摘要

作为大气细颗粒物（PM2.5）的两个重要组成部分，二次有机气溶胶（SOA）和硫酸盐在形成和演化过程中是紧密耦合的，但我们对两者互相联系的动力学和机理的认识还不完全。在这我们展示了α-蒎烯衍生的SOA与二氧化硫（SO2）的多相反应导致无机硫酸盐（85～90%）和有机硫化物（10～15%）的高效生成，以及SOA的化学演化。SO2在α-蒎烯SOA上的反应吸收系数（γSO2）为10-4—10-3，这取决于有机过氧化物含量（暗示SO2氧化中颗粒相过氧化物的重要作用），也受到气溶胶液体含水量的强烈影响。我们用α-蒎烯衍生过氧化物在弱酸性条件下（pH值4～5）估算S（Ⅳ）的水相反应速率常数（kII）为154～1545 M s- 1。这些KII值介于市售的异丙苯氢过氧化物和2-叔丁基过氧化氢，以及那些最小过氧化物如过氧化氢和甲基氢过氧化物的那些之间。基于我们估计的kII值的定量分析表明，SOA和SO2之间的多相反应是硫酸盐形成和SOA老化的重要途径，在气溶胶收支和对空气质量和气候影响的建模中需要考虑。

Figures:

Figure 1. Measured uptake coefficient of SO2 (γSO2) on α-pinene SOA under varying conditions. (A) The γSO2 as a function of RH. The γSO2 on deliquesced ammonium sulfate aerosol is also displayed for comparison. The peroxide content in SOA was determined to be 23 ± 4% (w/w) assuming an average molecular weight of 300 gmol–1 for organic peroxides. The solid line is an exponential fitting of the data. (B) The γSO2 as a function of total peroxide content in SOA at 80% RH. (C) The γSO2 versus the product of the particle-phase peroxide concentration and the aerosol volume-to-surface ratio. The line is a least-squares linear fit to the data (see text). The SO2 concentration in these uptake experiments is 12 ppb. The particle characteristics including the size distribution and peroxide contentare given in Table S1 (exp 1 for A and exps 1–9 for B and C). Errors are one standard deviation (1σ).

Figure 2. Evolution of aerosol composition upon 1–3 h of reaction with 40, 80, and 215 ppb SO2 at <5%, 40%, and 80% RH (filter-based reactor experiments). (A) Production of sulfate permass of SOA. (B) Production of organosulfate (OS) per mass of SOA. Only the most dominant OS series, i.e., C9 (dotted), C10 (dashed), C19 (solid), and C20 (dash-dotted) were quantified. (C) Decay of total peroxide content in SOA (blue bars and left axis) and the fraction of peroxides that decayed due to SO2 reaction (open diamond and right axis). The decay of peroxides in SOA in the absence of SO2 was also displayed for comparison (green bars). Errors are 1σ.

Atmospheric Implications:

The atmospheric importance of SOA–SO2 chemistry is determined together by the abundance and reactivity (i.e.,kII) of organic peroxides in ambient SOA. Since data regarding the peroxide content in ambient SOA are currently unavailable, we evaluate the atmospheric importance of organic peroxides relative to that of H2O2 for SO2 oxidation in typical clean (the southeastern United States, SEUS) and polluted (haze-days in Beijing) atmospheres by assuming an organic peroxide content of 10% and 1% in the SOA under these two extreme conditions. Using concentration values of gas-phase H2O2, SOA, aerosol liquid water, etc. typical for these two regions in summer time (see Table S4), we estimate the concentrations of H2O2 and total organic peroxides in aqueous aerosols to be 1.1 × 10–4 and 9 × 10–2 M,respectively, in the SEUS and 2.2 × 10–4 and 5 × 10–3 M, respectively, during haze-days in Beijing. As a result, despite the kII for S(IV) and organic peroxides as measured for α-pinene-derived peroxides in our study and commercially available peroxides in Wang et al.(32) being a factor of 5–100 smaller than that for S(IV) and H2O2, the much higher abundance of organic peroxides than H2O2 in aerosols makes SO2 oxidation by organic peroxides competitive with or even dominant over oxidation by H2O2, particularly in less polluted regions.

Recently, it has been shown that the known gas- and aqueous-phase chemistry in cloud droplets cannot explain rapid sulfate formation observed during haze events in northern China,(46,47) suggesting missing mechanisms for sulfate formation. Although several heterogeneous reaction pathways were proposed, including SO2 oxidationin aqueous aerosols by NO2,(46,48) H2O2,(49) and O2 catalyzed by transitionmetals,(50) there remains a gap between modeled and measured sulfate concentrations.(51,52) Our analysis above indicates that the multiphase reaction between SOA and SO2 is an important missing pathway for sulfate formation that needs to be considered in air quality modeling.

Our study also shows that SO2 reactions lead to a dramatic decay of total peroxides, significant formation of OS, as well as evolution of many other individual compounds in SOA. Previous studies have shown that organic peroxides play an important role in SOA aging.(20,21,53) However, atmospheric fates of peroxides in the absence and presence of SO2 are likely very different. For example, the decomposition of peroxides in SOA upon UV exposure(53) and interaction with liquid water or transition metals(44) could form substantial amounts of OH radicals that can result in oxidative aging of SOA, whereas the multiphase reaction of peroxides with SO2 forms presumably alcohol species, and inorganic and organic sulfate. Therefore, SO2 reactions have a strong impact on atmospheric fate of peroxides and aging of SOA. Furthermore, it has been shown that both laboratory-generated and ambient SOA are a significant source of particle-bound reactive oxygen species (ROS) including H2O2, OH, and organic radicals,(22,44,53,54) which are believed to play a critical role in adverse health effects of atmospheric particles.(4) Since organic peroxides are a potentially important source of these ROS, the alternation in its atmospheric fate by SO2 reactions may significantly influence the health effect of SOA particles.

Moving forward, more information about the kinetics and reaction mechanisms is needed to more accurately assess the atmospheric importance of SOA+SO2 chemistry. Future studies should further investigate the reactivity of SOA with varying chemical and physical properties. In particular, it is crucial to study the effect of inorganic aerosol components on SOA reactivity given that a large fraction of SOA in the atmosphere are internally mixed with inorganic aerosols. Also, since the peroxide content, hygroscopcity, and phase state of SOA greatly depend on the precursors, oxidants, and NOx levels,(4,34,38) further studies are warranted to elucidate the kinetics and mechanisms of SO2 reaction with SOA arising from different emission sources and oxidation regimes.

Published online: 03 December 2019

Reference：

Title: Multiphase Reactions between Secondary Organic Aerosol and Sulfur Dioxide: Kinetics and Contributions to Sulfate Formation and Aerosol Aging

Min Yao, Yue Zhao, Minghao Hu,Dandan Huang, Yuchen Wang, Jian Zhen Yu, and Naiqiang Yan: Environmental Science & Technology Letters Article ASAP DOI: 10.1021/acs.estlett.9b00657