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 HO
2 (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 HO
x production. Photolysis of HONO and alkene ozonolysis were also significant. Aerosol had a considerable impact on HO
x and O
x production, as revealed by sensitivity simulations, reducing the concentrations of OH and HO
2 by 56% and 25%, respectively. Among the reductions, that of OH was mostly through the radiative effect of aerosol, while HO
2 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 NO
2, CO and SO
2. This implies that aerosols may increase the transport distance of gaseous pollutants.
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