Quantifying black carbon in Arctic and Antarctic snow: Seasonal variability and implications for snowmelt

Snow and ice play a crucial role in the Earth’s radiation budget, not least due to their high albedo. The deposition of light-absorbing aerosols such as soot (black carbon, BC) reduces this reflectivity and can considerably accelerate melting processes. The primary focus of the present study is the development of an artefact-free method for quantifying BC in snow using a Single Particle Soot Photometer (SP2). This method forms the basis for investigating the seasonal and spatial variability of BC concentrations in snow in polar regions, with particular emphasis on comparing the Arctic and Antarctic, and subsequently quantifying the impact on snowmelt. The developed method specifically addresses the matrix effect, as described by Zanatta M. et al. (Atmos. Chem. Phys. 21: 9329–9342, 2021), whereby the increased electrical conductivity of saline samples—as frequently encountered in sea ice regions—leads to a significant underestimation of BC concentrations. This effect was systematically quantified and empirically based corrections were derived. For samples with high salinity levels that do not permit analysis with the SP2, a desalination procedure was established. In a methodological comparison of two approaches (electrodialysis and vacuum filtration), vacuum filtration was identified as the most cost-effective, efficient, and reproducible method, hence it was applied for the main analyses. The combination of mathematical correction and vacuum filtration enables, ideally, an unbiased analysis of saline snow samples. Surface and profile snow samples collected during the year-round MOSAiC expedition (Multidisciplinary drifting Observatory for the Study of Arctic Climate, 2019–2020) were analysed using the developed methodology. The resulting dataset represents the first continuous survey of BC concentrations in snow over almost an entire year in the central Arctic. The BC concentrations determined with the new method ranged between 0. 03 and 75. 11 ng/g. Without the salinity correction, BC concentrations would be underestimated by up to 40% in the MOSAiC samples. Higher concentrations were generally found in summer samples, indicating a seasonal trend. Potential superimposed BC contaminations from local activities are critically discussed, as they complicate the identification of genuine seasonal patterns and unbiased mean values. The hemispheric comparison was performed using snow samples collected during the REBCASA field campaign (Radiative Effect due to BC by combining Atmospheric and Snow measurements in Antarctica, 2021) at Neumayer Station III in East Antarctica. The concentrations measured there were approximately two to three orders of magnitude lower than in the Arctic samples and showed less horizontal variability. A possible annual cycle is suggested, but cannot be clearly identified. In the next step, the measured BC concentrations were fed into the radiative transfer model SNICAR (Snow, Ice, and Aerosol Radiative model) to quantify the impact on albedo and snowmelt processes. In addition, sensitivity studies were conducted covering the full range of BC concentrations observed during MOSAiC. The highest measured concentrations, presumably influenced by local activities, resulted in peak instantaneous radiative forcings of over 6W/m2 at maximum solar elevation. These values are considerably higher than those calculated for the remaining samples (0. 05–1. 86W/m2). Accordingly, for the highest BC concentrations, melt values of over 20 cm water equivalent were found, whereas assuming mean concentrations resulted in significantly lower values. For the Antarctic samples, albedo reductions were found in the order of Δα ≈ 10^ (−4) –10^ (−5), with a resulting instantaneous radiative forcing of less than 0. 1W/m2. The reasons for the hemispheric differences in the instantaneous radiative forcing caused by BC are the land–ocean distribution, the distribution of emission sources, and the respective characteristics of atmospheric transport processes. This work provides new insights into the role of BC in polar snow. The methods developed and their application to the MOSAiC snow samples contribute to improved quantification of aerosol-related feedbacks in cryospheric processes and provide a valuable basis for the modelling and interpretation of future measurement campaigns.