ASPECTS OF WET, ACIDIFYING DEPOSITION IN ARNHEM - SOURCE REGIONS, CORRELATIONS AND TRENDS (1984-1991)

A time series of wet deposition in Arnhem, the Netherlands, was analys ed for the period 1984-1991. Precipitation was collected with four sam plers on a daily basis. A comparative study by the Dutch National Prec ipitation Network showed significant biases for the observations of th e National Network station due to longer exposure to dry deposition. S imultaneous operation of wet-only and bulk collectors demonstrated a c oncentration bias of about 10% for daily bulk sampling. Using a cluste r analysis of backward trajectorics, clear distinctions could be made between precipitation from continental and maritime origin. Event-to-e vent variations in deposition seemed to be determined largely by meteo rological influences. As major anthropogenic source regions, the U.K., France, Belgium and the Netherlands itself were identified. The contr ibution of Dutch sources to wet acid deposition in Arnhem was estimate d at 30-40%. Trends and seasonal variations were analysed with an adva nced time-series model based on Kalman filtering. Similar seasonal var iations were found for SO42- and NH4+. Also, seasonal variations in th e concentrations of H+ and NO3- corresponded. Significant long-term ch anges in deposition and concentration were found for SO42- (about -3% yr-1) and H+ (about -9% yr-1) only. The analysed trends were decreasin g, but decreases were larger in the years 1984-1986 than in the follow ing years. The relative decrease in the wet deposition of SO42- was su bstantially smaller than the decrease in dry-deposited SO2 and SO42-.