Contributions from natural sources to the background aerosol in Canada

An initial assessment of the contributions of natural aerosols to the background PM levels in Canada has been performed using the NARCM model. Mass concentrations of four types of natural PM (soil dust, sea-salt, organic carbon [OC] and black carbon [BC]) were simulated for all of 1998 and for the spring of 2001 (March, April and May). To be able to clearly identify the carbonaceous PM from forest fires, no anthropogenic emissions of OC and BC were included in the 1998 simulations. All simulations also included anthropogenic sulphate as well as sulphate from DMS oxidation; however the latter is not isolated in this study due to the restrain to carry more tracers. The peak mass concentration of soil dust in the 2001 springtime occurs in the Prairies, where the dust comprised 60-80% of total PM. In the southwest US, the contributions of soil dust to total PM reached 90% during the spring of 2001. The 1998 simulations show that BC and OC from bio-mass burning in Canada and East Siberia have a major impact on the surface PM values during the summer. At the surface, the average mass concentrations of BC and POM during July 1998 are 90 ng m^-3 and 400 ng m^-3, respectively in the regions of the burning forests. This can be contrasted with average mass concentrations of anthropogenic sulphate of around 500 ng m^-3. Sea-salt mostly strongly impacts the Canadian coastal regions and the area around Hudson Bay. On the west coast, average surface sea-salt concentrations range from 0.5 to 6 µg m^-3 along the coast line and contribute from 5% to 70% to the total PM. On the east coast, the sea-salt surface concentrations reach 10 µg m^-3 with percentage contributions over Labrador, Nova Scotia and the tip of the St. Lawrence River ranging from 5% to 70% of the total PM during the spring of 2001. All natural aerosol components show strong seasonal variations and interannual variability.