On the direct radiative impact of upper tropospheric aerosol particles

Abstract. Despite the largest aerosol mass being located in the lower troposphere, the aerosol content of the upper troposphere (UT) plays an important role in the atmospheric radiation budget, both via direct and indirect aerosol-radiation interactions. The UT already has been identified as a highly sensitive region for the energy budget. This study analyses those interactions in this altitude relevant for climate change processes. For this purpose, global chemistry-climate model simulations are presented focussing on aerosol-radiation interactions. Employing a newly developed, highly flexible scheme for aerosol optical properties, the simulations depict the role of UT aerosol particles in the Earth's radiation budget. First, the fractional contribution of the UT aerosol to the total extinction is quantified, and the compounds relevant for the aerosol extinction are identified. It is shown, that the UT extinction contributes to more than 10 % in the global average, and that aerosol water is the most important component for the extinction. The sensitivity of the radiation budget to UT aerosols is analysed by scaling experiments of the upper tropospheric extinction. It is shown that the modified extinction results in significant changes in the radiative fluxes at the surface and throughout the whole atmosphere. A doubling of the extinction in the UT leads to a surface radiative disturbance of a global mean value of approximately 0.1 W/m2. Therefore, an increase in UT aerosols due to direct emissions, transport from near surface sources or space debris could result in a significant modification of the Earth’s radiation budget.