The Roles of Light Absorbing Particles in the Norwegian Arctic Snow

Abstract

Light absorbing particles (LAPs) such as black carbon (BC) and dust can reduce snow albedo and have a positive radiative forcing. Previous studies investigating LAP in snow in Tromsø focused on black carbon (BC) also called elemental carbon because it has human sources, but dust should also be considered because, while largely from natural sources, it is present in higher concentrations. To quantify the relative contribution of dust and BC in snow albedo reductions, snow samples were collected in Tromsø (Fjellheisen and Ekrehagen), Norway during the winter season from December 2022 to March 2023, both from surface snow and vertical profiles through the snow column. The samples were analyzed for particle concentration using gravimetric filtration, which provides a proxy of dust deposition. Snow reflectance was measured at both sites using a spectroradiometer to estimate the variation in snow reflectance from both sites. The SNICAR snow albedo model was used to evaluate the relative contribution of BC and dust to albedo reductions. The results from this study show that snow melt during warm periods, long dry periods of little additional accumulation of snowfall and wind all increase particle concentration in the surface layer of snowpack. Fresh snowfall generally has low gravimetric particle concentration. Snow particle concentrations are higher at Ekrehagen (70 m a.s.l.) than Fjellheisen (420 m a.s.l.) due to the 350 m elevation difference of the two sites. Ekrehagen has a higher occurrence of melt and rain, both of which increase particle concentrations, while Fjellheisen receives more snow and has a lower occurrence of times when particle concentrations are high on the surface of the snowpack. This has implications under a changing climate. As climate warms, more areas are more likely to get more rain and to have higher particle concentration surfaces, resulting in lower albedo, greater energy absorption, more melt of the snowpack and more energy kept in the Earth System. The SNICAR modeled spectral snow albedo indicates that dust is a greater driver of albedo reduction than BC. In both winter and spring dust only scenarios lower albedo more than BC only scenarios. However, the combination of BC and dust results in a further lower albedo reduction, but dust is the LAP that is most driving albedo reductions in the snow, with larger albedo reductions in late spring than in winter. The SNICAR modeled spectral snow albedo shows that 22% to 26% more of the incoming solar energy is being absorbed by the snowpack in spring relative to winter and contributing to melt. This is a large change in the surface energy balance. Therefore, it is important to include dust in analyses of LAP induced albedo reductions in snow.