Abstract: Biomass burning (BB) affects fine particulate matter (PM2.5) and ozone (O3) formations by emitting their gaseous precursors and primary aerosols. Impacts of BB in peninsular Southeast Asia (BB-PSEA) are evaluated on the PM2.5 and O3 formations in southern China, using a source-oriented WRF-Chem model to simulate an air pollution episode from 21 to 25 March 2015. The source-oriented model separates the emission from the BB-PSEA and other sources and is able to evaluate the effect of aerosol-radiation interactions (ARI) and aerosol-photolysis interactions (API) from the BB-PSEA. Comparisons with observations reveal that the model performs well in simulating the air pollution episode. Sensitivity experiments show that BB-PSEA increases PM2.5 concentrations by 39.3 μg m-3 (68.0%) in Yunnan Province (YNP) and 8.4 μg m-3 (24.1%) in other downwind areas (ODA) in southern China (including the provinces of Guizhou, Guangxi, Hunan, Guangdong, Jiangxi, Fujian, and Zhejiang) on the regional average. The PM2.5 enhancement is mainly contributed by primary aerosols in YNP but by secondary aerosols in the ODA. The BB-PSEA increases O3 concentrations of 18.1 μg m-3 (19.4%) in YNP and decreases O3 concentrations in the ODA by 3.7 μg m-3 (5.3%). The O3 increase in YNP is contributed by the gaseous emissions of the BB-PSEA, and the O3 decrease in the ODA is caused by the effects of ARI and API of the BB-PSEA. The NH3 emissions from the BB-PSEA plays a key role in enhancing secondary inorganic aerosols in southern China, and also determine the PM2.5 increase in the ODA.