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, Leiming Zhang
2016 Atmospheric Chemistry and Physics Discussions  
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&amp;ndash;2011. Scavenging ratios of inorganic ions and relative contributions of particulate- and gas-phase species to NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>&amp;minus;</sup>, and SO<sub>4</sub><sup>2&amp;minus;</sup> wet deposition were determined. Long-term median atmospheric NH<sub>4</sub><sup>+</sup>,
more » ... O<sub>3</sub><sup>&amp;minus;</sup>, and SO<sub>4</sub><sup>2&amp;minus;</sup> between sites ranged from 0.1&amp;ndash;1.7, 0.03&amp;ndash;2.0, and 0.6&amp;ndash;3.5&amp;thinsp;μg&amp;thinsp;m<sup>&amp;minus;3</sup>, respectively. Their median annual wet deposition varied from 0.2&amp;ndash;5.8, 0.8&amp;ndash;23.3, and 0.8&amp;ndash;26.6&amp;thinsp;kg&amp;thinsp;ha<sup>&amp;minus;1</sup>&amp;thinsp;a<sup>&amp;minus;1</sup>. Geographical patterns of atmospheric Ca<sup>2+</sup>, Na<sup>+</sup>, Cl<sup>&amp;minus;</sup>, NH<sub>4</sub><sup>+</sup>, NO<sub>3</sub><sup>&amp;minus;</sup>, and SO<sub>4</sub><sup>2&amp;minus;</sup> were similar to wet deposition and attributed to anthropogenic sources, sea-salt emissions, and agricultural emissions. Decreasing trends in atmospheric NH<sub>4</sub><sup>+</sup> (1994&amp;ndash;2010) and SO<sub>4</sub><sup>2&amp;minus;</sup> (1983&amp;ndash;2010) were prevalent. Atmospheric NO<sub>3</sub><sup>&amp;minus;</sup> increased from 1991&amp;ndash;2001 and declined from 2001&amp;ndash;2010. These results are consistent with SO<sub>2</sub>, NO<sub>x</sub> and NH<sub>3</sub> emission trends in Canada and the U.S. Widespread declines in annual NO<sub>3</sub><sup>&amp;minus;</sup> and SO<sub>4</sub><sup>2&amp;minus;</sup> wet deposition ranged from 0.07&amp;ndash;1.0&amp;thinsp;kg&amp;thinsp;ha<sup>&amp;minus;1</sup>&amp;thinsp;a<sup>&amp;minus;1</sup> (1984&amp;ndash;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 NH<sub>4</sub><sup>+</sup>, SO<sub>4</sub><sup>2&amp;minus;</sup> and NO<sub>3</sub><sup>&amp;minus;</sup> differed from literature values by 22&amp;thinsp;%, 44&amp;thinsp;% and a factor of 6, respectively, because of the exclusion of gas scavenging. Average gas and particle scavenging contributions to wet NO<sub>3</sub><sup>&amp;minus;</sup> deposition were 72±23&amp;thinsp;% for HNO<sub>3</sub> and 28±23&amp;thinsp;% for particulate NO<sub>3</sub><sup>&amp;minus;</sup>. SO<sub>2</sub> and particulate SO<sub>4</sub><sup>2&amp;minus;</sup> contributed 37±20&amp;thinsp;% and 63±20&amp;thinsp;% to wet SO<sub>4</sub><sup>2&amp;minus;</sup> deposition, respectively. NH<sub>3</sub> and particulate NH<sub>4</sub><sup>+</sup> contributed 30±19&amp;thinsp;% and 70±19&amp;thinsp;% to wet NH<sub>4</sub><sup>+</sup> deposition.
doi:10.5194/acp-2016-918 fatcat:gamaqdjdnnavpapuxihqsmxzla