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. Oil sands upgrading facilities in the Athabasca oil sands region (AOSR) inAlberta, Canada, have been reporting mercury (Hg) emissions to publicgovernment databases (National Pollutant Release Inventory (NPRI)) since theyear 2000, yet the relative contribution of these emissions to ambient Hgdeposition remains unknown. The impact of oil sands emissions (OSE) on Hglevels in and around the AOSR, relative to contributions from global(anthropogenic, geogenic and legacy) emissions and regional biomass burningemissions (BBE), was assessed using a global 3D-process-based Hg model,GEM-MACH-Hg, from 2012 to 2015. In addition, the relative importance ofyear-to-year changes in Hg emissions from the above sources andmeteorological conditions to inter-annual variations in Hg deposition wasexamined. Surface air concentrations of Hg species and annual snowpack Hgloadings simulated by the model were found comparable to measured levels inthe AOSR, suggesting consistency between reported Hg emissions from oilsands activities and Hg levels in the region. As a result of global-scaletransport and the long lifetime of gaseous elemental Hg (Hg(0)), surface airconcentrations of Hg(0) in the AOSR reflected the background Hg(0) levels inCanada. By comparison, average air concentrations of total oxidized Hg(efficiently deposited Hg species) in the AOSR were elevated up to 60 %within 50 km of the oil sands Hg emission sources. Hg emissions fromwildfire events led to episodes of high ambient Hg(0) concentrations anddeposition enrichments in northern Alberta, including the AOSR, during theburning season. Hg deposition fluxes in the AOSR were within the range ofthe deposition fluxes measured for the entire province of Alberta. On abroad spatial scale, contribution from imported Hg from global sourcesdominated the annual background Hg deposition in the AOSR, with present-dayglobal anthropogenic emissions contributing to 40 % (&lt; 1 % fromCanada excluding OSE) and geogenic and legacy emissions contributing to60 % of the background Hg deposition. In contrast, oil sands Hg emissionswere responsible for significant enhancements in Hg deposition in theimmediate vicinity of oil sands Hg emission sources, which were∼ 10 times larger in winter than summer (250 %–350 % inwinter and ∼ 35 % in summer within 10 km of OSE, 2012–2013).The spatial extent of the influence of oil sands emissions on Hg depositionwas also greater in winter relative to summer (∼ 100 km vs. 30 km from Hg-emitting facilities). In addition, inter-annual changes inmeteorological conditions and oil sands emissions also led to significantlyhigher inter-annual variations in wintertime Hg deposition compared tosummer. In 2015, within 10 km of major oil sands sources, relative to 2012,Hg deposition declined by 46 % in winter but 22 % annually, due to a largerOSE-led reduction in wintertime deposition. Inter-annual variations inmeteorological conditions were found to both exacerbate and diminish theimpacts of OSE on Hg deposition in the AOSR, which can confound theinterpretation of trends in short-term environmental Hg monitoring data. Hgrunoff in spring flood, comprising the majority of annual Hg runoff, ismainly derived from seasonal snowpack Hg loadings and mobilization of Hgdeposited in surface soils, both of which are sensitive to Hg emissions fromoil sands developments in the proximity of sources. Model results suggest thatsustained efforts to reduce anthropogenic Hg emissions from both global andoil sands sources are required to reduce Hg deposition in the AOSR.

Impact of Athabasca oil sands operations on mercury levels in air and deposition