Measurement-based aerosol forcing calculations: The influence of model complexity

Abstract

On the basis of ground-based microphysical and chemical aerosol measurements a simple 'two-layer-single-wavelength' and a complex 'multiple-layer-multiple-wavelength' radiative transfer model are used to calculate the local solar radiative forcing of black carbon (BC) and (NH4)2SO4 (ammonium sulfate) particles and mixtures (external and internal) of both materials. The focal points of our approach are (a) that the radiative forcing calculations are based on detailed aerosol measurements with special emphasis of particle absorption, and (b) the results of the radiative forcing calculations with two different types of models (with regards to model complexity) are compared using identical input data. The sensitivity of the radiative forcing due to key input parameters (type of particle mixture, particle growth due to humidity, surface albedo, solar zenith angle, boundary layer height) is investigated. It is shown that the model results for external particle mixtures (wet and dry) only slightly differ from those of the corresponding internal mixture. This conclusion is valid for the results of both model types and for both surface albedo scenarios considered (grass and snow). Furthermore, it is concluded that the results of the two model types approximately agree if it is assumed that the aerosol particles are composed of pure BC. As soon as a mainly scattering substance is included alone or in (internal or external) mixture with BC, the differences between the radiative forcings of both models become significant. This discrepancy results from neglecting multiple scattering effects in the simple radiative transfer model.