Assessment of Atmospheric Oxidizing Capacity Over the Beijing-Tianjin-Hebei (BTH) Area, China

The atmospheric oxidizing capacity (AOC) plays a key role in atmospheric chemistry, determining the removal of most reduced gases in the atmosphere. In this study, we assess the summertime AOC from 7 to 22 June 2014 associated with a field campaign conducted at Wangdu in the Beijing-Tianjin-Hebei region (BTH), China using the WRF-Chem model. The model reasonably well reproduces the incident solar radiation, ambient O₃ and fine particulate matter (PM_2.5) concentrations, the photolysis rates of O₃ and NO₂, and radical concentrations against measurements in the BTH. The model results show high daytime AOC levels in urban areas and relatively low AOC in remote areas of the BTH. The diurnal cycle of the AOC exhibits a unimodal pattern with peaks at noon. OH radical contributes about 68% of the daily average AOC in Beijing, followed by O₃ (27%) and NO₃ (5%). At Wangdu, the production rate of OH radical reaches around 13 ppb h⁻¹ at noon, which is mainly resulted from the reaction of HO₂ with NO. The OH radical loss is primarily contributed by reactions with volatile organic compounds, followed by CO and NO₂. Statistically, the population hourly or afternoon average O₃ and O_x (=O₃ + NO₂) concentrations are significantly correlated with the AOC in Beijing, constituting potential simplified tracers to indicate the AOC level.