TEMPORAL VARIATION OF SULFATE AND NITRATE CONCENTRATION IN PRECIPITATION IN EASTERN NORTH-AMERICA - 1979-1990

Precipitation chemistry data from the U.S. National Atmospheric Deposi tion Program/National Trends Network and the Canadian Air and Precipit ation Monitoring Network were used to investigate the temporal variati on 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 tempor al variations, and a geometric model fitted by least-squares. Of the 2 4 monitoring sites investigated, all produced statistically significan t long-term sulphate trends, while only 13 produced significant nitrat e trends. in contrast to previously published studies which assumed mo notonically increasing or decreasing temporal variations, only 2 of th e 48 long-term trends in this study exhibited monotonic behaviour. Lon g-term cycles with periods between 2 and 5.5 years were found at 58 an d 83% of the sites for SO42- and NO3-, respectively. Al more than 58% of the sites more complicated seasonal patterns than a simple sinusoid al 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. kerne l smoothing regression) be used to help in identifying the best parame tric model to use with least-squares techniques and to test for statis tical significance in the temporal model.