Aerosol Hygroscopic Growth as a Key Driver of Aerosol-Radiation Interactions in Radiation Fog

Abstract. Aerosol–radiation interactions (ARI) can alter the thermodynamic structure of the atmospheric boundary layer and thereby influence the evolution of radiation fog. Previous modeling studies have focused primarily on the shortwave radiative effect of aerosols and often assumed aerosols to be dry when interacting with radiation ("dry ARI"), neglecting the role of aerosol water in modifying particle size, optical properties, and longwave radiative transfer ("wet ARI"). Here, we use a large-eddy simulation model with explicit representations of dry aerosols, wet aerosols, and activated cloud droplets to systematically explore their radiative effects on a wintertime radiation fog case observed during the FAIRARI campaign in the Po Valley, Italy. The simulations show that the radiative effect of aerosols is weak if the aerosols are assumed to be dry. In contrast, the fog development is highly sensitive to the radiative effects of wet aerosols. Wet ARI enhances longwave radiative cooling at the fog top, strengthening in-fog turbulent mixing, promoting droplet activation, and increasing the vertical extent of the fog. This leads to a larger fog optical depth and stronger attenuation of shortwave radiation, resulting in improved agreement with observations of surface radiative fluxes and fog persistence. Increasing the accumulation-mode aerosol number concentration (5,000–25,000 cm^-3) generally enhances fog geometric height, optical depth, and liquid water path, effects that become substantially more pronounced when wet ARI are considered. Within the observed range of aerosol chemical composition, aerosol hygroscopicity (κ = 0.33–0.53) is the dominant chemical factor shaping the fog life cycle, while variations in black carbon content have a negligible effect. These results highlight the important role of wet ARI in understanding long-term fog trends and their responses to aerosol changes.