Surface snow bromide and nitrate at Eureka, Canada in early spring and implications for polar boundary layer chemistry

This study explores the role of snowpack in polar boundary layer chemistry, especially as a direct source of reactive bromine (BrOX=BrO+Br) and nitrogen (NOX=NO+NO2) in the Arctic springtime. Surface snow samples were collected daily from a Canadian high Arctic location at Eureka, Nunavut (80°N, 86°W) from the end of February to the end of March in 2018 and 2019. The snow was sampled at several sites representing distinct environments: sea ice, inland close to sea level, and a hilltop ~600 m above sea level.  At all sites, snow sodium and chloride concentrations increase by almost tenfold from the top 0.2 cm down to a depth of ~1.5 cm. Surface snow bromide at sea level is significantly enriched, indicating a net sink of atmospheric bromine. Moreover, surface snow bromide at sea level has an increasing trend over the measurement period, with mean slopes of 0.024 mM d-1 in the 0-0.2 cm layer and 0.016 mM d-1 in the 0.2-0.5 cm layer. Surface snow nitrate at sea level also shows a significant increasing trend, with mean slopes of 0.27, 0.20, and 0.07 mM d-1 in the top 0.2 cm, 0.2-0.5 cm, and 0.5-1.5 cm layers, respectively. Using these trends, an integrated net deposition flux of bromide of (1.01±0.48)×107 molecules cm-2 s-1 and an integrated net deposition flux of nitrate of (2.6±0.37)×108 molecules cm-2 s-1 were derived. In addition, the surface snow nitrate and bromide at inland sites were found to be significantly correlated (R=0.48-0.76) with the [NO3-]/[Br-] ratio of 4-7 indicating a possible acceleration effect of reactive bromine in atmospheric NOX-to-nitrate conversion. This is the first time such an effect has been seen in snow chemistry data obtained with a sampling frequency as short as one day. BrO partial column (0-4 km) data measured by MAX-DOAS show a decreasing trend in March 2019, which agrees with the derived surface snow bromide deposition flux. This indicates that bromine in the Eureka atmosphere and surface snow did not reach a photochemical equilibrium state and that the photochemical release flux of reactive bromine from snow must be a weak process and smaller than the derived bromide deposition flux of ~1×107 molecules cm-2 s-1.