We characterized water-soluble oxidative potential (OP) levels in wintertime PM_2.5 in the south and north of Beijing. Our results show that the volume-normalized dithiothreitol (DTT_v) in the north was comparable to that in the south, while the mass-normalized dithiothreitol (DTT_m) in the north was almost twice that in the south. Traffic-related emissions and biomass burning were the main sources of DTT_v in the south, and traffic-related emissions contributed the most to DTT_v in the north.