High variability of atmospheric mercury in the summertime boundary layer through the central Arctic Ocean

The Marginal Ice Zone as a dominant source region of atmospheric mercury during central Arctic summertime

Atmospheric gaseous elemental mercury (GEM) concentrations in the Arctic exhibit a clear summertime maximum, while the origin of this peak is still a matter of debate in the community. Based on summertime observations during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition and a modeling approach, we further investigate the sources of atmospheric Hg in the central Arctic. Simulations with a generalized additive model (GAM) show that long-range transport of anthropogenic and terrestrial Hg from lower latitudes is a minor contribution (~2%), and more than 50% of the explained GEM variability is caused by oceanic evasion. A potential source contribution function (PSCF) analysis further shows that oceanic evasion is not significant throughout the ice-covered central Arctic Ocean but mainly occurs in the Marginal Ice Zone (MIZ) due to the specific environmental conditions in that region. Our results suggest that this regional process could be the leading contributor to the observed summertime GEM maximum. In the context of rapid Arctic warming and the observed increase in width of the MIZ, oceanic Hg evasion may become more significant and strengthen the role of the central Arctic Ocean as a summertime source of atmospheric Hg.

Latitudinal Distribution of Gaseous Elemental Mercury in Tropical Western Pacific: The Role of the Doldrums and the ITCZ

The role of the tropical western Pacific in the latitudinal distribution of atmospheric mercury is still unclear. In this study, we conducted continuous measurements of gaseous elemental mercury (GEM) in the marine boundary layer (MBL) along a large latitudinal transect (∼60° S to ∼30° N) of the western Pacific, accompanied by measurements of dissolved gaseous mercury (DGM) in the surface seawater. We found that the GEM latitudinal gradient is the most significant in the tropical western Pacific, which to some extent might be attributed to the impact of the doldrums and the Intertropical Convergence Zone (ITCZ) in this area. For the doldrums, calm weather may delay the transport of GEM, facilitating its accumulation in the tropical western Pacific. Furthermore, the regional transport, and low O₃ and sea-salt aerosol levels in this area which would not favor the oxidation of GEM in the MBL, would intensify the accumulation of GEM in the tropical western Pacific. For the ITCZ, the vast wet deposition of Hg would drive elevated DGM in the surface seawater, which can increase the evasion flux and may further influence the spatial distribution of GEM. This study provides insight into the role of the tropical western Pacific in the regional atmospheric mercury cycle.

δ(13)C-CH4 reveals CH4 variations over oceans from mid-latitudes to the Arctic

The biogeochemical cycles of CH4 over oceans are poorly understood, especially over the Arctic Ocean. Here we report atmospheric CH4 levels together with δ(13)C-CH4 from offshore China (31°N) to the central Arctic Ocean (up to 87°N) from July to September 2012. CH4 concentrations and δ(13)C-CH4 displayed temporal and spatial variation ranging from 1.65 to 2.63 ppm, and from -50.34% to -44.94% (mean value: -48.55 ± 0.84%), respectively. Changes in CH4 with latitude were linked to the decreasing input of enriched δ(13)C and chemical oxidation by both OH and Cl radicals as indicated by variation of δ(13)C. There were complex mixing sources outside and inside the Arctic Ocean. A keeling plot showed the dominant influence by hydrate gas in the Nordic Sea region, while the long range transport of wetland emissions were one of potentially important sources in the central Arctic Ocean. Experiments comparing sunlight and darkness indicate that microbes may also play an important role in regional variations.