Temporal variation of oxides of sulphur and nitrogen in ambient air in eastern Canada: 1979-1994

The temporal variation of daily ambient concentrations of particulate SO² ₄-, NO₃ ^-, NH₄ ⁺ and gaseous SO₂ and HNO₃, and total-NO₃ (i.e. HNO₃ + NO₃) in eastern Canada was studied by fitting (using the maximum likelihood statistical technique) a temporal model that included long-term trend, long-term cycles, seasonal cycles and an auto-regressive model. The technique allowed for missing data and used kernel smoothing and spectral analysis to facilitate the selection of the right model. It was first found that the long-term trends were neither linear nor monotonic. Between the beginning of the 80s and 90s, SO₂ ambient air concentration decreased by 21 to 43% at 5 of the 8 sites. During the same period HNO₃ and total-NO₃ concentrations increased at 7 of the 8 sites. For the other compounds, no systematic variations were found for that period and any overall increases or decreases were small. Long-term cycles were found for 70% of the time-series but their amplitudes were small and they represented only a small fraction of the total variance for each time-series. Seasonal cycles with large amplitudes were found only for SO₂. For the other compounds, statistically significant seasonal cycles were obtained but they had small amplitudes. Auto-regressive models of order 2 were necessary for most of the time-series. For some SO₂and NO₃ ^- time-series, auto-regressive models of order 1 were sufficient. For the other compounds, auto-regressive models of order 3 were necessary for some of the time-series. These results indicate that the air masses that arrived at the sites were less homogeneous in the case of SO₂ and NO₃ ^- than for the other compounds.