Rapid Reduction and Reemission of Mercury Deposited into Snowpacks during Atmospheric Mercury Depletion Events at Churchill, Manitoba, Canada

Abstract

Mercury (Hg) in some Arctic marine mammals has increased to levels that may be toxic to Northern peoples consuming them as traditional food. It has been suggested that sunlight-induced atmospheric reactions called springtime atmospheric Hg depletion events (AMDEs) result in the loading of -150-300 tons of Hg to the Canadian Arctic archipelago each spring and that AMDEs are the ultimate source of Hg to Arctic foodwebs. AMDEs result from the oxidation of gaseous elemental Hg0 (GEM) in Arctic atmospheres to reactive gaseous Hg (RGM) and particulate Hg (pHg), both of which fall out of the atmosphere to snowpacks. We studied the springtime cycling of Hg between air and snowpacks near Churchill, Manitoba, for 2 years to determine the net input of Hg to Hudson Bay from AMDEs. In 2004, we monitored atmospheric concentrations of GEM, pHg, and RGM while simultaneously measuring concentrations of total Hg (THg) in surface snow collected over the sea ice on Hudson Bay. During numerous springtime AMDEs, concentrations of THg in surface snow increased, often to over 60 ng/L, demonstrating that AMDEs resulted in deposition of oxidized Hg (Hg(II)) to snowpacks. However, immediatelyfollowing AMDEs, average concentrations of THg in snow declined drastically from between 67.8+/-7.7 ng/L during AMDEs to only 4.25+/-1.85 ng/L four or more days following them. In 2003, we measured THg in surface snow collected daily over the sea ice and total gaseous Hg (TGM) concentrations in the interstitial airspaces of snowpacks. When concentrations of THg in the surface snow decreased, concentrations of TGM in interstitial airspaces of the snowpack increased sharply from between approximately 1.4-3.4 ng/m(3) to between approximately 20-150 ng/m(3), suggesting thatthere was a reduction of deposited Hg(II) to GEM, which then diffused out of snowpacks. At snowmelt in both 2003 and 2004, average concentrations of THg in meltwater collected over Hudson Bay were only 4.04+/-2.01 ng/L. Using concentrations of THg in meltwater and snow water equivalent, we estimated a net springtime loading of only 2.1+/-1.7 mg/ha of Hg to Hudson Bay from AMDEs, indicating that only a small portion of the Hg(II) deposited during AMDEs enters Hudosn Bay each spring.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[245] In contrast, it is now well established that the Hg deposited during AMDEs can be readily re-emitted from the snowpack during winter conditions following AMDEs [7,16,25,80,101,187,241,[246][247][248] and during snow metamorphism and melt. [20,79] Establishing a link between AMDEs and enhanced Hg levels in Arctic biota is complicated partly by a poor understanding of the net outcome of the AMDE deposition pathway, and partly by the complex Hg biogeochemistry of aquatic marine and fresh water ecosystems that contain inorganic Hg II from a variety of sources (of which AMDEs are but one) and methylate the inorganic Hg II into MeHg.…”

“…[252] Results from many Arctic locations suggest that 60-80 % of total deposited Hg is photo-reduced to volatile Hg 0 and re-emitted back to the atmosphere within days of AMDEs. [7,80,101,187,241,247,248] Fig. 10 includes a summary of the results from 10 different studies [247] that monitored the THg concentration in snow with time since an AMDE was active.…”

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

“…Measurements performed at Toolik, with Hg diss concentrations averaging 0.17 ng L −1 and ranging between 0.08 and 1.15 ng L −1 , showed very low levels compared to many other high-latitude studies. This is generally lower than Hg concentrations in interior Arctic sites reported by Douglas and Sturm (2004) (i.e., Hg diss concen- trations between 0.5 and 1.7 ng L −1 ) and at the low end of concentrations found in Arctic studies along the coastal zone (0.14-820 ng L −1 , for both Hg diss and Hg tot ; Douglas et al, 2005;Douglas and Sturm, 2004;Ferrari et al, 2004Ferrari et al, , 2005Kirk et al, 2006;Nerentorp Mastromonaco et al, 2016;St. Louis et al, 2005;Steffen et al, 2002).…”

“…An absence of direct solar radiation likely explains the lack of photochemical Hg 0 gas formation and volatilization between December and mid-January. However, springtime is a photochemically active period in the Arctic when strong Hg 0 gas volatilization from snow has been reported further north along the Arctic Ocean coast (Brooks et al, 2006;Kirk et al, 2006). Even in late spring, however, when abundant solar radiation is present (400-600 W m −2 ), Hg 0 gas volatilization losses were rare and largely limited to periods of active AMDEs.…”

“…Surface snow Hg concentration enhancements during AMDEs are commonly reported in polar regions, with at times Hg concentration enhancements up to 100 times the base concentration in the Arctic (Lalonde et al, 2002;Lindberg et al, 1998;Poulain et al, 2004;Steffen et al, 2002). The presence of AMDEs generally results in increased deposition of Hg to snow and ice surfaces, yet such additional deposition often is short-lived due to the photochemical re-emission of Hg 0 gas (Kirk et al, 2006). In our study, we did not have sufficient temporal resolution of snow sampling during the period of AMDEs to closely track the fate of Hg deposition during AMDEs and subsequent re-emissions.…”