Contribution of Emissions from the Oil Sands Activities in Alberta, Canada to Atmospheric Concentration and Deposition of Nitrogen and Sulfur Species at a Downwind Site

Oil and gas activities in the Athabasca Oil Sands Region in Alberta, Canada, are large sources of atmospheric nitrogen oxides (NO_x) and sulfur dioxide (SO₂) emissions. Most previous studies on nitrogen and sulfur deposition in this region focused on the area within 50 km of the facilities. This study investigates the atmospheric deposition of nitrogen and sulfur species at a downwind site in northern Saskatchewan, about 350 km away from the oil sands facilities, to quantify the impact of emissions from the oil sands activities at a greater distance. Measurement data in this study are from the Canadian Air and Precipitation Monitoring Network (CAPMoN) from 2015 to 2019 and include ambient concentrations of nitric acid (HNO₃), particulate nitrate (pNO₃^-), NO₂, total oxidized N (NO_y), particulate ammonium (pNH₄⁺), ammonia (NH₃), SO₂, particulate sulfate (pSO₄^2-) and base cations, as well as concentrations of NO₃^-, SO₄^2-, NH₄⁺, and base cations in the precipitation. Sector analysis of air mass back trajectories is conducted to distinguish measurements with different air mass origins. Results indicate that the median atmospheric concentrations and daily dry depositions of HNO₃, pNO₃^-, NO₂, pNH₄⁺, pSO₄^2-, and SO₂ in air masses coming from the oil sands sector are significantly greater than those from the background sector by 34 to 67 %. The difference between the median concentrations of NH₃ coming from the oil sands and the background sector is not significant. Dry deposition fluxes are calculated by the inferential method, and estimated contributions of the oil sands to dry depositions of HNO₃, pNO₃^-, NO₂, other NO_y, pNH₄⁺, SO₂, and pSO₄^2- are 9.8 ± 2.7 %, 4.5 ± 2.6 %, 13.1 ± 1.2 %, 7.6 ± 2.3 %, 3.8 ± 1.7 %, 10.4 ± 1.3 %, and 5.4 ± 2.3 %, respectively. In precipitation samples, the precipitation-weighted mean concentrations of NO₃^-, SO₄^2-, and NH₄⁺ on days when air masses came from the oil sands sector are 76 %, 65 % and 81 % greater than those from the background sector, respectively. Contributions of emissions from the oil sands activities to wet depositions of NO₃^-, SO₄^2-, and NH₄⁺ at this site are 12.5 ± 8.9 %, 8.7 ± 4.4 %, and 6.0 ± 3.3 %, respectively. The contributions of emissions from the oil sands activities to the total (wet + dry) depositions of nitrogen (2.0 kg-N ha^-1 y^-1) and sulfur (0.8 kg-S ha^-1 y^-1) at the site during 2015 - 2019 are 8.0 ± 3.5 % and 8.7 ± 3.6 %, respectively. Comparison of total depositions of sulfate and nitrogen at this site to the modeled critical loads of lakes in the region suggests that it is important to keep monitoring deposition of these species, especially N species, at sensitive regions in northern Saskatchewan.