investigate high RH effects on secondary aerosol production.2 Experimental description 2.1 Sample collection
Samples were collected at Lin’an Regional Atmosphere Background Station (30.3°N, 119.73°E, 138 m a.s.l.) (Fig. 1). The details about the station description can be found in Liang et al. (2017). A total of 86 daily 24 h-PM2.5 samples were collected from 7 June to 31 August 2015 by a mini-volume sampler operated at an ambient airflow rate of 5 L min–1.2.2 Chemical analysis
Carbonaceous components, i.e., organic carbon (OC) and element carbon (EC), were measured on a punch (0.495 cm2) of each quartz filter sample by a thermal/optical carbon analytical method (Liang et al., 2017). The detection limit of OC was 0.82 µg cm−2 (Liang et al., 2017). Water-soluble inorganic ions (K+, Na+, Ca2+, Mg2+,
The thermodynamic model ISORROPIA-II was applied in this study to predict aerosol pH. The forward mode in ISORROPIA-II was chosen, which is superior to the reverse mode, as assessed by prior studies (Hennigan et al., 2015; Bougiatioti et al., 2016). Considering the high RH in our case (mean ± standard deviation of 84 ± 10%), aerosol solutions were assumed to be metastable (only a liquid phase) when running the model, which often had better performance than the stable state solution (solid + liquid) and was commonly applied in previous pH predictions (Guo et al., 2015; Bougiatioti et al., 2016; Liu et al., 2017). ISORROPIA-II input data included water-soluble ions (K+, Na+, Ca2+, Mg2+,
During this campaign, surface ozone at Lin’an was observed by a Model 49C ozone analyzer and CO was observed by a Model 48C carbon monoxide (CO) analyzer (Thermo Fisher Scientific Co., Ltd.). Daily mean concentrations of surface O3 and CO were calculated from the raw data recorded every 1 min.3 Results and discussion 3.1 Ambient concentrations of SOC and associated meteorological parameters
Statistical results of carbonaceous components, i.e., OC and EC, and the ratios of SOC to OC in PM2.5 collected at Lin’an site during summer 2015 are listed in Table 1, and their temporal variation patterns are shown in Fig. 2. In this study, the average ratio of OC/EC was 4.75, varying from 1.80 to 12.0, of which most values were between 2.0 and 6.0 (84%) (Liang et al., 2017). If the OC/EC ratio exceeds 2.0, the presence of SOA is implied (Chow et al., 1996); and this indicates an obvious contribution of SOA to the ambient aerosol at the background site. In addition, secondary organic carbon (SOC) was estimated by the EC-tracer method (Lim and Turpin, 2002). Briefly, SOC was estimated by the following formulae:
|OC (µgC m−3)||14.30||3.95||8.02||32.80|
|EC (µgC m−3)||3.33||1.47||0.77||9.97|
|TC (µgC m−3)||17.60||4.90||10.00||36.00|
|POC (µgC m−3)||6.00||2.64||1.39||18.00|
|SOC (µgC m−3)||8.26||3.31||0.03||27.00|
|Relative humidity (%)||83.00||10.00||57.00||99.00|
|Average wind speed (m s−1)||2.10||0.90||1.10||7.00|
|Total solar radiation (W m−2)||289.00||150.00||37.90||56.40|
|Total cloud cover (%)||77.90||30.40||0.00||100.00|
Primary OC (POC) = EC × (OC/EC)min,
SOC = OC − POC,
where (OC/EC)min indicates the minimum ratio of OC/EC. The mean concentrations of POC and SOC in PM2.5 at Lin’an during the sampling time were 6.00 ± 2.64 and 8.26 ± 3.31 µgC m–3, respectively (Table 1). The ratios of SOC to OC varied from 20.8% to 85.0% with an average of 58.5 ± 12.6%. The frequency distribution of the SOC/OC ratios is shown in Fig. 3. The daily SOC/OC ratios had a primarily skewed distribution, and more than 60% of SOC/OC ratios were distributed in the range of 50%−70%. The SOC/OC ratios in our study were similar to those reported at different sites around the world (Zhang et al., 2007; Ge et al., 2017; Chen et al., 2018; Wu et al., 2018), illustrating that secondary organic aerosol is ubiquitous and presents a dominance of oxygenated species in organic aerosols in the atmosphere.
During the sampling time, almost all RH values were higher than 60%, except on one day, i.e., 4 August 2016 (RH = 57%), which was excluded in the following discussion. The average RH value was 84%. Based on meteorological data analysis, increases in clouds and precipitation induced higher RH and decreased total solar radiation (Fig. 4). Statistical analysis of RH and total solar radiation showed an obvious negative correlation (r = 0.89, p < 0.01, Fig. 5). In the case of low solar radiation and high RH, the surface temperature fluctuation was inhibited, causing the boundary layer height to decrease; as a consequence, aerosols accumulated, enhancing light scattering and thus decreasing visibility (Fig. 5). In the meantime, photochemical activity was suppressed under low solar radiation, and the formation of secondary aerosol from this pathway became less significant.3.2 High RH effects on SOC concentration
In order to explore the impact of higher RH on SOA concentration, the relationships of POC and SOC with RH are shown in Fig. 6. The concentration of POC showed no relationship with RH (p > 0.05, Fig. 6a). On the other hand, SOC exhibited an obvious negative relationship when RH was higher than 60% (r = 0.56, p < 0.01, Fig. 6b). This indicates that chemical processing of SOC appeared to be inhibited under high RH (> 60%) at Lin’an. This observation was similar to that reported byWu et al. (2018), i.e., the two less oxygenated SOA factors (LSOA and SVOOA) responded negatively to RH. These authors concluded that the two less oxygenated SOA products were mainly driven by photochemical process.
In addition, high SOC values were observed at high temperatures (red color, Fig. 6), demonstrating that higher temperatures can elevate SOA concentrations in the ambient environment, especially on days of more than 25°C. High temperatures in summer are typically associated with stronger solar radiation, and the concentrations of photochemical oxidants, i.e., O3 and OH, will increase and the photolysis rates will be larger accordingly. Since SOC is, to a large extent, a product of photochemical reactions, enhanced SOC production rates are expected on days with high temperatures and intense photochemistry. Furthermore, numerous studies have reported that high temperatures can increase the emission of biogenic SOA precursors, e.g., isoprene and monoterpenes (Monson et al., 1994; Dominguez-Taylor et al., 2007). Thus, under high temperatures, SOA formation rates will be higher due to increased biogenic precursor emission and enhanced photolysis rates in the ambient environment.
The variations of average concentrations of POC, SOC, EC, OC, CO, and aerosol pH as a function of RH throughout the study are shown in Fig. 7. SOC exhibited a decreasing trend when RH was higher than 60%. SOC showed the largest mass decreasing rate when RH increased from 60% to 80% (−2.98 μgC m−3/10% RH), whereas the decreasing rate of those species was reduced to −0.46 μgC m −3/10% RH at RH between 70% and 90% (Fig. 7, Table 2). Our explanation for this phenomenon is that the rate of SOA production via the photochemical oxidation reaction pathway was strongly decreased at RH between 60% and 80%; if the rate of SOA production via the heterogeneous reactions was not increased enough, the net value of SOA production rate was significantly decreased. However, with the increasing RH, heterogeneous reactions under the higher RH condition enhanced SOA yields, and compensated for the influence from the photochemical oxidation pathway at RH between 70% and 90%. Thus, the net decreasing rate at RH between 70% and 90% was lower than that at RH between 60% and 80%. It should be noted that POC and EC were expected to be non-hygroscopic, while they were observed still slightly elevated as RH increased from 60% to 80%, probably because of their continuous yet weak accumulation. Moreover, likely due to wet deposition, all carbonaceous components appeared to have similar decreasing rates when RH increased from the range between 80% and 90% to the range higher than 90%. Because of both concentrations of POC and SOC decreasing at the range of RH higher than 90%, OC concentrations exhibited a significant decreasing rate of −3.63 μgC m −3/10% RH (Fig. 7, Table 2). Compared to primary aerosol components, i.e., POC and EC, the primary gaseous species CO exhibited much more stable behavior with increasing RH (Fig. 7).
> 90% *
|* means the average concentration of the parameter in latter RH range minus that in the former RH range.|
Previous chamber studies have found that heterogeneous reactions under the high RH condition can significantly enhance SOA yields. Taking isoprene SOA chamber studies as an example, earlier work has demonstrated that under lower NOx conditions, the isoprene SOA yield was enhanced in the presence of wet and acidic sulfate aerosol, especially resulting in higher concentrations of isoprene-derived epoxydiols (IEPOX) SOA (Surratt et al., 2010; Zhang et al., 2011, 2014; Lin et al., 2012). To evaluate the effect of chemical production, SOC concentrations were normalized to EC (derived from only primary emissions and being rather inert to chemical reactions), in order to exclude accumulation and/or dilution effects. The EC-scaled method will largely eliminate the variations due to physical reactions, i.e., mixing and dilution, and better represent the contribution from chemical reactions (Zheng et al., 2015).
Compared to previous chamber studies, the results obtained in our ambient study show contrary patterns: SOC appeared to be inhibited under high RH (> 60%), as indicated by an obvious downward trend of SOC/EC with increasing RH (Figs. 8, 9). Apparently, SOA at our location had no heterogeneous formation pathway under the high RH condition compared to those reported in other studies (Kaul et al., 2011; Ge et al., 2012). On the contrary, there was a negative effect on the formation of SOA observed in our study. This result was similar to that reported by Zheng et al. (2015), i.e., SOC/EC ratios exhibiting constant behavior during winter in Beijing when RH increased from 50% to 80%, although the SOA concentrations were increasing rapidly with rising RH. By using HOA (hydrocarbon-like organic aerosol) instead of EC, a similar trend was also observed by Sun et al. (2013): although the OOA concentration was observed obviously elevated when RH rose from 50% to 90%, the HOA-normalized OOA was fairly constant across the high RH levels.
There might be several reasons for the results observed in our study, different from chamber simulations. First, our study was conducted at high RH conditions, which was typically accompanied by increased clouds and precipitation, resulting in reduced solar radiation at the ground level, while photochemical activity is weakened under low solar radiation. Accordingly, the total solar radiation was observed to sharply decrease with RH rise, and O3 was also detected to decrease when RH > 70%, indicating secondary aerosol formation via the photochemical oxidation reaction pathway became less important ( Fig. 9). On the other hand, Zheng et al. (2015) suggested that aerosol may be accumulated at the high RH stable synoptic condition and increased to support aqueous-phase production, which is not sufficient to compensate for the weakened photochemical activity influence and may lead to a net decrease in the formation of ambient SOA.
Second, based on concentrations of aerosol chemical species, temperature, and RH, the ISORROPIA-II model is able to offer a more rigorous approach to calculate particle pH (Guo et al., 2015). The average pH in this study was 1.86 ± 0.54, ranging from 0.38 to 4.17, predicted by the thermodynamic ISORROPIA-II model. These systematically low pH levels observed in this study may be significantly influencing acid-catalyzed reactions as part of SOA production. Prior studies have found that acidic sulfate particles can significantly enhance the isoprene SOA yields in the atmosphere (Gaston et al., 2014; Riedel et al., 2015). However, due to more water uptake under the higher RH condition, the aerosol droplet acidity decreased and the enhancement of SOA formation by acidity was accordingly reduced. Therefore, the variation of average aerosol pH in our study was observed to be elevated with the enhanced RH as well, increasing from 1.63 to 2.20 when RH changed from the range of 60%–70% to > 90%, indicating aerosol acidity was obviously weakened under the high RH condition (Fig. 7). It should be noted that the predicted aerosol pH in this study was underestimated by lack of the NH3 data, while it had no impact on the elevated pH trend with increasing RH, which was previously reported (Guo et al., 2015). Moreover, the simulated sulfate was in good agreement with observations (R2 = 0.98, slope 0.95), implying that the ISORROPIA-II model performed well.
In addition, it should be mentioned that recent studies have demonstrated that compared to dry conditions, where SOA is semisolid, high RH conditions can lower the particle’s microscopic viscosity, which affects the particle chemical reactivity, allowing for more rapid diffusion of reactants (Renbaum-Wolff et al., 2013; Grayson et al., 2016). Zhang et al. (2018) found that aerosol viscosity also affects the reactive uptake yield of SOA. The adverse effect of aerosol viscosity on reactive uptake is also a function of RH, which is due to RH affecting the viscosity of pre-existing SOA coatings. Moreover, different portions of SOA may have different predominant mechanisms in the ambient environment. Wu et al. (2018) found highly oxygenated SOA (LVOOA) was more associated with aqueous-phase processes, while the less oxygenated components (LSOA and SVOOA) were governed by photochemical processing in ambient aerosols.4 Conclusions
High RH conditions are typically associated with weaker photochemistry, leading to a low production rate of SOA via this pathway in ambient aerosols. Although the aerosol aqueous-phase production will be increased under the high RH condition, it is not sufficient to compensate for the weakened photochemical activity influence, leading to a net decrease in the formation of SOA when RH > 60%. Moreover, due to more water uptake, higher RH was more likely to induce relatively lower aerosol acidity, thus the enhancement of SOA formation by acidity was reduced. Due to measurement limitations, we were unable to identify the precursors and couldn’t separate RH effects from other meteorological factors, e.g., temperature, wind direction, solar radiation, etc., in the ambient environment. It may be true that in the highly controlled chamber studies, RH solely causes the increase of SOA tracers, but not so in all ambient environments. In fact, oxidant concentrations, acidity, viscosity, reaction rates, and chemical species in the atmosphere are highly variable, and these factors affect SOA chemical reactions under high RH conditions and render the processes to be very complex. Although our study results represent more likely a specific and localized event, they indicate that some organic oxidation products may behave differently at higher RH compared to low RH conditions in the ambient atmosphere, as some organic oxidation products, i.e., those produced under high-NO x and low-NOx conditions under actual ambient conditions in China, are different from the results obtained from chamber studies and ambient observations in the US (Ding et al., 2008; Paulot et al., 2009; He et al., 2018).
Overall, our investigations do show a negative response of SOA to RH. These findings, at least qualitatively, reveal that in moist ambient environments, SOA formation may be more associated with photochemical processes, while aqueous-phase chemistry is less important for some SOA production.
Acknowledgment. Financial support was also provided partly by the Ministry of Science and Technology (MOST) of Taiwan, China (MOST 103-2113-M-007-005). We are grateful to Professor Xiaobin Xu for providing the O3 and CO data.
|Bougiatioti, A., P. Nikolaou, I. Stavroulas, et al., 2016: Particle water and pH in the eastern Mediterranean: Source variability and implications for nutrient availability. Atmos. Chem. Phys., 16, 4579–4591. DOI:10.5194/acp-16-4579-2016|
|Boyd, C. M., J. Sanchez, L. Xu, et al., 2015: Secondary organic aerosol formation from the β-pinene+NO3 system: Effect of humidity and peroxy radical fate . Atmos. Chem. Phys., 15, 7497–7522. DOI:10.5194/acp-15-7497-2015|
|Chen, C.-L., S. J. Chen, L. M. Russell, et al., 2018: Organic aerosol particle chemical properties associated with residential burning and fog in wintertime San Joaquin Valley (Fresno) and with vehicle and firework emissions in summertime South Coast Air Basin (Fontana). J. Geophys. Res. Atmos., 123, 10707–10731. DOI:10.1029/2018JD028374|
|Cheng, Y., K. B. He, Z. Y. Du, et al., 2015: Humidity plays an important role in the PM2.5 pollution in Beijing . Environ. Pollut., 197, 68–75. DOI:10.1016/j.envpol.2014.11.028|
|Chow, J. C., J. G. Watson, Z. Q. Lu, et al., 1996: Descriptive analysis of PM2.5 and PM10 at regionally representative locations during SJVAQS/AUSPEX . Atmos. Environ., 30, 2079–2112. DOI:10.1016/1352-2310(95)00402-5|
|Ding, X., M. Zheng, L. P. Yu, et al., 2008: Spatial and seasonal trends in biogenic secondary organic aerosol tracers and water-soluble organic carbon in the southeastern United States. Environ. Sci. Technol., 42, 5171–5176. DOI:10.1021/es7032636|
|Dominguez-Taylor, P., L. G. Ruiz-Suarez, I. Rosas-Perez, et al., 2007: Monoterpene and isoprene emissions from typical tree species in forests around Mexico City. Atmos. Environ., 41, 2780–2790. DOI:10.1016/j.atmosenv.2006.11.042|
|Ervens, B., and R. Volkamer, 2010: Glyoxal processing by aerosol multiphase chemistry: Towards a kinetic modeling framework of secondary organic aerosol formation in aqueous particles. Atmos. Chem. Phys., 10, 8219–8244. DOI:10.5194/acp-10-8219-2010|
|Ervens, B., B. J. Turpin, and R. J. Weber, 2011: Secondary organic aerosol formation in cloud droplets and aqueous particles (aqSOA): A review of laboratory, field and model studies. Atmos. Chem. Phys., 11, 11069–11102. DOI:10.5194/acp-11-11069-2011|
|Ervens, B., A. Sorooshian, Y. B. Lim, et al., 2014: Key parameters controlling OH-initiated formation of secondary organic aerosol in the aqueous phase (aqSOA). J. Geophys. Res. Atmos., 119, 3997–4016. DOI:10.1002/2013JD021021|
|Gaston, C. J., T. P. Riedel, Z. F. Zhang, et al., 2014: Reactive uptake of an isoprene-derived epoxydiol to submicron aerosol particles. Environ. Sci. Technol., 48, 11178–11186. DOI:10.1021/es5034266|
|Ge, X. L., Q. Zhang, Y. L. Sun, et al., 2012: Effect of aqueous-phase processing on aerosol chemistry and size distributions in Fresno, California, during wintertime. Environ. Chem., 9, 221–235. DOI:10.1071/EN11168|
|Ge, X. L., L. Li, Y. F. Chen, et al., 2017: Aerosol characteristics and sources in Yangzhou, China resolved by offline aerosol mass spectrometry and other techniques. Environ. Pollut., 225, 74–85. DOI:10.1016/j.envpol.2017.03.044|
|Grayson, J. W., Y. Zhang, A. Mutzel, et al., 2016: Effect of varying experimental conditions on the viscosity of α-pinene derived secondary organic material . Atmos. Chem. Phys., 16, 6027–6040. DOI:10.5194/acp-16-6027-2016|
|Guo, H., L. Xu, A. Bougiatioti, et al., 2015: Fine-particle water and pH in the southeastern United States. Atmos. Chem. Phys., 15, 5211–5228. DOI:10.5194/acp-15-5211-2015|
|Hallquist, M., J. C. Wenger, U. Baltensperger, et al., 2009: The formation, properties and impact of secondary organic aerosol: Current and emerging issues. Atmos. Chem. Phys., 9, 5155–5236. DOI:10.5194/acp-9-5155-2009|
|He, Q. F., X. Ding, X. X. Fu, et al., 2018: Secondary organic aerosol formation from isoprene epoxides in the Pearl River Delta, South China: IEPOX- and HMML-derived tracers. J. Geophys. Res. Atmos., 123, 6999–7012. DOI:10.1029/2017JD028242|
|Heald, C. L., and D. V. Spracklen, 2009: Atmospheric budget of primary biological aerosol particles from fungal spores. Geophys. Res. Lett., 36, L09806. DOI:10.1029/2009GL037493|
|Hennigan, C. J., J. Izumi, A. P. Sullivan, et al., 2015: A critical evaluation of proxy methods used to estimate the acidity of atmospheric particles. Atmos. Chem. Phys., 15, 2775–2790. DOI:10.5194/acp-15-2775-2015|
|Jia, L., and Y. F. Xu, 2016: Ozone and secondary organic aerosol formation from Ethylene-NOx-NaCl irradiations under different relative humidity conditions . J. Atmos. Chem., 73, 81–100. DOI:10.1007/s10874-015-9317-1|
|Kanakidou, M., J. H. Seinfeld, S. N. Pandis, et al., 2005: Organic aerosol and global climate modelling: A review. Atmos. Chem. Phys., 5, 1053–1123. DOI:10.5194/acp-5-1053-2005|
|Kaul, D. S., T. Gupta, S. N. Tripathi, et al., 2011: Secondary organic aerosol: A comparison between foggy and nonfoggy days. Environ. Sci. Technol., 45, 7307–7313. DOI:10.1021/es201081d|
|Liang, L. L., G. Engling, Z. Y. Du, et al., 2016: Seasonal variations and source estimation of saccharides in atmospheric particulate matter in Beijing, China. Chemosphere, 150, 365–377. DOI:10.1016/j.chemosphere.2016.02.002|
|Liang, L. L., G. Engling, X. Y. Zhang, et al., 2017: Chemical characteristics of PM2.5 during summer at a background site of the Yangtze River Delta in China . Atmos. Res., 198, 163–172. DOI:10.1016/j.atmosres.2017.08.012|
|Lim, H. J., and B. J. Turpin, 2002: Origins of primary and secondary organic aerosol in Atlanta: Results of time-resolved measurements during the Atlanta Supersite Experiment. Environ. Sci. Technol., 36, 4489–4496. DOI:10.1021/es0206487|
|Lim, Y. B., Y. Tan, M. J. Perri, et al., 2010: Aqueous chemistry and its role in secondary organic aerosol (SOA) formation. Atmos. Chem. Phys., 10, 10521–10539. DOI:10.5194/acp-10-10521-2010|
|Lin, Y. H., Z. F. Zhang, K. S. Docherty, et al., 2012: Isoprene epoxydiols as precursors to secondary organic aerosol formation: Acid-catalyzed reactive uptake studies with authentic compounds. Environ. Sci. Technol., 46, 250–258. DOI:10.1021/es202554c|
|Liu, M. X., Y. Song, T. Zhou, et al., 2017: Fine particle pH during severe haze episodes in northern China. Geophys. Res. Lett., 44, 5213–5221. DOI:10.1002/2017GL073210|
|Monson, R. K., P. C. Harley, M. E. Litvak, et al., 1994: Environmental and developmental controls over the seasonal pattern of isoprene emission from aspen leaves. Oecologia, 99, 260–270. DOI:10.1007/BF00627738|
|Nguyen, T. B., P. J. Roach, J. Laskin, et al., 2011: Effect of humidity on the composition of isoprene photooxidation secondary organic aerosol. Atmos. Chem. Phys., 11, 6931–6944. DOI:10.5194/acp-11-6931-2011|
|Paulot, F., J. D. Crounse, H. G. Kjaergaard, et al., 2009: Unexpected epoxide formation in the gas-phase photooxidation of isoprene. Science, 325, 730–733. DOI:10.1126/science.1172910|
|Renbaum-Wolff, L., J. W. Grayson, A. P. Bateman, et al., 2013: Viscosity of α-pinene secondary organic material and implications for particle growth and reactivity . Proc. Natl. Acad. Sci. USA, 110, 8014–8019. DOI:10.1073/pnas.1219548110|
|Riedel, T. P., Y. H. Lin, S. H. Budisulistiorini, et al., 2015: Heterogeneous reactions of isoprene-derived epoxides: Reaction probabilities and molar secondary organic aerosol yield estimates. Environ. Sci. Technol. Lett., 2, 38–42. DOI:10.1021/ez500406f|
|Sun, Y. L., Z. F. Wang, P. Q. Fu, et al., 2013: The impact of relative humidity on aerosol composition and evolution processes during wintertime in Beijing, China. Atmos. Environ., 77, 927–934. DOI:10.1016/j.atmosenv.2013.06.019|
|Sun, Y. L., Q. Jiang, Z. F. Wang, et al., 2014: Investigation of the sources and evolution processes of severe haze pollution in Beijing in January 2013. J. Geophys. Res. Atmos., 119, 4380–4398. DOI:10.1002/2014JD021641|
|Surratt, J. D., A. W. H. Chan, N. C. Eddingsaas, et al., 2010: Reactive intermediates revealed in secondary organic aerosol formation from isoprene. Proc. Natl. Acad. Sci. USA, 107, 6640–6645. DOI:10.1073/pnas.0911114107|
|Varutbangkul, V., F. J. Brechtel, R. Bahreini, et al., 2006: Hygroscopicity of secondary organic aerosols formed by oxidation of cycloalkenes, monoterpenes, sesquiterpenes, and related compounds. Atmos. Chem. Phys., 6, 2367–2388. DOI:10.5194/acp-6-2367-2006|
|Wang, D. F., B. Zhou, Q. Y. Fu, et al., 2016: Intense secondary aerosol formation due to strong atmospheric photochemical reactions in summer: Observations at a rural site in eastern Yangtze River Delta of China. Sci. Total Environ., 571, 1454–1466. DOI:10.1016/j.scitotenv.2016.06.212|
|Wong, J. P. S., S. M. Zhou, and J. P. D. Abbatt, 2015: Changes in secondary organic aerosol composition and mass due to photolysis: Relative humidity dependence. J. Phys. Chem. A, 119, 4309–4316. DOI:10.1021/jp506898c|
|Wu, Y. Z., X. L. Ge, J. F. Wang, et al., 2018: Responses of secondary aerosols to relative humidity and photochemical activities in an industrialized environment during late winter. Atmos. Environ., 193, 66–78. DOI:10.1016/j.atmosenv.2018.09.008|
|Zhang, H., J. D. Surratt, Y. H. Lin, et al., 2011: Effect of relative humidity on SOA formation from isoprene/NO photooxidation: Enhancement of 2-methylglyceric acid and its corresponding oligoesters under dry conditions. Atmos. Chem. Phys., 11, 6411–6424. DOI:10.5194/acp-11-6411-2011|
|Zhang, H. F., Z. F. Zhang, T. Q. Cui, et al., 2014: Secondary organic aerosol formation via 2-methyl-3-buten-2-ol photooxidation: Evidence of acid-catalyzed reactive uptake of epoxides. Environ. Sci. Technol. Lett., 1, 242–247. DOI:10.1021/ez500055f|
|Zhang, Q., J. L. Jimenez, M. R. Canagaratna, et al., 2007: Ubiquity and dominance of oxygenated species in organic aerosols in anthropogenically-influenced Northern Hemisphere midlatitudes. Geophys. Res. Lett., 34, L13801. DOI:10.1029/2007GL029979|
|Zhang, X. L., J. M. Liu, E. T. Parker, et al., 2012: On the gas-particle partitioning of soluble organic aerosol in two urban atmospheres with contrasting emissions: 1. Bulk water-soluble organic carbon. J. Geophys. Res. Atmos., 117, D00V16. DOI:10.1029/2012JD017908|
|Zhang, Y., Y. Z. Chen, A. T. Lambe, et al., 2018: Effect of the aerosol-phase state on secondary organic aerosol formation from the reactive uptake of isoprene-derived epoxydiols (IEPOX). Environ. Sci. Technol. Lett., 5, 167–174. DOI:10.1021/acs.estlett.8b00044|
|Zhao, J., N. P. Levitt, R. Y. Zhang, et al., 2006: Heterogeneous reactions of methylglyoxal in acidic media: Implications for secondary organic aerosol formation. Environ. Sci. Technol., 40, 7682–7687. DOI:10.1021/es060610k|
|Zheng, G. J., F. K. Duan, H. Su, et al., 2015: Exploring the severe winter haze in Beijing: The impact of synoptic weather, regional transport and heterogeneous reactions. Atmos. Chem. Phys., 15, 2969–2983. DOI:10.5194/acp-15-2969-2015|
|Zhou, Y., H. F. Zhang, H. M. Parikh, et al., 2011: Secondary organic aerosol formation from xylenes and mixtures of toluene and xylenes in an atmospheric urban hydrocarbon mixture: Water and particle seed effects (II). Atmos. Environ., 45, 3882–3890. DOI:10.1016/j.atmosenv.2010.12.048|