Climate effect of black carbon aerosol in a Tibetan Plateau glacier

Abstract

h i g h l i g h t sWe sampled surface snow and snow pits in the snowmelt season in a Tibetan glacier. We depicted the variability of BC, OC, and dust concentrations. Scavenging efficiency and enrichment for BC and OC were derived. We evaluated BC radiative forcing in snow using SNICAR. a r t i c l e i n f o a b s t r a c tIn 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, meltscavenging, 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 À1 BC on 27 July; afterward, BC gradually increased, reaching 730.6 ng g À1 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 20e25 times depending on melt intensity. BC-in-snow radiative forcing (RF) was approximately 2.2 W m À2 for fresh snow and 18.1e20.4 W m À2 for aged snow, and was sometimes reduced by the presence of dust.

Concentrations and source regions of light-absorbing particles in snow/ice in northern Pakistan and their impact on snow albedo

“…Previous studies of ice cores and snow pits probably underestimated the albedo reduction and radiative forcing in glacier regions as samples were taken from high-elevation areas where there is less ageing and melting and thus lower surface enrichment of BC and dust than at lower elevation. Our results are higher than those reported in other studies on the northern slope of the Himalayas (Ming et al, 2012), on the western Tibetan Plateau (Yang et al, 2015), and the Tien Shan (Ming et al, 2016). However, they are comparable to values for radiative forcing reported more recently by others, for example for the Muji glacier (Yang et al, 2015), Zhadang glacier (Qu et al, 2014), in high Asia (Flanner et al, 2007;Nair et al, 2013), and in the Arctic Flanner, 2013).…”

“…In most cases, snow and ice samples were collected quite a long time after snow fall, and the concentration of pollutants would also have increased in the surface snow and ice due to dry deposition. It seems likely that the pollutants in surface samples would be affected by sublimation and deposition until the next melt season (Yang et al, 2015). In some of the cases in our study, the average concentration of BC, OC, and/or dust for a particular glacier/site was increased as a result of a single highly concentrated sample, reflecting the wide variation that results from the interplay of many factors.…”

Black carbon and mineral dust in snow cover on the Tibetan Plateau

“…The sample punch was placed in a quartz holder oriented in the direction of the carrier gas flow, and heated gradually to a specific temperature plateau (typically 140, 280, 480, 550 • C) in a pure helium atmosphere, which resulted in OC concentrations from the sample. Due to the influence of dust on the quantitative analysis of BC and OC in snow when using a thermal-optical method , we limited the initial temperature plateau (550 • C) to reduce the time that BC was exposed to a catalyzing atmosphere (Yang et al, 2015). The sample was reheated further in a stepwise fashion to near 840 • C in an oxygen-containing atmosphere (usually 2 % oxygen and 98 % helium) to burn out all remaining BC.…”

“…Quartz filters were then used to analyze BC and OC with a thermaloptical carbon analyzer (DRI 2001A model, Desert Research Institute, NV, USA) following a methodology (IMPROVE_A protocol) used in previous studies (Cao et al, 2003;Chow et al, 2004;Xu et al, 2009;Yang et al, 2015). Its function relies on the fact that OC components can be volatilized from the sample and deposited in a non-oxidizing helium atmosphere, in which BC can be combusted with an oxidizer.…”

“…Recently, the TP cryosphere has undergone rapid changes (Kang et al, 2010), including glacial shrinkage (Xu et al, 2009;Yao et al, 2012b), permafrost degradation (Cheng and Wu, 2007), and reduction in the annual duration of snow cover days (Ménégoz et al, 2014;Xu et al, 2017). Studies on the TP indicate that BC has been a significant contributing factor to the observed cryospheric change Ming et al, 2009;Niu et al, 2017;Xu et al, 2009;Yang et al, 2015;Zhang et al, 2017) and a major forcer of climate change Ji, 2016). Observations and simulations also found that dust deposition on snow/ice can change the surface albedo and perturb the surface radiation balance (Ji, 2016;Qu et al, 2014), resulting in a decrease of 5-25 mm snow water equivalent (mm w.e.)…”