Long-term declines in atmospheric nitrogen and sulfur deposition reduce critical loads exceedances at multiple Canadian rural sites, 2000–2018

Abstract

Daily air concentrations of inorganic nitrogen (N) species, including gaseous HNO₃ and particulate-bound (p)NH₄⁺ and pNO₃⁻, and sulfur (S) species, including SO₂ and pSO²⁻₄, and precipitation concentrations of NO⁻₃, NH⁺₄, and SO²⁻₄, have been routinely monitored by the Canadian Air and Precipitation Monitoring Network (CAPMoN) since 1983. Data at 15 rural sites from 2000–2018 were used to estimate dry and wet N and S deposition fluxes, which were then used to explore their spatiotemporal trends and assess ecosystem damage through a retrospective analysis of critical loads (CLs) exceedances. Total (dry + wet) N deposition ranged from 1.7–9.5 kgNha⁻¹ yr⁻¹ among the 15 sites, though dry deposition of NH₃ and some oxidized N species were not included due to lack of monitoring data. Based on additional N measurements in 2010 at one of the sites, annual total N deposition may be underestimated by up to 32 %. Total N deposition was dominated by wet NO⁻₃ and wet NH⁺₄ deposition, which together comprised 71%–95%. Contributions to dry N deposition were 40 %–74 % by HNO₃, 11%–40% by pNH⁺₄, and 5 %–25 % by pNO⁻₃. Total S deposition ranged from 1.3–8.5 kg S ha⁻¹yr⁻¹ and was dominated by wet deposition of SO²⁻₄and dry deposition of SO₂. Relative percentages of wet and dry S deposition were 45 %–89 % and 11 %–55 %, respectively. Acidic ion fluxes were greatest in southeastern Canada and were comparable among the west coast, prairie, remote, and eastern Canadian sites. Oxidized N (dry HNO₃, dry pNO⁻₃, wet NO⁻₃) deposition was greater than that of reduced N (dry pNH⁺₄, wet NH⁺₄) in the early 2000s. In 2014–2018, reduced N deposition surpassed that of oxidized N in southeastern Canada. Total N and S deposition decreased significantly at a rate of −0.03 to −0.25 kg N ha⁻¹yr⁻¹ (−1.1 % yr⁻¹ to −3.3 % yr⁻¹) and −0.08 to −0.66 kg S ha⁻¹yr⁻¹(−3.5 % yr⁻¹to −6.6 % yr⁻¹), respectively, among the sites. The weak declining trend in total N deposition at the west coast site was consistent with the slower decline in NO_x emissions in western Canada. Reductions in total N deposition were driven by its oxidized form as trends in reduced N were non-significant. As a result, reduced N contributions to total N deposition increased on average from 42 % in 2000–2004 to 53 % in 2014–2018. Anthropogenic NO_x and SO₂ emissions reductions in both eastern Canada and eastern US were highly effective in reducing total oxidized N and total S deposition, respectively, in eastern Canada. Acidic deposition exceeded terrestrial CL at 5 of the 14 sites and aquatic CL at 2 of the 5 sites in the early 2000s. However, exceedances have been trending downwards and acidic deposition fluxes were mostly near or below CL after 2012 for the subset of sites assessed, which support recovery from historical acidification. Further assessments of CL exceedances are required in other Canadian regions susceptible to acidification and affected by elevated or increasing N and S emissions.