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We quantify the effects of grain shape and multiple black carbon (BC)-snow internal mixing on snow albedo by explicitly resolving shape and mixing structures. Nonspherical snow grains tend to have higher albedos than spheres with the same effective sizes, while the albedo difference due to shape effects increases with grain size, with up to 0.013 and 0.055 for effective radii of 1,000m at visible and near-infrared bands, respectively. BC-snow internal mixing reduces snow albedo at wavelengths < similar to 1.5m, with negligible effects at longer wavelengths. Nonspherical snow grains show less BC-induced albedo reductions than spheres with the same effective sizes by up to 0.06 at ultraviolet and visible bands. Compared with external mixing, internal mixing enhances snow albedo reduction by a factor of 1.2-2.0 at visible wavelengths depending on BC concentration and snow shape. The opposite effects on albedo reductions due to snow grain nonsphericity and BC-snow internal mixing point toward a careful investigation of these two factors simultaneously in climate modeling. We further develop parameterizations for snow albedo and its reduction by accounting for grain shape and BC-snow internal/external mixing. Combining the parameterizations with BC-in-snow measurements in China, North America, and the Arctic, we estimate that nonspherical snow grains reduce BC-induced albedo radiative effects by up to 50% compared with spherical grains. Moreover, BC-snow internal mixing enhances the albedo effects by up to 30% (130%) for spherical (nonspherical) grains relative to external mixing. The overall uncertainty induced by snow shape and BC-snow mixing state is about 21-32%.

Plain Language Summary Pure snow strongly reflects sunlight, the degree of which is regulated by grain size and shape. Observations have shown that snow can be significantly darkened by impurities, such as black carbon (BC), which is the most important light-absorbing aerosol. However, the combined effects of the two critical factors, snow grain shape and BC-snow mixing structure, have not been previously investigated, the neglect of which could introduce large uncertainties in the estimates of snow albedo in terms of BC-induced darkening. We have developed a snow model to quantify the impact of the preceding two factors on snow albedo by means of resolving the structures of BC-snow mixtures for different grain shapes. Both snow grain shape and multiple BC-snow internal mixing play important roles in their impacts on snow albedo. For application to climate models, we construct a scheme to parameterize snow albedo and its darkening in terms of snow grain size, shape, and BC content.