Seasonal progression of the deposition of black carbon by snowfall at Ny-Alesund, Spitsbergen.

Wet and dry deposition of black carbon (BC) aerosol in the Arctic lowers snow albedo, thus contributing to warming in the Arctic. However, the processes and impacts associated with BC deposition in the Arctic are poorly understood because of the scarcity and uncertainties of measurements of BC in snow with adequate spatiotemporal resolution. We sampled snowpack at two sites (11 m and 300 m above sea level) at Ny-Alesund, Spitsbergen, in April 2013. We also collected falling snow near the surface with a windsock from September 2012 to April 2013. The size distribution of BC in snowpack and falling snow was measured using a single-particle soot photometer combined with a nebulizer. The BC size distributions did not show significant variations with depth in the snowpack, suggesting stable size distributions in falling snow. The BC number and mass concentrations (CNBC and CMBC) at these sites agreed to within 19% and 10%, respectively, despite the sites difference in snow water equivalent (SWE). This indicates the small influence of the amount of SWE (or precipitation) on these quantities. Average CNBC and CMBC in snowpack and falling snow at nearly the same locations agreed to within 5% and 16%, after small corrections for artifacts associated with the sampling of the falling snow. This comparison shows that the relative contribution of dry deposition to total deposition to be smaller than the uncertainty of their measurements. CNBC and CMBC in falling snow and BC concentrations in ambient air were highest in winter.北极地区黑碳（Black Carbon, BC）气溶胶的干湿沉降会降低积雪反照率，进而加剧北极升温。然而，由于现有积雪黑碳观测在时空分辨率上存在不足与不确定性，学界对北极地区黑碳沉降相关的过程与影响仍知之甚少。本研究于2013年4月在斯匹次卑尔根岛的新奥尔松（Ny-Ålesund）两处海拔分别为11米与300米的站点开展积雪采样；同时于2012年9月至2013年4月期间，利用风向袋在近地表处收集降雪样品。本研究采用单颗粒炭黑光度计（single-particle soot photometer）结合雾化器（nebulizer），对积雪与降雪中的黑碳粒径分布进行了测定。积雪中的黑碳粒径分布未随深度呈现显著变化，表明降雪中的黑碳粒径分布较为稳定。尽管两处站点的积雪水当量（Snow Water Equivalent, SWE）存在差异，但其黑碳数浓度（BC Number Concentration, CNBC）与黑碳质量浓度（BC Mass Concentration, CMBC）的相对偏差分别控制在19%与10%以内。这表明积雪水当量（或降水量）对上述两类浓度的影响较小。在对降雪采样引入的系统误差进行小幅校正后，近相同位置处积雪与降雪中的黑碳数浓度与质量浓度的平均相对偏差分别控制在5%与16%以内。该对比结果表明，干沉降对总沉降的相对贡献小于其观测本身的不确定性。降雪中的黑碳数浓度、质量浓度以及环境空气中的黑碳浓度均在冬季达到峰值。