Understanding mercury oxidation and air–snow exchange on the East Antarctic Plateau: a modeling study

Abstract

Abstract. Distinct diurnal and seasonal variations of mercury (Hg) have been observed in near-surface air at Concordia Station on the East Antarctic Plateau, but the processes controlling these characteristics are not well understood. Here, we use a box model to interpret the Hg0 (gaseous elemental mercury) measurements in thes year 2013. The model includes atmospheric Hg0 oxidation (by OH, O3, or bromine), surface snow HgII (oxidized mercury) reduction, and air–snow exchange, and is driven by meteorological fields from a regional climate model. The simulations suggest that a photochemically driven mercury diurnal cycle occurs at the air–snow interface in austral summer. The fast oxidation of Hg0 in summer may be provided by a two-step bromine-initiated scheme, which is favored by low temperature and high nitrogen oxides at Concordia. The summertime diurnal variations of Hg0 (peaking during daytime) may be confined within several tens of meters above the snow surface and affected by changing mixed layer depths. Snow re-emission of Hg0 is mainly driven by photoreduction of snow HgII in summer. Intermittent warming events and a hypothesized reduction of HgII occurring in snow in the dark may be important processes controlling the mercury variations in the non-summer period, although their relative importance is uncertain. The Br-initiated oxidation of Hg0 is expected to be slower at Summit Station in Greenland than at Concordia (due to their difference in temperature and levels of nitrogen oxides and ozone), which may contribute to the observed differences in the summertime diurnal variations of Hg0 between these two polar inland stations.

Diurnal cycle of iodine, bromine, and mercury concentrations in Svalbard surface snow

“…The emitted gas phase iodine would then readily form reservoir species (HOI, IONO 2 , HI) (Saiz-Lopez et al, 2014) that, once photochemistry ceases, could deposit and accumulate in the snow or ice until the following sunrise, when re-emission starts again. Active mercury recycling from the snowpack has already been suggested or observed by several authors (Dommergue et al, 2012;Durnford and Dastoor, 2011;Song et al, 2018;Steffen et al, 2008). Mercury in its oxidized forms can be deposited onto the snowpack, increasing total Hg concentrations in the upper snow strata.…”

“…Nighttime mercury reactions have been thought to occur. Angot et al (2016b) suggested that mercury deposition onto the snow surface in the dark could be due to several mechanisms, including gas phase oxidation, heterogeneous reactions, or dry deposition of Hg(0) (Angot et al, 2016b;Song et al, 2018). This hypothesis, however, is based on results obtained at Dome C on the Antarctic Plateau over the entire winter season, conditions very different to those in Svalbard.…”

“…Polar regions are being increasingly studied for their important roles in global climate and atmospheric chemical cycles. Multiple studies have improved our understanding of atmospheric processes in polar regions, ranging from new particle formation processes (Dall'Osto et al, 2017;Sipilä et al, 2016), ozone destruction processes (Saiz-Lopez et al, 2007;Simpson et al, 2007), the role of halogens in polar atmospheric processes Spolaor et al, 2013a), the mercury cycle (Angot et al, 2016a;Aspmo et al, 2005;Dommergue et al, 2003a;Durnford and Dastoor, 2011;Skov et al, 2006) to atmospheric transport and deposition of natural and anthropogenic compounds (Moroni et al, 2015(Moroni et al, , 2017Udisti et al, 2016;Zangrando et al, 2013). The polar regions are characterized by periods with 24 h of continuous solar radiation (April to September in the Arctic), periods when the night and day cycle is present (February to March and September to October in the Arctic) and periods of continuous darkness (November to January in the Arctic), the so-called polar night.…”

“…Mercury in its oxidized form can be deposited onto the snowpack, increasing Hg concentrations in the upper snow strata (Obrist et al, 2017). Once present in the snowpack, Hg is very labile, and it can be reduced back to elemental Hg (Hg(0)) and undergo dynamic exchange with the atmosphere (Song et al, 2018;Spolaor et al, 2018;Steffen et al, 2002). The role of the snowpack is crucial in the mercury cycle in polar regions since it acts as both a sink (deposition, accumulation) and a source (re-emission).…”

Diurnal cycle of iodine and mercury concentrations in Svalbard surface snow

“…Mercury in its oxidized form can be deposited onto the snowpack, increasing Hg concentrations in the upper snow strata (Obrist et al, 2017). Once present in the snowpack, Hg is very labile, and it can be reduced back to elemental Hg (Hg(0)) and undergo dynamic exchange with the atmosphere above (Song et al, 2018;Spolaor et al, 2018;Steffen et al, 2002). The role of the snowpack is crucial in the mercury cycle in Polar Regions since it acts as both a sink (deposition, accumulation) and a source (re-emission).…”