Spatial distribution and sources of winter black carbon and brown carbon in six Chinese megacities

The light-absorbing carbonaceous aerosols, including black carbon (BC) and brown carbon (BrC), influenced heavily on aerosol environmental quality and the Earth's radiation. Here, a winter campaign to characterize BC and BrC in PM_2.5 was conducted simultaneously in six Chinese megacities (i.e., Harbin, Beijing, Xi'an, Shanghai, Wuhan, and Guangzhou) using continual aethalometers. The combinations of advanced aethalometer and generalized additive model (GAM) were used to precisely quantify the BC and BrC sources in these megacities. The averaged light-absorbing coefficients of BC (b_abs-BC) and BrC (b_abs-BrC) were 28.6 and 21.8 Mm⁻¹ in northern cities, they were 1.4 and 2.7 times higher than those in southern cities. The BrC dominated the total b_abs (>40%) in northern cities but low to 20% in southern cities. On the other hand, the BC fractions were high in the southern cities, with the contributions of 62.4–79.7%, whereas much lower values of 53.7–59.4% in the northern cities. Source apportionment showed that the combustion of liquid fuels (e.g., gasoline or diesel) was highly dominant to b_abs-BC (>80%) in Guangzhou and Wuhan. This was further supported by the high NO₂ loadings in the GAM model. Solid fuels (i.e., biomass or coal) contributed a substantial portion to total b_abs-BC in the other four cities where the high abundances of primary b_abs-BrC were observed. The diurnal trend showed the peaks of secondary-BrC (b_abs-BrC_S) and b_abs-BrC_S/ΔCO in the northern cities occurred at high relative humidity in nighttime, implying the secondary BrC formation was possibly related to aqueous reactions in winter. In contrast, in the southern cities of Shanghai and Guangzhou, the accumulation of vehicle emissions during the morning traffic rush hours lead the formation of secondary BrC through photochemical reactions. The results of this work can be applied for the development of more effective practices to control BC and BrC on regional scale.