Previous studies have indicated that black carbon (BC) potentially induces snow albedo reductions across northern China. However, the effects of other light-absorbing particles (LAPs, e.g., mineral dust, MD), snow grain shape, or BC-snow mixing state on snow albedo have been largely ignored. Here we evaluate the BC- and MDinduced snow albedo reductions and radiative forcings (RFs) using an updated Snow, Ice, and Aerosol Radiation radiative transfer model, considering all of the potential factors that can be derived from the field observations across northern China. The results highlight that the LAP-induced albedo reductions for nonspherical snow grains are 2%-30% less than those for spherical grains. Furthermore, BC-snow internal mixing can significantly enhance albedo reduction by a factor of 1.42-1.48 relative to external mixing, with snow grain radius ranging from 100 to 1000 mu m. The mean regional BC + MD-induced snow albedo reductions are amplified by the increase of snow grain radius, ranging from 0.012 to 0.123 for fresh snow to 0.016-0.227 for old snow. Finally, we discuss the relative contributions of BC and MD to the albedo reductions and RFs, highlighting the dominant role of BC in reducing snow albedo across northern China.

Purpose of Review Black carbon (BC) deposition in snow can trigger a significant reduction in snow albedo and accelerate snowmelt. As a result, numerous snow surveys have performed to measure BC concentrations in snow across the polar regions, the Tibetan Plateau, and other high-mountain regions. This review is aimed to synthesize the current progresses of the potential feedbacks of snow albedo and its sensitivity by BC in snow across the Northern Hemisphere. Recent Findings Generally, BC concentrations in snow are highest in the mid-latitudes of Northern China and North America, and reduce toward higher latitudes (e.g., Greenland and the rest of the Arctic). We found that the snow albedo reduction attributed to low BC contamination (< 20 ng g(-1)) in older snow (200 mu m snow grains) is 1.2%, compared with 0.6% in fresh snow (50 mu m snow grains). Non-spherical snow grains exhibit a significantly lower snow albedo reduction (2-6%) due to BC contamination compared with spherical snow grains with 100-500 ng g(-1)of BC in the snowpack. Snow-BC-internal mixing reduces the snow albedo (< 10%) more substantially than does external mixing in the case of 50-200 mu m snow grains and a given BC concentration (< 2000 ng g(-1)). Besides the BC and other light-absorbing particles (LAPs), the mixing state of LAPs in snow, snow grain properties, and the scavenging\washing effects are also major challenges in determining snow albedo, which need to be further investigated on a global scale.

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.