Daytime Atmospheric Oxidation Capacity of Urban Beijing under Polluted Conditions during the 2008 Beijing Olympic Games and the Impact of Aerosols

Understanding atmospheric photochemical oxidation capacity is key to the evaluation of human impacts on the atmosphere. In this study, an observation-based box model constrained by in-situ measurements was used to analyze budgets of OH and HO2 (two key oxidants) in polluted conditions during August 2008 in urban Beijing. The results showed that photolysis of HCHO and other VOCs were the dominant factors influencing HOx production. Photolysis of HONO and alkene ozonolysis were also significant. Aerosol had a considerable impact on HOx and O-x production, as revealed by sensitivity simulations, reducing the concentrations of OH and HO2 by 56% and 25%, respectively. Among the reductions, that of OH was mostly through the radiative effect of aerosol, while HO2 reduction was from both radiative and heterogeneous chemical effects of aerosol. This impact of aerosol on atmospheric oxidation capacity led to a 100-200% increase in the lifetimes of NO2, CO and SO2. This implies that aerosols may increase the transport distance of gaseous pollutants.