Effects of surface coating on the shortwave and longwave radiative effects of dust aerosol in comparison with external mixing: A theoretical study

Abstract

Dust particles can be coated with a surface layer of pollutants such as sulfate and nitrate after mixing with local pollution during long-range transport. Previous studies investigated the effects of surface coating on scattering properties and direct radiative effects (DRE) of dust in the solar shortwave (SW) spectral region. In this research, we carried out a theoretical study of the surface coating effects in both solar SW and the terrestrial longwave (LW) and compared the results with external mixing. Three dust coating schemes were developed to study a hypothetical sulfate-dust coating case, in which the thickness of the coating sulfate layer is proportional to the size, surface area, and mass of the dust core, respectively. We found that at the 0.55 µm the aerosol optical thickness (AOD) of the externally mixed dust-sulfate increases more efficiently with the increasing sulfate mass than the coated dust cases, whereas the opposite is true at the 10 µm. This is because at 0.55 µm the difference in the total geometrical cross section is the dominant factor for the AOD difference, while at the 10 µm the dominant factor is the difference in extinction cross section. The differences in dust DRE at the top of atmosphere and surface between the external mixing and coated dust cases can be largely explained by the differences in AOD. Dust absorption in the SW is found to be significantly enhanced by the surface coating of non-absorptive sulfate due to the so-called “lensing effect”. When SW and LW DREs are combined, the volume- and area-proportional coating schemes have a significantly weaker cooling effect than externally mixed dust-sulfate. This research provides the theoretical understanding of how surface coating affects the SW, LW and total dust DREs.