Abstract
Abstract. A new aerosol-optics model is implemented in which realistic morphologies and mixing states are assumed, especially for black carbon particles. The model includes both external and internal mixing of all chemical species, it treats externally mixed black carbon as fractal aggregates, and it accounts for inhomogeneous internal mixing of black carbon by use of a novel "core-grey-shell" model. Simulated results of aerosol optical properties, such as aerosol optical depth, backscattering coefficients and the Ångström exponent, as well as radiative fluxes are computed with the new optics model and compared with results from an older optics-model version that treats all particles as externally mixed homogeneous spheres. The results show that using a more detailed description of particle morphology and mixing state impacts the aerosol optical properties to a degree of the same order of magnitude as the effects of aerosol-microphysical processes. For instance, the aerosol optical depth computed for two cases in 2007 shows a relative difference between the two optics models that varies over the European region between −28 and 18 %, while the differences caused by the inclusion or omission of the aerosol-microphysical processes range from −50 to 37 %. This is an important finding, suggesting that a simple optics model coupled to a chemical transport model can introduce considerable errors affecting radiative fluxes in chemistry-climate models, compromising comparisons of model results with remote sensing observations of aerosols, and impeding the assimilation of satellite products for aerosols into chemical-transport models.
Highlights
Aerosol-optics models are employed in large-scale chemical transport models (CTMs) in mainly two contexts, namely, in chemistry-climate modelling (CCM), and in conjunction with remote sensing observations
Replacing one optics model by another will not offset the resulting optical properties by some common factor; it will introduce a significant change in modelled optical properties, of which the magnitude and even the sign can be dependent on local conditions, such as the size distribution and the chemical composition of the aerosol particles
The new model differs from an earlier optics model described in Kahnert (2008) in three essential points. (i) While the old model treats all chemical components as externally mixed, the new model accommodates both external and internal mixtures of aerosol species. (ii) The old model treats black carbon particles as homogeneous spheres; the new model assumes a fractal aggregate morphology with fractal parameters based on observations
Summary
Aerosol-optics models are employed in large-scale chemical transport models (CTMs) in mainly two contexts, namely, in chemistry-climate modelling (CCM), and in conjunction with remote sensing observations. The aerosoloptics model provides the observation operator that maps the CTM output to radiometric variables that can be compared to satellite observations or satellite retrieval products This allows us to either employ satellite observations for evaluating CTM model results, or to assimilate satellite data into a CTM-based air-quality forecasting system. It is clear that the aerosol-optics model has a pivotal role in these kinds of applications It may constitute an additional source of error that could compromise the reliability of CCMs, impair the reliability of CTM evaluations, or degrade chemical data assimilation results.
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