Effect Of Light-absorbing Particles On Snow Albedo

The surface energy budget of snow-covered regions,and the ablation rate of the snowpack in particular,are significantly affected by snow albedo.Numerous observations and model simulations have shown that light-absorbing particles（LAPs）,such as black carbon（BC）and mineral dust（MD）within the snowpack can reduce snow albedo and accelerate snow melting by enhancing the absorption of solar radiation,which has important implications for regional climate,hydrology,and ecological systems.This thesis systematically analyzes the effect of LAPs on snow albedo characteristics from the observation and simulation perspectives,based on the snow field survey across northern Xinjiang,China in 2018.In addition,the effects of coated BC,dust–snow mixing state,and dust morphological characteristics on snow albedo are quantified based on theoretical models,respectively.Finally,we evaluate the BC-and MD-induced snow albedo reductions and associated uncertainties due to snow grain shape and BC–snow mixing state,using the field observations across northern China and the updated Snow,Ice,and Aerosol Radiation radiative transfer（SNICARv2）model.（1）We measured BC concentration in snow at 28 sites and snow albedo at 18 sites in a vast region across northern Xinjiang,China in January 2018.The BC concentration was in a wide range of 40–1850 ng g^-1.The presence of the BC reduced the snow albedo by 0.01–0.20 at the visible wavelength band（400–750 nm）.The reduction differed from sites to sites with large values close to industrial areas that are characterized by high pollutants emission.Albedos simulated with the SNICAR model based on the measured BC agreed well with the measured albedos,with the deviation within±0.03 and the average underestimation of<0.002.It worth noting that the retrieved optical effective snow grain radius was the key factor in snow albedo simulation.Finally,the BC-induced albedo reductions were largely in the 0.003–0.090 range,and the associated radiative forcings were largely in the 0.2–6.9 W·m^-2,both of which were higher than most of the results from other snow-covered areas.This indicates strong acceleration of snowmelt due to BC in northern Xinjiang,China.（2）Although the BC coating effect on light absorption enhancement in the atmosphere has been broadly acknowledged,little research has been carried out on snow albedo.Here we have explicitly resolved the BC coating effect on snow albedo using the core–shell Mie theory and SNICAR model.The results indicated that the BC coating effect enhances the reduction in snow albedo by a factor ranging from 1.1–1.8for a nonabsorbing shell and 1.1–1.3 for an absorbing shell,depending on the BC concentration,snow grain radius,and core?/shell ratio.We developed parameterizations of the BC coating effect for application to climate models,which provides a convenient way to accurately estimate the climate impact of BC in snow.Finally,based on a comprehensive set of in situ measurements across the Northern Hemisphere,we found that the enhancement of snow albedo reduction ranged from 1.13–1.27,and the enhancement of radiative forcing was 1.14–1.27,which exceeds the contribution of dust to snow light absorption over most areas of northern China.（3）The effects of dust–snow mixing state on absorption coefficients and snow albedo were theoretically analyzed by combining SAMDS（Spectral Albedo Model for Dirty Snow）and（core–shell）Mie theory.In general,snow albedo was substantially reduced at wavelengths of<1.0?μm by internal dust–snow mixing,with stronger reductions at higher dust concentrations and larger snow grain sizes.Moreover,calculations showed that a nonuniform distribution of dust in snow grains can lead to significant differences in the values of the absorption coefficient and albedo of dust-contaminated snowpack at visible wavelengths relative to a uniform dust distribution in snow grains.Finally,using comprehensive in situ measurements across the Northern Hemisphere,we found that broadband snow albedo was further reduced by 5.2?%and9.1?%due to the effects of internal dust–snow mixing on the Tibetan Plateau and North American mountains.This was higher than the reduction in snow albedo caused by black carbon in snow over most North American and Arctic regions.Our results suggest that significant dust–snow internal mixing is important for the melting and retreat of Tibetan glaciers and North American mountain snowpack.（4）The combined effects of mineral dust nonsphericity and size on snow albedo reduction are quantified based on the MOPSMAP（Modeled optical properties of ensembles of aerosol particles）package and SAMDS,with the consideration of dust from Sahara,Greenland,San Juan Mountains,and Tibetan Plateau.Results indicate that the dust-induced albedo reduction decreases by up to 30%as the effective radii of dust particles increase in 1–5μm.Nonspherical dust enhances snow albedo reduction relative to spherical dust by up to 20%.Stronger enhancements are obtained for higher dust concentration and larger dust size.Furthermore,the dust nonsphericity-induced enhancement of snow albedo reduction is more pronounced for more-absorptive dust.Finally,we developed parameterizations for the dust-induced snow albedo reduction as a function of the dust concentration in snow,snow grain size,and dust size for different dust shapes,and provide a convenient way for assessing the climate impacts of dust in sno（5）Previous studies have indicated that black carbon（BC）potentially induces snow albedo reductions across northern China.However,the effects of other LAPs（e.g.,MD）,snow grain shape,or BC–snow mixing state on snow albedo have been largely ignored.Hence we evaluate the BC-and MD-induced snow albedo reductions and radiative forcings（RFs）based on the field observations across northern China during2010-2015 and SNICARv2 model.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 1000μm.The mean regional BC+MD-induced snow albedo reductions are amplified by the increase of snow grain radius,ranging from 0.012 to0.123 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.