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+ pNO3-, sulfur (S) SO2 pSO42-, precipitation NO3-, NH4+, SO42-, have been routinely monitored by the Canadian Air Precipitation Monitoring Network (CAPMoN) since 1983. Data at 15 rural sites from 2000–2018 were used to estimate dry wet N S deposition fluxes, which then explore their spatiotemporal trends assess ecosystem damage through a retrospective analysis critical loads (CLs) exceedances. Total (dry + wet) ranged 1.7–9.5 kgNha-1yr-1 among sites, though NH3 some oxidized species not included due lack monitoring data. Based on additional measurements in 2010 one annual total may be underestimated up 32 %. was dominated NO3- NH4+ deposition, together comprised 71 %–95 Contributions 40 %–74 % HNO3, 11 %–40 pNH4+, 5 %–25 pNO3-. 1.3–8.5 kgSha-1yr-1 SO42- SO2. Relative percentages 45 %–89 %–55 %, respectively. Acidic ion fluxes greatest southeastern Canada comparable west coast, prairie, remote, eastern sites. Oxidized NO3-) greater than that reduced NH4+) early 2000s. In 2014–2018, surpassed Canada. decreased significantly rate −0.03 −0.25 (−1.1 yr−1 −3.3 yr−1) −0.08 −0.66 (−3.5 −6.6 yr−1), respectively, The weak declining trend coast site consistent with slower decline NOx emissions western Reductions driven its form as non-significant. As result, contributions increased average 42 2000–2004 53 2014–2018. Anthropogenic reductions both US highly effective reducing exceeded terrestrial CL 14 aquatic 2 However, exceedances trending downwards acidic mostly near or below after 2012 for subset assessed, support recovery historical acidification. Further assessments are required other regions susceptible acidification affected elevated increasing emissions.