Effects of SO 2 on optical properties of secondary organic aerosol generated from photooxidation of toluene under different relative humidity conditions

Zhang, Wenyu; Wang, Weigang; Li, Junling; Peng, Chao; Li, Kun; Zhou, Li; Shi, Bo; Chen, Yan; Liu, Mingyuan; Ge, Maofa

Secondary organic aerosol (SOA) has great impacts on air quality, climate change and human health. The composition and physicochemical properties of SOA differ greatly because they form under different atmospheric conditions and from various precursors as well as differing oxidation. In this work, photooxidation experiments of toluene were performed under four conditions (dry, dry with inline-formulaSO2, wet and wet with inline-formulaSO2) to investigate the effect of inline-formulaSO2 under different relative humidities on the composition and optical properties of SOA at wavelengths of 375 and 532 inline-formulanm. According to our results, the increase in humidity enhances not only light absorption but also the scattering property of the SOA. Oligomers formed through multiphase reactions might be the reason for this phenomenon. Adding inline-formulaSO2 slightly lowers the real part of the complex refractive index, RI(inline-formulan), of toluene-derived SOA (RI(inline-formulan)inline-formula M10inlinescrollmathml normal dry , nobreak0.125em normal SO normal 2 < RI ( n ) normal dry 81pt15ptsvg-formulamathimg3f549125f962304b53ec80913e32d618 acp-20-4477-2020-ie00001.svg81pt15ptacp-20-4477-2020-ie00001.png , RI(inline-formulan)inline-formula M12inlinescrollmathml normal wet , 0.125emnobreak normal SO normal 2 < RI ( n ) normal wet 83pt15ptsvg-formulamathimg899330242d643de8fd6914049145569f acp-20-4477-2020-ie00002.svg83pt15ptacp-20-4477-2020-ie00002.png ), which might be a result of the partitioning of low-oxidation-state products. The imaginary part of the complex refractive index, RI(inline-formulak), is enhanced under dry conditions with inline-formulaSO2 compared to that of only dry conditions, which might be due to acid-catalyzed aldol condensation reactions. Wet conditions with inline-formulaSO2 shows the combined effect of inline-formulaSO2 and humidity. The extinction properties of toluene-derived SOA under wet conditions with inline-formulaSO2 increased by approximately 30 % compared to that of toluene-derived SOA formed under dry conditions. Our results suggest that various atmospheric conditions will affect the composition and optical proprieties of SOA, which has significant implications for evaluating the impacts of SOA on the rapid formation of regional haze, global radiative balance and climate change.

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Zhang, Wenyu / Wang, Weigang / Li, Junling / et al: Effects of SO2 on optical properties of secondary organic aerosol generated from photooxidation of toluene under different relative humidity conditions. 2020. Copernicus Publications.

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