Fractal Geometry of Aerosol Particles and Its Impact on Atmospheric Optical Properties: Development and Validation of the Fractal Aerosol Cluster Model in a Heavy Haze Event

Abstract. Aerosol particles, as crucial atmospheric components, significantly influence optical properties and play an important role in climate change. Based on Lorenz-Mie's theory, the optical equivalent radius of aerosol particle affects the atmosphere visibility. Traditional aerosol models, which assume spherical or other geometrically regular particle shapes, significantly deviate from observations in visibility simulations. In this study, the mechanism of random diffusion and aggregation of monomeric particles into fractal geometrical clusters was reproduced. The Fractal Aerosol Cluster Model (FACM) was developed to parameterize the optical and aerodynamic sizes of the fractal geometry of aerosol particles. Sensitivity experiments were conducted to simulate a severe haze event in northern China in November 2018 by coupling FACM into WRF-Chem as the experimental case (EXP) while the control case (CTR) by the original WRF-Chem. The simulated near-ground PM_2.5 concentrations in both EXP and CTR are similar to the observations (OBS). However, EXP simulated the larger extinction coefficients and lower atmospheric visibility (AV), which is more closely to OBS. The average normalized mean error of AV by EXP to OBS is 163.39 %, compared to 421.62 % by CTR. Thus, considering the fractal geometry of aerosol particles significantly improves simulated AV. Furthermore, a reduction of approximately 60 W·m⁻² in land surface shortwave radiation in EXP than those by CTR was also confirmed by observations. This study of the optical properties of the fractal aerosol cluster will contribute to future research of atmospheric environment and climate change forcing.