Transboundary transport of air pollution in eastern Canada†

We assess the GEOS-Chem chemical transport model against observations from the Quebec Air Quality Monitoring Network (RSQAQ) for carbon monoxide (CO), nitrogen oxides (NO_X; nitrogen oxide (NO) and nitrogen dioxide (NO₂)), fine particles having a diameter of less than 2.5 μm (PM_2.5), ozone (O₃), sulphur dioxide (SO₂), and elemental carbon (EC), a component of PM_2.5 known to have effects on both human health and climate. These pollutants have lifetimes that span from hours to weeks, allowing them to cross national borders and affect air quality far from their emission sites. We then investigate the sources of air pollution in Quebec through two complementary methods: back trajectory analysis using potential source contribution function (PSCF) and chemical transport modelling using GEOS-Chem. We perform three sensitivity studies with GEOS-Chem to determine the contributions from three source regions (Quebec, the rest of Canada, and the United States) to the concentrations of each of the investigated pollutants in Quebec. The PSCF calculations show that southern Quebec (local sources), the east coast of the United States, and southeastern Ontario are associated with days of high concentrations of several pollutants. Depending on the season, southern Quebec is associated with high concentrations of NO_X, SO₂, and CO; the east coast of the United States with high concentrations of PM_2.5, NO_X, O₃, and CO; and southeastern Ontario with high concentrations of PM_2.5 and EC. The GEOS-Chem results reveal that anthropogenic emissions from Quebec contribute the greatest amount (53%, 58%, 30%, and 44%) to concentrations of NO_X, SO₂, PM_2.5, and EC in Quebec. Anthropogenic emissions from the US were the greatest contributor to CO concentrations (11%) and summertime O₃ concentrations (17%). We find that removing all anthropogenic emissions from Quebec would reduce the fraction of the population of Quebec living in regions that exceed the recommended annual mean WHO PM_2.5 concentration threshold of 5.0 μg m⁻³ from 87.7% to about 0.0%. While an absolute cessation of anthropogenic emissions is neither feasible nor desirable, our results suggest that substantial improvements in air quality in Quebec would be possible through reductions in local emissions alone despite the strong influence of transboundary transport.