We highlight some of the research at Environment and Climate Change Canada (ECCC) on atmospheric pollutant deposition in Canada.  The Canadian Atmospheric Precipitation Monitoring Network (CAPMoN) of ECCC has been measuring daily wet deposition of acidifying pollutants and base cations since 1983.  These measurements are necessary for monitoring the spatial and temporal trends in acid rain, evaluating environmental policies as part of the Canada-U.S. Air Quality Agreement, and studying the acidification of ecosystems.  Wet deposition of inorganic ions including Ca²⁺, Mg²⁺, K⁺, Na⁺, NH₄⁺, Cl^-, NO₃^-, and SO₄^2- measured between 1984 and 2011 were analyzed at 31 sites in Canada.  Annual wet deposition fluxes (kg ha^-1 a^-1) ranged from 0.08-3.6 for Ca²⁺, 0.02-1.6  for Mg²⁺, 0.01-0.7 for K⁺, 0.03-12.0 for Na⁺, 0.1-6.4 for NH₄⁺,  0.06-23 for Cl^-, 0.4-27 for NO₃^-, and 0.5-33 for SO₄^2-.  Most of the sites observed statistically significant decreases in the annual NO₃^- and SO₄^2- wet deposition at a rate of 0.07-1.0 kg ha^-1 a^-1 over the 1984-2011 period.  The rate of decline in annual SO₄^2- wet deposition was particularly larger at southern Canadian sites likely because of decreasing SO₂ emissions leading to lower sulfur air concentrations.  In contrast, there were no significant temporal trends in base cations and NH₄⁺.  In terms of acid rain, pH levels in precipitation were lower in southern and eastern Canada than western Canada.  Our analysis indicates that precipitation has become less acidic over time. 

In addition to acidifying pollutants, the atmospheric deposition of 43 polycyclic aromatic compounds (PACs) was estimated in the oil sands region of Alberta, Canada and surrounding communities.  PACs, which include routinely-monitored polycyclic aromatic hydrocarbons (PAHs) and other lesser known compounds, are a concern in this region because these pollutants are typically emitted from crude oil production and biomass burning and are known for their carcinogenic effects.  Our model results estimates the annual total deposition of PACs in 2011 ranged from 55 to 175,000 µg m^-2 a^-1.  The highest deposition fluxes were found over the surface mining area and at a few sites in the in-situ production region.  The contributions from wet and dry deposition were 64% and 36%, respectively.  Alkylated PAHs were the dominant PACs in deposition and the fluxes for these compounds were higher over a larger area than parent PAHs.  The deposition estimates provide insight into wildlife exposure to PACs and supplement gaps in deposition monitoring in this remote region.