Characterizing the deposition of black carbon by snowfall at Ny-Ålesund, Spitsbergen

Wet deposition of black carbon (BC) in the Arctic lowers snow albedo, thus contributing to warming in the Arctic. However, key processes and the magnitude of the effect of the BC deposition on radiative forcing in the Arctic are poorly understood due to uncertainties in the measurements of BC in snow. We measured the size distribution of BC in snowpack and falling snow using a single particle soot photometer combined with a nebulizer. We sampled snowpack at two sites (11 m and 300 m asl) at Ny-Ålesund, Spitsbergen, in April 2013. The BC size distributions did not show significant variations throughout the columns of the snowpack, suggesting stable size distributions in falling snow. The number and mass concentrations (C_NBC and C_MBC) at these sites agreed to within 19% and 10%, respectively, despite the sites’ difference in snow water equivalence. This indicates the small influence of the amount of precipitation on these quantities. We also sampled falling snow near the surface using a windsock during the same snow accumulation period. Column-averaged C_NBC between snowpack and falling snow agreed to within 15%, after corrections for artifacts associated with the sampling of the falling snow. From the comparison of C_NBC and C_MBC in snowpack and falling snow, we estimated the relative contribution of dry deposition to total deposition to be about 22±6% at Ny-Ålesund during the snow accumulation period. C_NBC in falling snow and BC mass concentrations in ambient air showed maxima in winter.