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 di-rect source of reactive bromine (BrO_X=BrO+Br) and nitrogen (NO_X=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 (asl).

At the inland sites, surface snow salinity has a double-peak distribution with the first and low-est peak at 0.001–0.002 practical salinity unit (psu), which corresponds to the precipitation ef-fect, and the second peak at 0.01–0.04 psu, likely due to the condensation effect. 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 0.2–0.4 psu can be clearly attributed to sea water contamination.

At all sites, sodium and chloride concentrations in surface snow increase by almost 10-fold from the top 0.2 cm to ~1 cm in depth. Bromide in surface snow is significantly enriched, indi-cating that surface snow at Eureka is a net sink of atmospheric bromine. Moreover, daily data show that top surface snow bromide at all sampling sites has an increasing trend over the measurement time period (late February to late March), with mean slopes of 1.9 and 1.3 ppb d^-1 in the 0–0.2 cm and the 0.2–0.5 cm layers, respectively. At the sea level sites, snow nitrate also shows a significant increasing trend, with mean slopes of 12.1, 12.4, and 4.3 ppb d^-1 in the top 0.2 cm, 0.2–0.5 cm, and 0.5–1.5 cm layers, respectively. Using these trends, we derive a novel method to calculate deposition flux of bromide and nitrate to the snowpack. For bromide, the integrated deposition flux is 1.29×10⁷ molecules cm^-2 s^-1 at sea level and 1.01×10⁷ molecules cm^-2 s^-1 at ~600 m. For nitrate, the integrated deposition flux is 2.4×10⁸ molecules cm^-2 s^-1 at sea level and -1.0×10⁸ molecules cm^-2 s^-1 at ~600 m; the negative flux indicates that snow at the hilltop sites is losing nitrate. The smaller vertical gradient of bromide deposition flux strongly indicates that local snowpack emission on sea ice and inland is not likely to be a large source of reactive bromine. In contrast, nitrate deposition flux has a large vertical gradient, e.g., with a positive flux at sea level and a negative flux at ~600 m, indicating that snowpack at sea level is a large source of reactive nitrate.

In addition, we found a significant correlation (with coefficient R values of 0.48-0.76) between surface snow nitrate and bromide at the inland sites. The [NO₃-] / [Br^-] ratio ranges from 4 to 7, highlighting the effect of reactive bromine in accelerating the atmospheric NO_X-to-nitrate con-version. This is the first time we see such an effect over the course of one day.