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 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 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.