Abstract. Oil sands upgrading facilities in the Athabasca oil sands region (AOSR) in
Alberta, Canada, have been reporting mercury (Hg) emissions to public
government databases (National Pollutant Release Inventory (NPRI)) since the
year 2000, yet the relative contribution of these emissions to ambient Hg
deposition remains unknown. The impact of oil sands emissions (OSE) on Hg
levels in and around the AOSR, relative to contributions from global
(anthropogenic, geogenic and legacy) emissions and regional biomass burning
emissions (BBE), was assessed using a global 3D-process-based Hg model,
GEM-MACH-Hg, from 2012 to 2015. In addition, the relative importance of
year-to-year changes in Hg emissions from the above sources and
meteorological conditions to inter-annual variations in Hg deposition was
examined. Surface air concentrations of Hg species and annual snowpack Hg
loadings simulated by the model were found comparable to measured levels in
the AOSR, suggesting consistency between reported Hg emissions from oil
sands activities and Hg levels in the region. As a result of global-scale
transport and the long lifetime of gaseous elemental Hg (Hg(0)), surface air
concentrations of Hg(0) in the AOSR reflected the background Hg(0) levels in
Canada. 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 from
wildfire events led to episodes of high ambient Hg(0) concentrations and
deposition enrichments in northern Alberta, including the AOSR, during the
burning season. Hg deposition fluxes in the AOSR were within the range of
the deposition fluxes measured for the entire province of Alberta. On a
broad spatial scale, contribution from imported Hg from global sources
dominated the annual background Hg deposition in the AOSR, with present-day
global anthropogenic emissions contributing to 40 % (< 1 % from
Canada excluding OSE) and geogenic and legacy emissions contributing to
60 % of the background Hg deposition. In contrast, oil sands Hg emissions
were responsible for significant enhancements in Hg deposition in the
immediate vicinity of oil sands Hg emission sources, which were
∼ 10 times larger in winter than summer (250 %–350 % in
winter and ∼ 35 % in summer within 10 km of OSE, 2012–2013).
The spatial extent of the influence of oil sands emissions on Hg deposition
was also greater in winter relative to summer (∼ 100 km vs. 30 km from Hg-emitting facilities). In addition, inter-annual changes in
meteorological conditions and oil sands emissions also led to significantly
higher inter-annual variations in wintertime Hg deposition compared to
summer. 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 larger
OSE-led reduction in wintertime deposition. Inter-annual variations in
meteorological conditions were found to both exacerbate and diminish the
impacts of OSE on Hg deposition in the AOSR, which can confound the
interpretation of trends in short-term environmental Hg monitoring data. Hg
runoff in spring flood, comprising the majority of annual Hg runoff, is
mainly derived from seasonal snowpack Hg loadings and mobilization of Hg
deposited in surface soils, both of which are sensitive to Hg emissions from
oil sands developments in the proximity of sources. Model results suggest that
sustained efforts to reduce anthropogenic Hg emissions from both global and
oil sands sources are required to reduce Hg deposition in the AOSR.
The impact of atmospheric acid deposition on tree growth and forest understory vegetation in the Athabasca Oil Sands Region
