Sensitivity of aerosol radiative effects to different mixing assumptions in the AEROPT 1.0 submodel of the EMAC atmospheric-chemistry–climate model

摘要

The modelling of aerosol radiative forcing is a major cause of uncertainty inthe assessment of global and regional atmospheric energy budgets and climatechange. One reason is the strong dependence of the aerosol optical propertieson the mixing state of aerosol components, such as absorbing black carbon and,predominantly scattering sulfates. Using a new column version of the aerosoloptical properties and radiative-transfer code of the ECHAM/MESSy atmospheric-chemistry–climate model (EMAC), we study the radiative transfer applyingvarious mixing states. The aerosol optics code builds on the AEROPT (AERosol OPTical properties)submodel, which assumes homogeneous internal mixing utilising the volume averagerefractive index mixing rule. We have extended the submodel to additionallyaccount for external mixing, partial external mixing and multilayeredparticles. Furthermore, we have implemented the volume averagedielectric constant and Maxwell Garnett mixing rule. We performed regionalcase studies considering columns over China, India and Africa, corroboratingmuch stronger absorption by internal than external mixtures. Well-mixedaerosol is a good approximation for particles with a black-carbon core,whereas particles with black carbon at the surface absorb significantly less.Based on a model simulation for the year 2005, we calculate that the globalaerosol direct radiative forcing for homogeneous internal mixing differs fromthat for external mixing by about 0.5 W m−2.