Thermodynamic Modeling Suggests Declines in Water Uptake and Acidity of Inorganic Aerosols in Beijing Winter Haze Events during 2014/2015–2018/2019

Abstract

During recent years, aggressive air pollution mitigation measures in northern China have resulted in considerable changes in gas and aerosol chemical composition. But it is unclear whether aerosol water content and acidity respond to these changes. The two parameters have been shown to affect heterogenous production of winter haze aerosols. Here, we performed thermodynamic equilibrium modeling using chemical and meteorological data observed in urban Beijing for four recent winter seasons and quantified the changes in the mass growth factor and pH of inorganic aerosols. We focused on high relative humidity (> 60%) conditions when submicron particles have been shown to be in the liquid state. From 2014/2015 to 2018/2019, the modeled mass growth factor decreased by about 9%-17% due to changes in aerosol compositions (more nitrate and less sulfate and 18 Hence, it is useful to characterize these two parameters for understanding haze formation. 48 A general approach to measuring aerosol water content is by perturbing RH and detecting the 49 changes in aerosol physical properties, e.g., using nephelometers or hygroscopicity tandem 50 differential mobility analyzers, 3,19,20 but such measurements are not performed routinely in China. Another common approach involves thermodynamic equilibrium analyses between gas and aerosol phase for semivolatile species, requiring composition measurements and a thermodynamic model (e.g., E-AIM or ISORROPIA). 2,21,22 Intercomparisons indicate that water contents estimated

Chemical Differences Between PM1 and PM2.5 in Highly Polluted Environment and Implications in Air Pollution Studies

“…As indicated in Figure S8, the RH during P2 was generally low and rarely exceeded 80%; meanwhile, aerosol composition was dominated by less hygroscopic OA, and the contribution of SIA (27 ± 8%) was less than 40% for 95% time of P2 ( Figure S9). Previous studies suggested that SIA played a key role in uptake of aerosol liquid water (AWC; Song et al, 2019). We then estimated AWC due to SIA by using the ISORROPIA-II thermodynamic model (Nenes et al, 1998), and the results showed that AWC was below 40 μg m −3 for most of the time except the night on 16 December when it exceeded 100 μg m −3 .…”

Global modeling of heterogeneous hydroxymethanesulfonate chemistry

“…Such an exponential-like relationship has been interpreted to support the hypothesis that HMS is produced through aerosol water (Song et al, 2019a;Ma et al, 2020). This is because the amount of aerosol water also exhibits an exponential relationship with RH (Song et al, 2018(Song et al, , 2019b. Interestingly, our model simulations using aqueous clouds as the only medium can obtain a similar relationship between HMS and RH, which reduces the credibility of this aerosol water hypothesis.…”

“…The higher pH in JJA is related to more abundant gaseous NH3 (Fig. S8), given the buffer capacity of NH3 in moderating the acidity of atmospheric condensed water (Song et al, 2019b).…”

An interlaboratory comparison of aerosol inorganic ion measurements by ion chromatography: implications for aerosol pH estimate

“…It was applied in this study to calculate the aerosol water content (AWC) and pH. Aerosol pH in this study (pH i ) was defined as the molality-based hydrogen ion activity on a logarithmic scale, calculated applying the following equation (Jia et al, 2018;Song et al, 2019):…”