Long-term air concentrations, wet deposition, and scavenging ratios
of inorganic ions, HNO3 and SO2 and assessment of aerosol and precipitation acidity at Canadian rural locations
Irene Cheng and Leiming Zhang
Air Quality Research Division, Science and Technology Branch, Environment and Climate Change Canada, 4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada
Received: 12 Oct 2016 – Accepted for review: 27 Nov 2016 – Discussion started: 29 Nov 2016
Abstract. This study analyzed long-term air concentrations and annual wet deposition of inorganic ions and aerosol and precipitation acidity at 30 Canadian sites from 1983–2011. Scavenging ratios of inorganic ions and relative contributions of particulate- and gas-phase species to NH4+, NO3−, and SO42− wet deposition were determined. Long-term median atmospheric NH4+, NO3−, and SO42− between sites ranged from 0.1–1.7, 0.03–2.0, and 0.6–3.5 μg m−3, respectively. Their median annual wet deposition varied from 0.2–5.8, 0.8–23.3, and 0.8–26.6 kg ha−1 a−1. Geographical patterns of atmospheric Ca2+, Na+, Cl−, NH4+, NO3−, and SO42− were similar to wet deposition and attributed to anthropogenic sources, sea-salt emissions, and agricultural emissions. Decreasing trends in atmospheric NH4+ (1994–2010) and SO42− (1983–2010) were prevalent. Atmospheric NO3− increased from 1991–2001 and declined from 2001–2010. These results are consistent with SO2, NOx and NH3 emission trends in Canada and the U.S. Widespread declines in annual NO3− and SO42− wet deposition ranged from 0.07–1.0 kg ha−1 a−1 (1984–2011). Acidic aerosols and precipitation impacted southern and eastern Canada more than western Canada; however both trends have been decreasing since 1994. Scavenging ratios of particulate NH4+, SO42− and NO3− differed from literature values by 22 %, 44 % and a factor of 6, respectively, because of the exclusion of gas scavenging. Average gas and particle scavenging contributions to wet NO3− deposition were 72±23 % for HNO3 and 28±23 % for particulate NO3−. SO2 and particulate SO42− contributed 37±20 % and 63±20 % to wet SO42− deposition, respectively. NH3 and particulate NH4+ contributed 30±19 % and 70±19 % to wet NH4+ deposition.
Cheng, I. and Zhang, L.: Long-term air concentrations, wet deposition, and scavenging ratios
of inorganic ions, HNO3 and SO2 and assessment of aerosol and precipitation acidity at Canadian rural locations, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2016-918, in review, 2016.