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Long-Term Trends in Atmospheric Reactive Nitrogen across Canada: 19882007

Antoni Zbieranowski* and Julian Aherne
Environmental and Resource Studies
Trent University, Peterborough, ON K9J 7B8

The long-term trends in atmospheric reactive nitrogen (Nr) species at 12 Canadian Air and Precipitation Monitoring Network (CAPMoN) stations (9 with air and precipitation observations) across Canada were evaluated during the period 19882007. The non-parametric Mann-Kendall test was used to determine monotonic trends in the annual chemistry of gaseous nitric acid (HNO3), particulate nitrate (pNO3), particulate ammonium (pNH4+), wet ammonium (NH4 +) and wet nitrate (NO3) in response to emission reductions primarily driven by the Canada-United States Air Quality Agreement. The (trend) slope was estimated using the non-parametric Sen's method, and trend significance was assumed at the 0.05 confidence level in the current study. Annual air concentrations (19882007) of pNH4+ and HNO3 significantly decreased at all CAPMoN stations, while pNO3 concentrations increased at 7 of 9 stations. Precipitation NH4+ had no significant or consistent trend whereas precipitation NO3 concentrations significantly decreased at 9 of the 11 stations and increased at one (non-significant). Normalized temporal sequences showed consistent temporal patterns across Canada for several Nr species. Annual average air concentrations of pNH4+ and HNO3 had synchronous time-series of consistently decreasing concentrations across all CAPMoN stations (19882007), in contrast, pNO3 had a complex temporal pattern, dominated by an initial period of no change (19881993), followed by a period of steep increase (19932002) and then a period of steep decrease (20022007). The period of steep decrease started around 2002 and was observed at all CAPMoN stations (all Nr species except wet NH4+). The steep decrease was consistent with the observed decrease in NOx emissions from power plants and on-road vehicles in the United States. Southern Ontario consistently had the highest concentrations of all Nr species measured across Canada. These stations are located in (or close to) agricultural areas that are in close proximity to the most concentrated industrialized region in eastern Canada; moreover, they are located downwind of the largest anthropogenic emissions sources in North America (Ohio valley), therefore emissions in the eastern United States drive deposition trends observed in eastern Canada.

*E-mail: , Telephone: (705) 748-1011 ext. 7959 ERS, Trent University, 1600 West Bank Drive, Peterborough, Ontario, Canada K9J 7B8