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

On the basis of ground-based microphysical and chemical aerosol measurement s a simple 'two-layer-single-wavelength' and a complex 'multiple-layer-mult iple-wavelength' radiative transfer model are used to calculate the local s olar radiative forcing of black carbon (BC) and (NH4)(2)SO4 (ammonium sulfa te) particles and mixtures (external and internal) of both materials. The f ocal points of our approach are (a) that the radiative forcing calculations are based on detailed aerosol measurements with special emphasis of partic le absorption, and (b) the results of the radiative forcing calculations wi th two different types of models (with regards to model complexity) are com pared 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 mixt ures (wet and dry) only slightly differ from those of the corresponding int ernal mixture. This conclusion is valid for the results of both model types and for both surface albedo scenarios considered (grass and snow). Further more, 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 forcin gs of both models become significant. This discrepancy results from neglect ing multiple scattering effects in the simple radiative transfer model.