A Summary of Decadal Trends of Various Pollutants Monitored across Canada

Evaluating long-term trends in air quality and atmospheric deposition of key pollutants can demonstrate effectiveness of emission control strategies and guide future air quality management planning. Decadal trends of various pollutants monitored across Canada are analyzed in a series of studies with some of the studies also including some U.S. data. Decreasing trends in annual air concentrations of SO₄^2- (1983-2010) and NH₄⁺ (1994-2010) were prevalent, while annual air concentrations of NO₃^- increased prior to 2001 and then declined afterwards at 16 Canadian sites monitoring ambient pollutants. Widespread declines in annual NO₃^- and SO₄^2- wet deposition ranged from 0.07-1.0 kg ha^-1 a^-1 (1984-2011) at 31 Canadian sites monitoring precipitation chemistry. In eastern regions of Canada and U.S., annual air concentrations of total sulfur (SO₂+SO₄^2-) decreased by ~80%, total nitrate (NO₃^- + HNO₃) by ~50%, and NH₄⁺ by >60% from 1990 to 2015. However, increased annual air concentrations of NH₃ were observed in the past decade at the majority of the 15 sites in the U.S. and seven sites in Canada, despite the reported decreasing NH₃ emissions. The changing atmospheric chemistry conditions due to emissions reductions of SO₂ and NO_X, enhanced drought condition and increased ambient temperature all contributed to the increasing trends in annual NH₃ concentrations. Air concentrations of organic carbon (OC) and elemental carbon (EC) in PM_2.5 decreased, while OC/EC ratio increased from 2004 to 2016 at 11 Canadian sites, suggesting the effective reduction of primary anthropogenic emissions and increasingly important role of secondary organic aerosol formation. Analysis of decadal trends of CO, NO₂ and O₃ air concentrations and dry and total deposition of acidifying pollutants at 30+ sites is ongoing. The next steps are to examine similarities, differences and links between the trends of the multiple pollutants and associated causes and changing atmospheric chemistry conditions on the decadal scale.