Long-term trends in atmospheric reactive nitrogen across Canada: 1988–2007

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 1988–2007. The non-parametric Mann–Kendall test was used to determine monotonic trends in the annual average concentrations of gaseous nitric acid (HNO3), particulate nitrate ( pNO 3 − ) , particulate ammonium ( pNH 4 + ) , wet ammonium ( NH 4 + ) and wet nitrate ( NO 3 − ) in response to emission reductions primarily driven by the Canada–United States Air Quality Agreement. Annual air concentrations (1988–2007) of pNH 4 + and HNO3 significantly decreased at all CAPMoN stations, while pNO 3 − concentrations increased at 6 of 9 stations. Precipitation NH 4 + had no significant or consistent trend; in contrast precipitation NO 3 − concentrations significantly decreased at 9 of the 11 stations and increased at one (non-significant). Normalized temporal sequences showed consistent patterns across Canada for several Nr species. Annual average air concentrations of pNH 4 + and HNO3 had synchronous time-series with consistently decreasing concentrations across all CAPMoN stations (1988–2007), in contrast, pNO 3 − had a complex temporal pattern, characterised by an initial period of no change (1988–1993), followed by a period of steep increase (1993–2002) and then a period of steep decrease (2002–2007). The period of steep decrease started around 2002 and was mirrored by all Nr species (except wet NH 4 + ) at all CAPMoN stations. The steep decrease was consistent with the observed decrease in NOx emissions from power plants and on-road vehicles in the United States and Canada. Legislated emission reductions have resulted in significant decreases in Nr concentrations except for NH 4 + , as such uncontrolled ammonia emissions continue to be a significant source of Nr.