Mercury in the Arctic tundra snowpack: temporal and spatial concentration patterns and trace gas exchanges

Abstract

Abstract. In the Arctic, the snowpack forms the major interface between atmospheric and terrestrial cycling of mercury (Hg), a global pollutant. We investigated Hg dynamics in an interior Arctic tundra snowpack in northern Alaska during two winter seasons. Using a snow tower system to monitor Hg trace gas exchange, we observed consistent concentration declines of gaseous elemental Hg (Hg 0 gas ) from the atmosphere to the snowpack to soils. The snowpack itself was unlikely a direct sink for atmospheric Hg 0 gas . In addition, there was no evidence of photochemical reduction of Hg II to Hg 0 gas in the tundra snowpack, with the exception of short periods during late winter in the uppermost snow layer. The patterns in this interior Arctic snowpack thus differ substantially from observations in Arctic coastal and temperate snowpacks. We consistently measured low concentrations of both total and dissolved Hg in snowpack throughout the two seasons. Chemical tracers showed that Hg was mainly associated with local mineral dust and regional marine sea spray inputs. Mass balance calculations show that the snowpack represents a small reservoir of Hg, resulting in low inputs during snowmelt. Taken together, the results from this study suggest that interior Arctic snowpacks are negligible sources of Hg to the Arctic.

Insights from mercury stable isotopes on terrestrial–atmosphere exchange of Hg(0) in the Arctic tundra

“…The climate of Toolik Field Station is char-acterised by low mean annual temperatures of −8.5 • C and a mean annual precipitation of 312 mm a −1 (Cherry et al, 2014). During the 1-year Hg isotope campaign from October 2015 to September 2016, the tundra was snow-covered for a total of 248 d (Agnan et al, 2018), leading to a relatively short snow-free growing season.…”

“…In recent years, a number of studies using Hg(0) flux measurements based on micrometeorological methods directly quantified net ecosystem exchange fluxes of Hg(0) over terrestrial ecosystems (i.e. at the ecosystem level), including atmosphere-vegetation exchange and underlying soil/litter contributions (Lindberg et al, 1998;Fritsche et al, 2008;Bash and Miller, 2009;Castro and Moore, 2016;Osterwalder et al, 2017). Measurements of multi-level Hg(0) gradients and interstitial snow air and soil pore air provided additional constraints on the terrestrial surface exchange flux (Sigler and Lee, 2006;Moore and Castro, 2012;Faïn et al, 2013;Obrist et al, 2014;Fu et al, 2016b;Agnan et al, 2018).…”

“…In Obrist et al 2017, we performed a 2-year mass balance of terrestrial-atmosphere exchange over the Arctic tundra. We also investigated the spatial distribution of Hg in tundra soils (Olson et al, 2018), and spatial and temporal patterns of Hg in snow (Agnan et al, 2018) and in vegetation (Olson et al, 2019). In our previous work, we showed that the uptake of atmospheric Hg(0) by vegetation and soil represents 70 % of total atmospheric deposition and has led to high Hg levels in Arctic soils (Obrist et al, 2017(Obrist et al, , 2018.…”

Environmental controls on ecosystem-scale cold season methane and carbon dioxide fluxes in an Arctic tundra ecosystem

“…A snow tower (Seok et al, 2009;Faïn et al, 2013) was installed prior to the first snowfall and recorded temperatures at 0, 10, 20, 30, 40 and 110 cm above the soil surface, thus measuring temperatures within the snowpack as it developed above each measurement height. The average snowpack depth over the site was measured daily using a camera set to automatically record images of reference snow stakes (Agnan et al, 2018). These depth measurements began in November 2014, and so the first snowfalls in that year were not recorded.…”

“…Arctic tundra ecosystems are highly heterogeneous within the scale of micrometeorological flux footprints (typically 10s to 100s metres Kljun et al, 2015;Fox et al, 2008), and during the winter, the combined effects of wind and topography lead to even greater spatial heterogeneity in snow depths and snow physical properties (Agnan et al, 2018). Sub-surface soil temperatures, which are further influenced by air temperature and downwelling radiation; overlying vegetation and snow; and soil properties and moisture, are likely highly spatially variable within the footprint as well.…”