Measurement Report: Rapid decline of aerosol absorption coefficient and aerosol optical properties effects on radiative forcing in urban areas of Beijing from 2018 to 2021

Abstract. Reliable observations of aerosol optical properties are crucial for quantifying the radiative forcing of climate. The simultaneous measurements of aerosol optical properties at three wavelengths for PM1 and PM10 were conducted in urban Beijing from March 2018 to February 2022. The results showed considerable reductions in aerosol absorption coefficient (σab) at 550 nm of PM10 and PM1 by 55.0 % and 53.5 % from 2018 to 2021. SSA increased from 0.89 ± 0.04 for PM10 (0.87 ± 0.05 for PM1) in 2018 to 0.93 ± 0.03 for PM10 (0.91 ± 0.04 for PM1) in 2021. These results indicated that the absorbing aerosols were more effectively controlled than scattering aerosols due to pollution control measure-taking. The annual average submicron absorption ratio (Rab) increased from 86.1 % in 2018 to 89.2 % in 2021, suggesting that fine particles are the main contributors to total PM10 absorption and that the fine particles to absorption became more important. Absorption Angstrom exponent (AAE) in winter decreased from 2018 to 2021, implying a decreasing contribution from brown carbon to light absorption, which may relate to the decreased emissions of biomass burning and coal combustion. During the study period, aerosol radiative forcing efficiency became more negative and were −27.0 and -26.2 W m−2 AOD−1 for PM10 and PM1 in 2021, which was mainly influenced by increasing SSA. Higher σab and PM2.5 mass concentrations were mainly distributed in clusters 4 and 5 transported from the south and the west of Beijing in each year. σab and PM2.5 corresponding to clusters 4 and 5 decreased evidently from 2018 to 2021, which may result from the control of source emissions in surrounding regions of Beijing. The 4-year data presented in this study provide critical optical parameters for radiative forcing assessment within two size ranges and are helpful for evaluating the effectiveness of clean air action.