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

Abstract

This study explores the role of snowpack in polar boundary layer chemistry, especially as a direct source of reactive bromine (BrOxgCombining double low linegBrOg+gBr) and nitrogen (NOxgCombining double low linegNOg+gNO2) in the Arctic springtime. Surface snow samples were collected daily from a Canadian high Arctic location at Eureka, Nunavut (80°gN, 86°gW) 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 g1/4g600gm above sea level (a.s.l.). At the inland sites, surface snow salinity has a double-peak distribution with the first and lowest peak at 0.001-0.002 practical salinity unit (psu), which corresponds to the precipitation effect, and the second peak at 0.01-0.04gpsu, which is likely related to the salt accumulation effect (due to loss of water vapour by sublimation). Snow salinity on sea ice has a triple-peak distribution; its first and second peaks overlap with the inland peaks, and the third peak at 0.2-0.4gpsu is likely due to the sea water effect (a result of upward migration of brine). At all sites, snow sodium and chloride concentrations increase by almost 10-fold from the top 0.2 to g1/4g1.5gcm. 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 0.024gμMd-1 in the 0-0.2gcm layer and 0.016gμMd-1 in the 0.2-0.5gcm layer. Surface snow nitrate at sea level also shows a significant increasing trend, with mean slopes of 0.27, 0.20, and 0.07gμMd-1 in the top 0.2, 0.2-0.5, and 0.5-1.5gcm layers, respectively. Using these trends, an integrated net deposition flux of bromide of (1.01g±g0.48)g×g107gmolec.cm-2s-1 and an integrated net deposition flux of nitrate of (2.6g±g0.37)g×g108gmolec.cm-2s-1 were derived. In addition, the surface snow nitrate and bromide at inland sites were found to be significantly correlated (RgCombining double low lineg0.48-0.76) 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 1gd. BrO partial column (0-4gkm) data measured by MAX-DOAS show a decreasing trend in early spring, which generally agrees with the derived surface snow bromide deposition flux indicating that bromine in Eureka atmosphere and surface snow did not reach a photochemical equilibrium state. Through mass balance analysis, we conclude that the average release flux of reactive bromine from snow over the campaign period must be smaller than the derived bromide deposition flux of g1/4g1g×g107gmolec.cm-2s-1. Note that the net mean fluxes observed do not completely rule out larger bidirectional fluxes over shorter timescales.