Canada-United States Transboundary Transport and Wet Deposition of Sulphur and Nitrogen Oxides – A Mass Balance Approach

Chul-Un Ro and Robert Vet
Environment Canada, Science and Technology Branch, Air Quality Research Division
4905 Dufferin Street, Toronto, Ontario, M3H 5T4, Canada

Past acid deposition studies in North America have shown that SO2 and NOx emissions from Canada and the United States can be transported across the Canada-United States border and deposited in the other country. In this study, we estimate the percentage contribution of eastern US emissions to wet deposition in eastern Canada, and vice versa, in two five-year periods, 1990-1994 and 1996-2000. This is done using a set of mass balance equations derived from five-year-average total emissions in eastern Canada and the eastern US (taken from emission inventories) and fiveyear spatially-integrated wet deposition values (based on wet deposition measurements) in the same area. In deriving the mass balance equations, it was found that eastern Canada was responsible for only 8-9% of the total eastern North American SO2 emissions but received 28-29% of the non-sea-salt-sulphate wet deposition, even though the area of integration in eastern Canada was approximately one half that of the eastern US. Similar figures were found for NOx emissions and nitrate wet deposition. This emission/deposition imbalance is assumed to be due to transboundary transport and deposition – a term for which was included in the mass balance equations. The results suggest that SO2 and NOx emissions in the eastern US were responsible for 45 to 80% of the non-sea-salt-SO4 2- (0.87-1.89 MT) and NO3 - (0.84-1.51 MT) wet deposition in eastern Canada. Conversely, eastern Canadian emissions were estimated to be responsible for 0 to 16% of the non-sea-salt-SO4 2- and NO3 - wet deposition in the eastern US. Better resolution within these ranges is not possible given the limitations of the analysis. These results are consistent with other mass balance studies.