Characterising the levels and sources of the historical metal contamination in the atmosphere of Montreal (Canada) from 1973 to 2013 by coupling chemistry and Lead and Osmium isotope ratios

摘要

Since 1969 the city of Montreal monitors the air quality on its island through a network of sampling stations. Some of these stations collect aerosols, including total suspended particles (TSP), at various locations, representative of specific environments: downtown, harbour, road traffic... Between 1973 and 2013 the implementation of mitigation measures by the government resulted in successive decreases in the TSP concentrations. Here, the study of the Pb atmospheric concentrations suggests three distinct periods: 1) 1973, with expected high atmospheric Pb concentrations, 2) between 1978 and 1988, a period corresponding to the phasing out of lead in gasoline in Canada (officially from 1974 to 1990), where Pb concentrations decreased at an approximate rate of 0.05 mu g/m(3)/year, and 3) 1993 to 2013, a period during which atmospheric Pb concentrations still decreased but at a slower rate. The Pb and Os stable isotope analysis allows us to identify the major sources of metal contamination during these periods as well as to characterise the evolution of their respective contributions. The combustion of leaded gasoline mainly explains the Pb isotope ratios of the first period, followed by contributions corresponding to emissions from the US industries between 1988 and 2002. After 2002, Pb isotopes identify a higher contribution from local Pb emitters. In contrast, Os atmospheric concentrations remained low over the same period, although Os isotopes suggest an anthropogenic contribution from automobile catalytic converters and/or waste industries for the highest concentrations.