Secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor during wintertime in Beijing, China.

Secondary organic aerosols (SOA) constitute a large fraction of atmospheric aerosols, yet our knowledge of the formation and aging processes of SOA in megacities of China is still limited. In this work, the formation and aging processes of SOA in winter in Beijing was investigated using a high-resolution aerosol mass spectrometer (AMS) and an oxidation flow reactor (OFR). Our results showed that the OA enhancement from OH aging peaked at ∼3.9 equivalent days with an average enhancement of 0.9 (±0.3) μg m-3. Positive matrix factorization analysis of AMS-OFR data identified three primary OA (POA) and two SOA factors. While the concentrations of POA factors decreased as a function of photochemical age, the two SOA factors showed clear enhancements by 2.5 and 4.3 μg m-3 at ∼3.9 and ∼2.6 days of equivalent photochemical age, respectively. The average contribution of SOA to the total OA was 47% in ambient air and 87% in OFR-oxidized ambient air. The elevated oxygen-to-carbon (O/C) ratio from 0.49 to 0.77-0.82 and the decreased hydrogen-to-carbon (H/C) from 1.37 to ∼1.1 highlighted the formation of more oxidized SOA during photochemical aging in winter in Beijing. The ubiquitous SOA enhancement as a function of OA levels indicated the significant formation potential of SOA in winter, and it varied differently among different episodes. In particular, we observed a maximum SOA enhancement of 38.6 μg m-3 during a biomass burning event. This result demonstrates that photochemical oxidation of ubiquitous biomass burning emissions can be a large source of SOA in winter in North China Plain.