Atmospheric oxidizing capacity in autumn Beijing: Analysis of the O and PM episodes based on observation-based model.

Atmospheric oxidizing capacity (AOC) is the fundamental driving factors of chemistry process (e.g., the formation of ozone (O) and secondary organic aerosols (SOA)) in the troposphere. However, accurate quantification of AOC still remains uncertainty. In this study, a comprehensive field campaign was conducted during autumn 2019 in downtown of Beijing, where O and PM episodes had been experienced successively. The observation-based model (OBM) is used to quantify the AOC at O and PM episodes. The strong intensity of AOC is found at O and PM episodes, and hydroxyl radical (OH) is the dominating daytime oxidant for both episodes. The photolysis of O is main source of OH at O episode; the photolysis of nitrous acid (HONO) and formaldehyde (HCHO) plays important role in OH formation at PM episode. The radicals loss routines vary according to precursor pollutants, resulting in different types of air pollution. O budgets and sensitivity analysis indicates that O production is transition regime (both VOC and NO-limited) at O episode. The heterogeneous reaction of hydroperoxy radicals (HO) on aerosol surfaces has significant influence on OH and O production rates. The HO uptake coefficient (γHO) is the determining factor and required accurate measurement in real atmospheric environment. Our findings could provide the important bases for coordinated control of PM and O pollution.