Time trend of wet deposition acidifying potential at five ecological monitoring sites in eastern Canada 1981?1993

Abstract

The Acidifying Potential (AP) of wet deposition has been previously defined by the expression AP = SO4−-[Ca++ + Mg++]. The time series over the period 1981 to 1993 for sulphate, calcium plus magnesium and AP concentrations are examined at five ecological monitoring research sites in the eastern half of Canada. At all sites there was a reduction in the concentrations of all 3 parameters over the whole period. As a result the reduction in AP was between 70 and 80% of the sulphate reduction at the 4 sites where precipitation acidity is highest. The annual wet deposition of AP showed a substantial decline in the first half of the period, but has levelled off in the second half, in spite of continuing sulphate decline. In assessing the impact of acidic deposition on surface water systems it is, thus, important to account for the counteraction of sulphate decreases by base cation decreases. This is particularly important for simulation modelling, either diagnostic using past data, or predictive using future sulphur dioxide emission scenarios, where in general cations have been assumed to remain constant.