Contribution of Canada–United States transboundary transport to wet deposition of sulphur and nitrogen oxides—A mass balance approach

Past acid deposition studies in North America have demonstrated that SO2 and NOx emissions from Canada and the United States are transported across the Canada–United States border and deposited in the other country. In this study, we use an observation-based mass balance approach to estimate the percentage contribution of eastern US (EUS) emissions to wet deposition in eastern Canada (EC) during two 5-year periods: 1990–1994 and 1996–2000. The estimates are made using a set of mass balance equations derived from 5-year-average integrated emissions in EC and the EUS (taken from emission inventories) and 5-year-average wet deposition values integrated over the same areas (based on wet deposition measurements). In deriving the mass balance equations, it was found that EC emitted, on average, 8–9% of eastern North American SO2 emissions but received 28–29% of eastern North American non-sea-salt-SO42− wet deposition, even though the integration area of EC was approximately half that of the EUS. Similar figures were found for NOx emissions and nitrate wet deposition. This emission-versus-deposition imbalance is assumed to be due to the transboundary transport of EUS emissions into EC, which is factored into the mass balance equations. The results of the mass balance calculations indicate that SO2 and NOx emission sources in the EUS are responsible for between 56% and 83% of the non-sea-salt-SO42− and NO3− wet deposition in EC. This range is consistent with previously published, model-based, mass balance studies in this region and provides an independent, observation-based confirmation of the model results.