Tracing the origin of black carbon deposition over the Greenland ice sheet to forest fires in Canada.

Abstract : The processes that result in aerosol deposition within the Arctic are currently a key uncertainty in our ability to understand Arctic change. Aerosol deposition is a dominant source of light-absorbing impurities, including black carbon, found in Arctic ice and snow. Trace amounts of light absorbing impurities in snow are important because they are used to interpret past pollution trends (e.g. fire frequency) using ice cores and because they have important climate impacts (warming) due to their modification of snow and ice albedo. Here, we focus on the role of biomass burning in controlling the amount of black carbon deposited on the Greenland ice sheet. We study a specific case of aerosol deposition to the Greenland ice sheet by combining extensive snow pit measurements with simulations using the regional model WRF-Chem. Light absorbing impurities were measured in snow pits (in 2014) and snow accumulation rates (2013-2014) were monitored at several remote sites on the Greenland ice sheet as part of the SAGE project. The largest black carbon deposition quantity measured was traced to a snow accumulation event that occurred in late July and early August 2013. In order to understand the origin and identify the processes controlling the observed deposition event, the regional model WRF-Chem is used (run from 17 July – 5 August 2013) combined with satellite observations (MODIS and CALIPSO/CALIOP). The model simulation includes anthropogenic and fire emissions in North America as well as transport and chemical/physical transformations of aerosols. Model results show that the observed deposition event can be traced to fires burning in northern Canada in late July 2013. The processes controlling aerosol deposition will be discussed including the critical role of transport pathways and wet removal processes, which are essential in controlling the fate of emissions within the Arctic region.