Source apportionment of black carbon during winter in Beijing.

Black carbon (BC) in PM2.5 was measured at an urban site in Beijing during winter 2015 using an aethalometer. The characteristics and sources of BC during pollution episodes and clean days were analyzed. The average hourly mass concentration of BC during the study period was 5.31±6.27μg/m3. BC was highly correlated with PM2.5 (R2=0.80), with its concentration ranging from 0.17μg/m3 in clean days to 35.33μg/m3 in haze days. Source apportionment results showed that the average contribution of liquid fuel source (e.g., vehicle emission) to BC was around 50% in clean days. While during the pollution episodes, solid fuel sources including coal combustion and biomass burning were the predominant sources, accounting for 61-83% of BC. Specific source tracers suggested that coal combustion and biomass burning dominated in different pollution episodes. Ratios of BC/PM2.5 and BC/CO as well as source tracers provided further supportive evidences for the source apportionment results. Our findings suggest that it is more important to control solid fuel sources such as coal combustion for BC abatement in Beijing during haze days, while liquid fuel source (e.g., vehicle emission) plays a relatively more important role in clean days compared to pollution episodes.