Aerosol dry deposition fluxes on snow during the ALPACA campaign in Fairbanks, Alaska

Abstract. A comprehensive study of aerosol exchange surface fluxes was conducted at a suburban site in Fairbanks (Alaska) during the Arctic winter as part of the ALPACA experiment. Aerosol fluxes were measured by an eddy covariance system on a snow-covered field located at the University of Alaska Fairbanks (UAF) Farm site from 26 January to 17 February 2022. Overall, the flux measurements indicate that the site acted mainly as an emission source for ultrafine particles, while the fluxes for larger particle sizes were substantially bidirectional. Median deposition velocities were 0.61, 0.04, and 8.73 mm s−1 for ultrafine (< 50 nm), accumulation (0.25–0.8 µm), and quasi-coarse (0.8–3 µm) particles, respectively. Anticyclonic synoptic meteorological conditions enhanced atmospheric stagnation and favoured pollutant accumulation near the surface, whereas cyclonic conditions increased aerosol dispersion, thus reducing deposition rates. Despite the frequent conditions of atmospheric stability and pronounced temperature inversions resulting from the strong surface radiative cooling, turbulence was generated mechanically by wind friction, leading to particle deposition. Our findings provide quantitative evidence that wintertime aerosol dry deposition in Arctic urban areas contributes significantly to pollutant accumulation in the snowpack, potentially enhancing contaminant remobilization during snowmelt. Finally, this study provides data for improving aerosol transport models and understanding pollutant-snow interactions in cold urban regions.