Long-term declines in atmospheric nitrogen and sulfur deposition reduce critical loads exceedances at multiple Canadian rural sites, 2000–2018
Authors: Cheng Irene , Zhang Leiming , He Zhuanshi , Cathcart Hazel , Houle Daniel , Cole Amanda , Feng Jian , O'Brien Jason , Macdonald Anne Marie , Aherne Julian , Brook Jeffrey 
Published: November 2022

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 pSO42-, and precipitation concentrations of NO3-, NH4+, and SO42-, 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-1yr-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 NO3- and wet NH4+ deposition, which together
comprised 71 %–95 %. Contributions to dry N deposition were 40 %–74 % by HNO3, 11 %–40 % by pNH4+,
and 5 %–25 % by pNO3-. Total S deposition ranged from 1.3–8.5 kgSha-1yr-1 and was dominated by wet
deposition of SO42- and 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 pNO3-, wet NO3-) deposition was greater than that of
reduced N (dry pNH4+, wet NH4+) 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 kgNha-1yr-1 (−1.1 % yr−1 to −3.3 % yr−1) and −0.08 to −0.66 kgSha-1yr-1
(−3.5 % yr−1 to −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 NOx 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 NOx 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.