28 Jun 2022
28 Jun 2022
Status: this preprint is currently under review for the journal ACP.

Long-term Atmospheric Deposition of Nitrogen and Sulfur and Assessment of Critical Loads Exceedances at Canadian Rural Locations

Irene Cheng1, Leiming Zhang1, Zhuanshi He1, Hazel Cathcart1, Daniel Houle2, Amanda Cole1, Jian Feng1, Jason O'Brien1, Anne Marie Macdonald1, Julian Aherne3, and Jeffrey Brook4 Irene Cheng et al.
  • 1Air Quality Research Division, Atmospheric Science and Technology Directorate, Science and Technology Branch, Environment and Climate Change Canada, Toronto, Ontario, M3H 5T4, Canada
  • 2Aquatic Contaminants Research Division, Water Science and Technology Directorate, Science and Technology Branch, Environment and Climate Change Canada, Montréal, Quebec, H2Y 2E7, Canada
  • 3School of Environment, Trent University, Peterborough, Ontario, K9L 0G2, Canada
  • 4Dalla Lana School of Public Health and Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, M5T 3M7, Canada

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 (CL) exceedances. Total (dry+wet) N deposition ranged from 1.7–9.5 kg N ha-1 yr-1 among the 15 sites, though dry deposition of NH3 and some oxidized N species were not included due to a lack of 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 kg S ha-1 yr-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 kg N ha-1 yr-1 (-1.1 % to -3.3 % yr-1) and -0.08 to -0.66 kg S ha-1 yr-1 (-3.5 % 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 U.S. 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.

Irene Cheng et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-400', Anonymous Referee #1, 01 Aug 2022
  • RC2: 'Comment on acp-2022-400', Gregory Wetherbee, 02 Aug 2022
  • RC3: 'Comment on acp-2022-400', Anonymous Referee #3, 03 Aug 2022
  • RC4: 'Comment on acp-2022-400', Anonymous Referee #4, 12 Aug 2022

Irene Cheng et al.

Irene Cheng et al.


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Short summary
Nitrogen (N) and sulfur (S) deposition decreased significantly at 14 Canadian sites during 2000–2018. The greatest decline was observed in southeastern Canada owing to regional SO2 and NOx reductions. Wet deposition was more important than dry deposition, comprising 71–95 % of total N and 45–89 % of total S deposition. While critical loads (CL) were exceeded at a few sites in the early 2000s, acidic deposition has declined below CL after 2012, which signifies recovery from legacy acidification.