Climate effect of black carbon aerosol in a Tibetan Plateau glacier

In the Tibetan Plateau, the black carbon (BC) concentration in surface snow and snow pits has received much attention, whereas the seasonal behavior of aerosol-in-snow concentration, vertical profile, melt-scavenging, and enrichment have received relatively little attention. Here we investigate these processes and their impacts on radiative forcing on the Muji glacier in the westernmost Tibetan Plateau during the 2012 snowmelt season. Increasing impurity concentrations were mostly due to post-deposition effects rather than new deposition. On 5 July, BC concentrations in the surface snow were higher than those of fresh snow, implying enrichment via sublimation and/or melting of previous snow. Fresh snow contained 25 ng g−1 BC on 27 July; afterward, BC gradually increased, reaching 730.6 ng g−1 in September. BC, organic carbon (OC), and dust concentrations co-varied but differed in magnitude. Melt-scavenging efficiencies were estimated at 0.19 ± 0.05 and 0.04 ± 0.01 for OC and BC, respectively, and the BC in surface snow increased by 20-25 times depending on melt intensity. BC-in-snow radiative forcing (RF) was approximately 2.2 W m−2 for fresh snow and 18.1-20.4 W m−2 for aged snow, and was sometimes reduced by the presence of dust.