Research progress of light-absorbing impurities in glaciers of the Tibetan Plateau and its surroundings

Abstract

Light-absorbing impurities (LAIs, including black carbon, organic carbon, and dust) deposited on glacier surface can have strong effects on mass and energy balance of the cryosphere. Widespread with amount of glaciers, the Tibetan Plateau (TP) is considered to be an ideal region to study the impact of LAIs to glacier melt. Based on recent studies of LAIs in glacial snow of the TP, we summarized the research progress of LAIs in glacial snow, including their concentrations and spatial distributions, especially focusing on discussion of sources of black carbon, and documenting the albedo reduction and radiative forcing caused by LAIs. Additionally, we identified research gaps and suggested future research directions. At present, spatial distribution of LAIs in glacial snow and ice from TP and its surroundings exist a significant difference due to natural/anthropogenic emission sources, local topography, atmospheric transportation, and different glacial surface (e.g., fresh snow, aged snow, granular ice, bare ice). In general, higher concentrations of BC occurred in the northern and southern regions; LAIs in aged snow or granular ice were higher by 1–2 order of magnitude than that in the fresh snow or snowpit. Characteristics of LAIs from different glacial surfaces were still limited especially in Tienshan regions, which need to be further studies. Since Industrial Revolution, BC in ice cores increased in recent decades which may be likely caused by increasing emissions of fossil fuel combustion and biomass burning in the South Asia and Central Asia. The process of snowmelt can enrich the BC concentrations in surface and change the mixture of BC with snow grains, which will further enhance the albedo reduction. Studies on post-deposition processes of other LAIs and their effects on snow surface albedo are urgent to be strengthened. Simulation results indicated that BC aerosol of the southern TP mainly originated from the South Asia, which can contributed to approximately 50% in non-monsoon season and 30% in monsoon season; anthropogenic sourced BC can contributed to 30%–70%. However, studies on enrichment and transport processes of LAIs in glacial surface snow still should be emphasized. Although LAIs initiated powerful snow albedo feedbacks and glacier melt, estimation of this effect was associated with a large uncertainty. Based snow ice and aerosol radiation (SNICAR) model, effects of BC and dust on albedo reduction were estimated to be less than 10% in the southern TP and more than 25% in the central and northern TP influencing by snow types. Simulation of BC on albedo effects were affected by mixture of BC, coatings with other components, and snow grains, which lead to different radiative forcing in different regions. For aged snow or granular ice, effects of BC and dust can give localized instantaneous radiative forcing to about near or more than 100 W m−2; for fresh or snowpit, BC and dust can result in a minor radiative forcing (mostly less than 10 W m−2). These effects of LAIs on albedo reduction and radiative forcing enhanced glacier melt, which will further affect the hydrological processes. Estimation represented that LAIs in snow can lead to approximately 15% of the total glacier melt in the southeast TP and 6.3% increasing glacier melt in the Pamir regions. However, there still existed differences between the observations and simulations of LAIs in snow, which was mostly related to the different deposition flux and snow accumulation. In the future, mixing state of LAIs in snow, and their synergistic effects of biogeochemical process and climate change will be a new research direction.