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 PM1 and PM2.5 were comparably measured in a winter field campaign in urban Beijing. On average, aerosol absorption by PM1 at 880 nm wavelength contributes to approximately half of aerosol absorption by PM2.5, mainly because aerosol absorption at this wavelength was dominated by EC and nearly half of the total EC mass in PM2.5 existed in PM1. The average MAE of EC at 880 nm (MAEEC,880nm) was 3.9 +/- 0.7 m2 g-1 for PM2.5, lower than that for PM1 (4.4 +/- 0.8 m2 g-1) likely because of the larger EC cores and lower coating degrees of EC particles in PM1-2.5. Variation in MAEEC,880nm was related to the bulk mass fractions of nitrate in PM1 and PM2.5, 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 PM1 and PM2.5 at 370 nm. The mean MAE of OC at 370 nm (MAEOC,370nm) was 2.4 +/- 0.9 m2 g-1 for PM1, higher than that of for PM2.5 (1.9 +/- 0.6 m2 g-1). The high value of MAEOC,370nm might be associated with regionally transported aerosols during clean and polluted periods. Enlarged particle sizes might have considerably weakened MAEOC,370nm for PM2.5 while exerting negligible impact on MAEOC,370nm for PM1.