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Using field observations, we perform radiative transfer calculations on snowpacks in the Arctic, China, and North America to quantify the impact of light-absorbing particles (LAPs) on snow albedo and its sensitivity to different factors. For new snow, the regional-averaged albedo reductions caused by all LAPs in the Arctic, North America, and China are 0.009, 0.012, and 0.077, respectively, of which the albedo reductions caused by black carbon (BC) alone are 0.005, 0.005, and 0.031, corresponding to a positive radiative forcing of 0.06, 0.3, and 3Wm(-2). For the same particulate concentrations, the albedo reduction for old melting snow is larger than that of new snow by a factor of 2; this leads to 3-8 times larger radiative forcing, in part due to higher solar irradiance in the melting season. These calculations used ambient snowpack properties; if all snowpacks were instead assumed to be optically thick, the albedo reduction would be 20-50% larger for new snow in the Arctic and North America and 120-300% larger for old snow. Accounting for non-BC LAPs reduces the albedo reduction by BC in the Arctic, North America, and China by 32%, 29%, and 70%, respectively, for new snow and 11%, 7%, and 51% for old snow. BC-in-snow albedo reduction computed using a two-layer model agrees reasonably with that computed using a multilayer model. Biases in BC concentration or snow depth often lead to nonlinear biases in BC-induced albedo reduction.

Plain Language Summary The presence of certain particles in snow darkens the snowpack by increasing sunlight absorption, which leads to earlier snowmelt. This darkening can also affect regional climate. In this work, we use field observational data gathered from the Northern Hemisphere (the Arctic, China, and North America) and a physical model to quantify the impact of these particles on snow reflectivity, and we examine the dependence of these impacts on different factors such as snow depth and particle concentration.