Black carbon in a glacier and snow cover on the northeastern Tibetan Plateau: Concentrations, radiative forcing and potential source from local topsoil

Abstract

Black carbon (BC), which consists of the strongest light-absorbing particles (LAP) in snow/ice, has been regarded as a potential factor accelerating the melting of glaciers and snow cover over the Third Pole. During the winter and summer of 2016, snow, ice and topsoil were sampled from the Laohugou basin located on the northeastern Tibetan Plateau. Concentrations of BC in Laohugou Glacier No. 12 (LG12) and snow cover in this basin (LSC) varied broadly (21.7-2700.1 and 89.6 to 6326.2 ng g^-1, respectively), indicating large spatiotemporal variability in wet, dry and post depositional conditions. Further, internally mixed BC in snow grains enhanced the albedo reduction (15.0-26.3%) more than externally mixed BC in LG12 and LSC. Dust played a more important role than BC in accelerating the melting of LG12, whereas these components played comparable roles in accelerating the melting of LSC. In total, externally mixed BC and dust reduced the albedo by 0.075-0.423, with an associated mean radiative forcing (RF) of 97.5 ± 41.5 Wm^-2 in LSC. This level was lower than those in the ablation zone (354.1 ± 81.2 Wm^-2) and accumulation zone (145.6 ± 76.7 Wm^-2) of LG12 because of discrepancies in LAP concentrations, solar zenith angles and incoming shortwave radiation. Furthermore, we observed that topsoil containing abundant BC was transported along the slope from the debris to the LG12 surface ice, and topsoil in this region could be lifted by strong mountain-valley winds and then deposited on snow/ice surfaces, which affected the LAP concentrations. Therefore, this study is important for understanding the role of BC and dust in the melting of snow/ice in the northeastern Tibetan Plateau.

Keywords: Black carbon; Glacier; Local source; Radiative forcing; Snow cover; Tibetan Plateau.