Wintertime aerosol optical properties in Lanzhou, Northwest China: Emphasis on the rapid increase of aerosol absorption under high particulate pollution

The evolution of aerosol absorption and the contribution of absorbing species under different severities of particulate pollution are poorly understood, though absorption is key in aerosol radiative forcing. To resolve the problems, aerosol absorbing properties from low to high particulate pollution were investigated by using intensive observations of aerosol optical properties in the winter of 2019-2020 in Lanzhou, Northwest China. The aerosol scattering coefficient increased linearly with increasing particulate matter <2.5 mu m in diameter (PM2.5) and the absorption coefficient increased more rapidly under higher particulate pollution, leading to rapid decline in single scattering albedo (SSA) and sharp increase in mass absorption efficiency of PM2.5 (MAEPM(2.5)). The SSA (MAEPM(2.5)) decreased (increased) from 0.87 (0.76) in the lowest PM2.5 bin to 0.82 (1.11) in the highest PM2.5 bin. The linear relationship between the scattering coefficient and PM2.5 was attributed to decreasing aerosol hygroscopicity with increasing PM2.5. Elemental carbon (EC), fine soils (FS), and organic carbon (OC) accounted for 77.4%, 16.6%, and 6.0% of the total aerosol absorption, respectively. From low to high particulate pollution levels, the contribution of EC absorption increased from 68.3% to 80.5% while that of FS decreased from 25.5% to 13.9%. The aerosol radiative forcing efficiency was strongly correlated with SSA. Our results show a unique rapid increase in aerosol absorption under high particulate pollution during winter in Lanzhou, which is opposite to the trends observed in eastern Chinese cities, where SSA increases with increasing PM2.5.