Modeling simulation of aerosol light absorption over the Beijing-Tianjin-Hebei region: the impact of mixing state and aging processes

Abstract. The mixing state and aging characteristics of black carbon (BC) aerosols are the key factors in calculating their optical properties and quantifying their impacts on radiation balance and global climate change. Considerable uncertainty still exists in the absorption properties of BC-containing aerosols and the absorption enhancement (E_abs) due to the lensing effect. It is crucial to reasonably represent the mixing of BC with other aerosol components to reduce the uncertainty. In this study, the absorption properties of PM_2.5 were investigated based on the nested air quality prediction model system (NAQPMS) with different assumptions of the aerosol mixing state. The absorption coefficient (b_abs) is highest under uniform internal mixing, lower under core-shell mixing, and lowest under the assumption of external mixing. The result under core-shell mixing is closest to the observation. The aging process and coating thickness were well produced by the advanced particle microphysical module (APM) in NAQPMS. Then the fraction of embedded BC and secondary components coating aerosols was used to constrain the mixing state. The E_abs at 880 nm over the Beijing-Tianjin-Hebei region was 2.0~2.5 under core-shell mixing. When the fraction of coated BC and the coating layer are resolved, the E_{abs_880} caused by the lensing effect can decrease by 30~43 % to 1.2~1.7, which is close to the range reported in previous studies. This study highlights the importance of representing the microphysical processes governing the mixing state and aging of BC and provides a reference for quantifying its radiative effect.