Contrasting mass absorption efficiency of carbonaceous aerosols between PM1 and PM2.5 in urban Beijing.

Carbonaceous aerosol, mainly comprising elemental carbon (EC) and organic carbon (OC), plays an important role in aerosol–climate interaction owing to its light-absorbing feature. The radiative forcing of carbonaceous aerosol is largely determined by its mass absorption efficiency (MAE). To depict the MAE of carbonaceous aerosol and its variations with aerosol size and chemical composition, light absorption coefficients and the main chemical composition of PM 1 and PM 2.5 were comparably measured in a winter field campaign in urban Beijing. On average, aerosol absorption by PM 1 at 880 nm wavelength contributes to approximately half of aerosol absorption by PM 2.5 , mainly because aerosol absorption at this wavelength was dominated by EC and nearly half of the total EC mass in PM 2.5 existed in PM 1. The average MAE of EC at 880 nm (MAE EC,880nm) was 3.9 ± 0.7 m2 g−1 for PM 2.5 , lower than that for PM 1 (4.4 ± 0.8 m2 g−1) likely because of the larger EC cores and lower coating degrees of EC particles in PM 1–2.5. Variation in MAE EC,880nm was related to the bulk mass fractions of nitrate in PM 1 and PM 2.5 , implying the important impact of secondary nitrate formation on the modification of EC mixing states and enhancing EC absorption efficiency in Beijing. Absorption by OC took up about 40% of the absorption coefficients for PM 1 and PM 2.5 at 370 nm. The mean MAE of OC at 370 nm (MAE OC,370nm) was 2.4 ± 0.9 m2 g−1 for PM 1 , higher than that of for PM 2.5 (1.9 ± 0.6 m2 g−1). The high value of MAE OC,370nm might be associated with regionally transported aerosols during clean and polluted periods. Enlarged particle sizes might have considerably weakened MAE OC,370nm for PM 2.5 while exerting negligible impact on MAE OC,370nm for PM 1. • Light absorption capacity of elemental carbon and organic carbon aerosols was compared between PM 1 and PM 2.5. • Nitrate was found to be the main aerosol chemical composition affecting the mass absorption efficiency (MAE) of EC. • MAE of EC in PM 2.5 was lower than that in PM 1 owing to large core sizes and low coating degrees of EC particles in PM 1–2.5. • Enlarged particle sizes in polluted periods might have weakened MAE of OC for PM 2.5. [ABSTRACT FROM AUTHOR]