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

Abstract. This study analyzed long-term air concentrations and annual wet deposition of inorganic ions and aerosol and precipitation acidity at 31 Canadian sites from 1983 to 2011. Scavenging ratios of inorganic ions and relative contributions of particulate- and gas-phase species to NH₄⁺, NO₃⁻, and SO₄²⁻ wet deposition were determined. 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 prior to 2001 and then declined afterwards. These results are consistent with SO₂, NO_x and NH₃ emission trends in Canada and the USA. Widespread declines in annual NO₃⁻ and SO₄²⁻ wet deposition ranged from 0.07 to 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 in previous studies. Average gas and particle scavenging contributions to total wet deposition were estimated to be 72 % for HNO₃ and 28 % for particulate NO₃⁻, 37 % for SO₂ and 63 % for particulate SO₄²⁻, and 30 % for NH₃ and 70 % for particulate NH₄⁺.