Combining atmospheric and snow layer radiative transfer models toassess the solar radiative effects of black carbon in the Arctic

Abstract. Solar radiative effects (cooling or warming) of black carbon (BC) particles suspended in the Arctic atmosphere and surface snow layer were explored by radiative transfer simulations on the basis of BC mass concentrations measured in pristine early summer and polluted early spring conditions under cloudless and cloudy conditions. To account for the radiative interactions between the black carbon containing snow surface layer and the atmosphere, a snow layer and an atmospheric radiative transfer model were coupled iteratively. For pristine summer conditions (no atmospheric BC) and a representative BC particle mass concentration of 5 ng g−1 in the surface snow layer, a positive solar radiative effect of +0.2 W m−2 was calculated for the surface radiative budget. Contrarily, a higher load of atmospheric BC representing springtime conditions, results in a slightly negative radiative effect of about −0.05 W m−2, even when the same BC mass concentration is suspended in the surface snow layer. This counteracting of atmospheric BC and BC suspended in the snow layer strongly depends on the snow optical properties determined by the snow specific surface area. However, it was found, that the atmospheric heating rate by water vapor or clouds is one to two orders of magnitude larger than that by atmospheric BC. Similarly, the total heating rate (6 K day−1) within a snow pack due to absorption by the ice water, was found to be more than one order of magnitude larger than the heating rate of suspended BC (0.2 K day−1). The role of clouds in the estimation of the combined direct radiative BC effect (BC in snow and in atmosphere) was analyzed for the pristine early summer and the polluted early spring BC conditions. Both, the cooling effect by atmospheric BC, as well as the warming effect by BC suspended in snow are reduced in the presence of clouds.