Modeling aerosols on the mesoscale-γ: Treatment of soot aerosol and its radiative effects

Aerosol particles modify the transfer of solar radiation in the atmosphere. To quantify this impact, the composition and the size distribution of the aerosol have to be known. In this study, simulations with a comprehensive model system including meteorological processes, gas phase chemistry, and aerosol dynamics and chemistry were carried out to investigate the impact of aerosol particles on global radiation. The model system is applied to a summer situation in the southwestern part of Germany. The aerosol model represents the aerosol population by several overlapping modes. It originally accounted for the species sulfate, nitrate, ammonium, and water and now has been extended to treat soot. Since the optical properties of soot particles depend critically on their mixing state, the transfer of soot from the external into the internal mixture is parameterized. On the basis of the simulated aerosol distributions the optical properties of the aerosol were derived using Mie calculations. These results serve as input data for the radiative transfer calculations, where we considered the wavelength interval from 280 nm to 700 nm. The results of the radiative transfer calculations show that for the summer situation presented, the aerosol in the boundary layer reduces the downward flux of the solar radiation by up to 25 W m^-1. Sensitivity studies showed that up to 50% of this effect can be attributed to the impact of soot.