Temporal variation of sulphate and nitrate concentration in precipitation in eastern North America: 1979–1990

Abstract

Precipitation chemistry data from the U.S. National Atmospheric Deposition Program/National Trends Network and the Canadian Air and Precipitation Monitoring Network were used to investigate the temporal variation of sulphate and nitrate concentrations in precipitation at 24 sites across eastern North America. Two different techniques were applied—a kernel regression smoother to investigate the character of the temporal variations, and a geometric model fitted by least-squares. Of the 24 monitoring sites investigated, all produced statistically significant long-term sulphate trends, while only 13 produced significant nitrate trends. In contrast to previously published studies which assumed monotonically increasing or decreasing temporal variations, only 2 of the 48 long-term trends in this study exhibited monotonic behaviour. Long-term cycles with periods between 2 and 5.5 years were found at 58 and 83% of the sites for SO 2− 4 and NO − 3 , respectively. At more than 58% of the sites more complicated seasonal patterns than a simple sinusoidal cycle were obtained for both ions. These results indicate that more complicated models than the ones that have been used up to now should be employed. It is recommended that a smoothing technique (e.g. kernel smoothing regression) be used to help in identifying the best parametric model to use with least-squares techniques and to test for statistical significance in the temporal model.