Preprints
https://doi.org/10.5194/egusphere-2022-696
https://doi.org/10.5194/egusphere-2022-696
 
18 Oct 2022
18 Oct 2022
Status: this preprint is open for discussion.

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

Xin Yang1, Kimberly Strong2, Alison S. Criscitiello3, Marta Santos-Garcia1,a, Kristof Bognar2,b, Xiaoyi Zhao4, Pierre Fogal2, Kaley A. Walker2, Sara M. Morris5, and Peter Effertz6,7 Xin Yang et al.
  • 1British Antarctic Survey, Natural Environment Research Council, Cambridge, UK
  • 2Department of Physics, University of Toronto, Toronto, ON, Canada
  • 3Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
  • 4Air Quality Research Division, Environment and Climate Change Canada, Toronto, ON, Canada
  • 5NOAA Earth System Research Laboratories, Physical Sciences Laboratory, Boulder, CO, USA
  • 6Cooperative Institute for Research in Environmental Science - CU Boulder, Boulder, CO, USA
  • 7NOAA Earth System Research Laboratories, Global Monitoring Laboratory, Boulder, CO, USA
  • anow at: School of Geosciences, University of Edinburgh, Edinburgh, UK
  • bnow at: 3v Geomatics Inc., Vancouver, BC, Canada

Abstract. This study explores the role of snowpack in polar boundary layer chemistry, especially as a di-rect 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 (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 NOX-to-nitrate con-version. This is the first time we see such an effect over the course of one day.

Xin Yang et al.

Status: open

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Xin Yang et al.

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Short summary
Snow pack in high Arctic plays a key role in polar atmospheric chemistry, especially in spring when photochemistry becomes active. By sampling surface snow from a Canadian high Arctic location at Eureka, Nunavut (80° N, 86° W), we demonstrate that surface snow is a net sink rather than a source of atmospheric reactive bromine and nitrate. This finding is new and opposite to previous conclusions that snowpack is a large and direct source of reactive bromine in polar spring.