Insights into aqueous-phase and photochemical formation of secondary organic aerosol in the winter of Beijing

Haze pollution has become a major public concern of China in recent years, especially in Beijing and surrounding areas, and in the fall and winter seasons. Implementation of Clean Air Action Plan in 2013 targeted the distinct improvement of air quality in Beijing since 2017. However, there is the absence of detailed investigation of SOA evolutionary processes in winter 2017. Here we deployed an Aerodyne high-resolution aerosol mass spectrometer during winter 2017 in Beijing. Our results show that PM1 average mass concentrations decreased from 81.7 mu g/m3 in 2013 to 24.5 mu g/m3 in 2017. OA was the most abundant component in PM1, accounting for 37%. Five OA factors, including two SOA factors, less-oxidized oxygenated OA (LO-OOA) and more-oxidized oxygenated OA (MO-OOA), were resolved using positive matrix factorization analysis. The mass fraction of SOA in OA was higher than 58%, underlining the important role of secondary formation in OA pollution. The contribution of MO-OOA to OA increased from -20% to more than 40% as a function of relative humidity and odd oxygen (OX=O3+NO2), indicating the promotion of aqueous-phase processing and photochemical oxidation on SOA formation. O/CSOA ratio also presented similar trends, increasing from -0.8 to around 1.0. Comparison of SOA evolution at daytime and nighttime shows that SOA aging at daytime was more active than that at nighttime, implying the potential of aqueous photoformation. Further analysis of the joint influence of RH and OX indicates that aqueous-phase processing was more significant than photochemical formation in MO-OOA formation and SOA aging. This study helps better understand the rapid decrease of PM1 with increasing contribution of SOA in Beijing winter. Summary: This work focused on the detailed characteristics of SOA formation in Beijing winter, and found the potential significance of the intense photochemical aqueous-phase oxidation on SOA formation.

Automated summary

This study focused on the detailed characteristics of SOA formation in Beijing winter, and found the potential significance of the intense photochemical aqueous-phase oxidation on SOA formation.