Canopy Interception of Acid Deposition in Southern Ontario

The impact of tree canopies on acid deposition was examined. Differences in the chemical composition of unintercepted precipitation (dustfall) and canopy was intercepted precipitation (throughfall) at 18 southern Ontario forests, collected during the summers of 1995-1996, were chemically analyzed. The methodology of collection and analysis validated using consistency checks for interception loss, maintenance of electrical neutrality and ion correlation. T-test analyses found throughfall fluxes of K+, Ca2+, Mg2+, and NO3- were significantly higher than dustfall flux (p < 0.05), consistent with other studies. Barrie and nearby sites at Orillia and Bracebridge had larger dustfall depositions of base cations and Cl- suggesting a nearby source of these ions. T-tests revealed large exceedances of pH and sulphate concentration in dustfall over throughfall at the two Scarborough sites; a local point source of sulphates in the Greater Toronto Area was suspected.