Accounting for Black Carbon Aging Process in a Two-way Coupled Meteorology – Air Quality Model

Abstract. Black carbon (BC) exerts significant impacts on both climate and environment. BC aging process alters its hygroscopicity and light absorption properties. Current models, like the Weather Research and Forecasting – Community Multiscale Air Quality (WRF-CMAQ) two-way coupled model, inadequately characterize these alterations. In this study, we accounted for BC aging process in the WRF-CMAQ model (WRF-CMAQ-BCG). We introduced two new species (Bare BC and Coated BC) in the model and implemented a module to simulate the conversion from Bare BC to Coated BC, thereby characterizing the aging process. Furthermore, we improved the cloud chemistry and aerosol optics modules to analyze the effects of BC aging on hydrophobicity and light absorption. The simulated results indicate a spatial distribution pattern with Bare BC prevalent near emission sources and Coated BC more common farther from sources. The average Number Fraction of Coated BC (NF_coated) is approximately 57 %. Temporal variation exhibits a distinct diurnal pattern, with NF_coated increasing during the daytime. The spatial distribution of wet deposition varies significantly between Bare and Coated BC. Bare BC exhibits a point-like deposition pattern, whereas Coated BC displays a zonal distribution. Notably, Coated BC dominates the BC wet deposition process. Additionally, incorporating BC aging process reduces BC wet deposition by 17.7 % and increases BC column concentration by 10.5 %. The simulated Mass Absorption Cross-section (MAC) value improved agreement with observed measurements. Overall, the WRF-CMAQ-BCG model enhances the capability to analyze aging-related variables and BC mixing state, while also improving performance in wet deposition and optical properties.