Abstract
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.
Highlights
Black carbon (BC) deposited in snow absorbs more sunlight than pure snow due to significant differences between the optical properties of BC and ice (Bond and Bergstrom, 2006; Warren and Brandt, 2008)
The prevailing winds over the study region are westerlies, ice core and atmospheric aerosol studies in the Muztagh Ata region show that BC, organic carbon (OC) and dust concentrations peak in summer, rather than in winter as in other Tibetan Plateau (TP) regions (Kaspari et al, 2014; Xu et al, 2009a)
The Muji Glacier is far from known emission sources, so we assume that BC/OC/dust deposition is homogeneous in our small study area
Summary
Black carbon (BC) deposited in snow absorbs more sunlight than pure snow due to significant differences between the optical properties of BC and ice (Bond and Bergstrom, 2006; Warren and Brandt, 2008). Conway et al (1996) conducted an experiment on Snowdome on the Blue glacier in the Olympic Range, USA and found that hydrophilic BC was more likely flushed by meltwater than hydrophobic BC Both types had a scavenging efficiency
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