Atmospheric mercury in the Canadian Arctic. Part II: insight from modeling.

A review of mercury in the Canadian Arctic with a focus on field measurements is presented in part I (see Steffen et al., this issue). Here we provide insights into the dynamics of mercury in the Canadian Arctic from new and published mercury modeling studies using Environment Canada's mercury model. The model simulations presented in this study use global anthropogenic emissions of mercury for the period 1995-2005. The most recent modeling estimate of the net gain of mercury from the atmosphere to the Arctic Ocean is 75 Mg year(-1) and the net gain to the terrestrial ecosystems north of 66.5° is 42 Mg year(-1). Model based annual export of riverine mercury from North American, Russian and all Arctic watersheds to the Arctic Ocean are in the range of 2.8-5.6, 12.7-25.4 and 15.5-31.0 Mg year(-1), respectively. Analysis of long-range transport events of Hg at Alert and Little Fox Lake monitoring sites indicates that Asia contributes the most ambient Hg to the Canadian Arctic followed by contributions from North America, Russia, and Europe. The largest anthropogenic Hg deposition to the Canadian Arctic is from East Asia followed by Europe (and Russia), North America, and South Asia. An examination of temporal trends of Hg using the model suggests that changes in meteorology and changes in anthropogenic emissions equally contribute to the decrease in surface air elemental mercury concentrations in the Canadian Arctic with an overall decline of ~12% from 1990 to 2005. A slow increase in net deposition of Hg is found in the Canadian Arctic in response to changes in meteorology. Changes in snowpack and sea-ice characteristics and increase in precipitation in the Arctic related with climate change are found to be primary causes for the meteorology-related changes in air concentrations and deposition of Hg in the region. The model estimates that under the emissions reduction scenario of worldwide implementation of the best emission control technologies by 2020, mercury deposition could potentially be reduced by 18-20% in the Canadian Arctic.

Atmospheric mercury in the Canadian Arctic. Part I: a review of recent field measurements.

Long-range atmospheric transport and deposition are important sources of mercury (Hg) to Arctic aquatic and terrestrial ecosystems. We review here recent progress made in the study of the transport, transformation, deposition and reemission of atmospheric Hg in the Canadian Arctic, focusing on field measurements (see Dastoor et al., this issue for a review of modeling studies on the same topics). Redox processes control the speciation of atmospheric Hg, and thus impart an important influence on Hg deposition, particularly during atmospheric mercury depletion events (AMDEs). Bromine radicals were identified as the primary oxidant of atmospheric Hg during AMDEs. Since the start of monitoring at Alert (NU) in 1995, the timing of peak AMDE occurrence has shifted to earlier times in the spring (from May to April) in recent years, and while AMDE frequency and GEM concentrations are correlated with local meteorological conditions, the reasons for this timing-shift are not understood. Mercury is subject to various post-depositional processes in snowpacks and a large portion of deposited oxidized Hg can be reemitted following photoreduction; how much Hg is deposited and reemitted depends on geographical location, meteorological, vegetative and sea-ice conditions, as well as snow chemistry. Halide anions in the snow can stabilize Hg, therefore it is expected that a smaller fraction of deposited Hg will be reemitted from coastal snowpacks. Atmospheric gaseous Hg concentrations have decreased in some parts of the Arctic (e.g., Alert) from 2000 to 2009 but at a rate that was less than that at lower latitudes. Despite numerous recent advances, a number of knowledge gaps remain, including uncertainties in the identification of oxidized Hg species in the air (and how this relates to dry vs. wet deposition), physical-chemical processes in air, snow and water-especially over sea ice-and the relationship between these processes and climate change.

Arctic Ocean: is it a sink or a source of atmospheric mercury?

High levels of mercury in marine mammals threaten the health of Arctic inhabitants. Whether the Arctic Ocean (AO) is a sink or a source of atmospheric mercury is unknown. Given the paucity of observations in the Arctic, models are useful in addressing this question. GEOS-Chem and GRAHM, two complex numerical mercury models, present contrasting pictures of atmospheric mercury input to AO at 45 and 108 Mg yr(-1), respectively, and ocean evasion at 90 and 33 Mg yr(-1), respectively. We provide a comprehensive evaluation of GRAHM simulated atmospheric mercury input to AO using mercury observations in air, precipitation and snowpacks, and an analysis of the discrepancy between the two modeling estimates using observations. We discover two peaks in high-latitude summertime concentrations of atmospheric mercury. We show that the first is caused mainly by snowmelt revolatilization and the second by AO evasion of mercury. Riverine mercury export to AO is estimated at 50 Mg yr(-1) based on measured DOC export and at 15.5-31 Mg yr(-1) based on simulated mercury in meltwater. The range of simulated mercury fluxes to and from AO reflects uncertainties in modeling mercury in the Arctic; comprehensive observations in all compartments of the Arctic ecosystem are needed to close the gap.

Mercury in Arctic marine ecosystems: sources, pathways and exposure.

Mercury in the Arctic is an important environmental and human health issue. The reliance of Northern Peoples on traditional foods, such as marine mammals, for subsistence means that they are particularly at risk from mercury exposure. The cycling of mercury in Arctic marine systems is reviewed here, with emphasis placed on the key sources, pathways and processes which regulate mercury levels in marine food webs and ultimately the exposure of human populations to this contaminant. While many knowledge gaps exist limiting our ability to make strong conclusions, it appears that the long-range transport of mercury from Asian emissions is an important source of atmospheric Hg to the Arctic and that mercury methylation resulting in monomethylmercury production (an organic form of mercury which is both toxic and bioaccumulated) in Arctic marine waters is the principal source of mercury incorporated into food webs. Mercury concentrations in biological organisms have increased since the onset of the industrial age and are controlled by a combination of abiotic factors (e.g., monomethylmercury supply), food web dynamics and structure, and animal behavior (e.g., habitat selection and feeding behavior). Finally, although some Northern Peoples have high mercury concentrations of mercury in their blood and hair, harvesting and consuming traditional foods have many nutritional, social, cultural and physical health benefits which must be considered in risk management and communication.