Highly oxidized organic aerosols in Beijing: Possible contribution of aqueous-phase chemistry

The oxidation state of organic aerosol (OA) substantially influences the optical and hygroscopic properties, toxicity, and atmospheric lifetime of OA. Comprehensive field measurements have been carried out in Beijing to understand the possible mechanism relevant to the evolution of OA oxidation state in the ambient atmosphere. We found that OA was more oxidized in pollution events in autumn than in winter, evidenced by the higher ratios of OA/OC (2.50 +/- 0.69) and f(44)/f(43) (1.90 +/- 0.54) in autumn than the corresponding values (1.80 +/- 0.39 and 1.64 +/- 0.45) in winter. Plumes with highly oxidized OA were identified to be transported from the southern regions of Beijing and strongly correlated with the high concentration of aerosol water content (AWC), which was dominantly affected by the high concentration of ammonium nitrate and the high RH. This suggests that aqueous-phase chemistry, as regulated by ammonium nitrate and RH, plays a crucial role in the formation of highly oxidized OA in Beijing.