Impacts of the 2021 Northwestern Ontario and Manitoba Wildfires on the Chemical Composition and Oxidative Potential of Airborne Particulate Matter in Montréal, Canada

Abstract

In July and August 2021, wildfire smoke transported from Northwestern Ontario and Manitoba impacted the air quality in Montréal, Québec, Canada. To investigate the impact of the wildfire smoke on PM₁₀ composition in an urban environment, samples were collected and analyzed for organic carbon (OC), elemental carbon (EC), elements, water-soluble ions, sugars, and polycyclic aromatic hydrocarbons (PAHs) during contrasting periods of biomass burning and nonbiomass burning-influenced conditions. Biomass burning tracers in PM₁₀ (e.g., levoglucosan, mannosan, galactosan, rubidium, and water-soluble potassium) and other compounds associated with biomass burning emissions (e.g., OC, EC, oxalate, succinate, and NH₄⁺) increased by a factor of 2.0–5.0 during biomass burning periods. The influence of wildfires had little impact on trace elements (e.g., Ba, Co, Cu, Mn, Ni, Pb, Sr, and V) concentrations which did not increase significantly compared to the urban background. Major PM₁₀ constituents were carbonaceous matter, followed by crustal matter and secondary inorganic ions during both biomass and nonbiomass burning days. The contribution of carbonaceous matter increased significantly during biomass burning events representing up to 71% of the total PM₁₀ mass concentration. The ascorbic acid assay found no notable difference in intrinsic oxidative potential between biomass burning and nonbiomass burning days despite decreasing proportions of redox-active metals in PM₁₀ during episodes of biomass burning smoke. This observation indicates that other components of biomass burning PM₁₀ such as organic matter and sulfate may directly or indirectly contribute to the oxidative potential in a way that compensates for the decreasing proportion of redox-active metals that normally dominate the oxidative potential measured by the ascorbic acid assay.