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 (Eabs) due to the lensing effect. It is crucial to reasonably represent the mixing of BC with other aerosol components to reduce this uncertainty. In this study, the absorption properties of PM2.5 were investigated based on the Nested Air Quality Prediction Modeling System (NAQPMS) with different assumptions of the aerosol mixing state. The absorption coefficient (babs) is the highest under the assumption of uniform internal mixing, lower under core-shell mixing, and the lowest under external mixing. The result under core-shell mixing is closest to the observations. The aging process and coating thickness were well reproduced by an advanced particle microphysics (APM) module in NAQPMS. Following this, the fraction of embedded BC and secondary component coating on aerosols was used to constrain the mixing state. Eabs 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, Eabs-880 - caused by the lensing effect - decreases 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 their radiative effects.