An Approach to Inform Air Quality Management Through Receptor Source Apportionment and Thermodynamic Modelling of Fine Particulate Matter in Red Deer, Alberta, Canada

Fine particulate matter (PM2.5) concentrations in Red Deer, Alberta, Canada, exceeded the national Canadian Ambient Air Quality Standards (CAAQS) for 2011–2013. In response, a monitoring campaign was deployed to measure PM2.5 composition and precursor gases. The objective of the study was to assess PM2.5 sources, and ultimately to support management actions to improve air quality. The elevated PM2.5 concentrations were regional scale events, with similar and correlated concentrations observed at two stations in Red Deer, and at a site located 9.4 km upwind. Receptor source apportionment identified 9 factors, with secondary organics/aged smoke, sulphate and nitrate factors making up 60% of the PM2.5 mass. When wildfire-impacted samples are removed, seasonal average PM2.5 mass concentrations are largest in the spring, and are dominated by the sulphate and nitrate factors (74%). The relationship between the sulphate factor and meteorological conditions is consistent with a regional source, which could include the coal-fired power plants and the smaller upstream oil and gas operations in the area. Alberta's power plants are switching from coal to natural gas, sulphate levels are expected to decrease. The elevated nitrate factors appeared to be affected by local emissions sources that differed at each monitoring site, and could include urban and industrial sources. The elevated nitrate factors occurred under regional meteorological conditions that promote nitrate formation, with higher contributions observed for sample days with relative humidity >65%, temperature between 0 to 10 °C, and snow cover. The nitrate factor enhancements support a non-negligible contribution from the heterogeneous reactions forming nitric acid. A thermodynamic analysis showed that the ammonia-sulphate-nitrate system is nitric acid limited for this study, suggesting that decreases in nitrogen oxide precursors would help to manage PM2.5 concentrations. This study demonstrates the complexity of managing PM2.5 concentrations in an urban environment with nearby industrial sources.