Heavy metals in the near-road environment: Results of semi-continuous monitoring of ambient particulate matter in the greater Toronto and Hamilton area

Six heavy metals - Mn, Fe, Cu, Zn, Se, and Pb among other elemental species were monitored in ambient PM2.5 at three near-road ambient air monitoring locations in the Greater Toronto and Hamilton Area (GTHA) with semi-continuous X-ray fluorescence (XRF) instrumentation over a period spanning January 1st, 2014 to June 30th, 2017. Land use in these air monitoring locations includes residential, institutional and industrial, thus, air monitoring is representative of typical urban areas. Ambient metal concentrations were found below Ontario's ambient air quality criteria. Temporal trends however indicated that high concentrations of Fe and Cu correlated with peak commuting and working hours on weekdays. To further understand the potential sources of these metals, scatterplots of metal concentrations and criteria pollutant gases were made on weekdays and weekends. These scatterplots reveal edges that are due to multiple sources of these metals. When these scatterplots are colour-coded by the hour of day, edges associated with the morning rush hour on weekdays for Fe and Cu (also Mn and Zn to a lesser extent) likely due to traffic-related emissions are more clearly-delineated from other edges arising from industrial or regional sources that were prevalent during other times of the day. Finally, an auxiliary receptor model was used to explore the potential source regions of these metals. It was observed that Mn, Fe and Cu had intense potential source regions within the GTHA on weekdays that diminished on the weekends, and in the case of Fe, the potential source regions in the GTHA were sensitive to the morning rush hour period, indicating that traffic-related emissions are a major source of Fe. Other metals, especially Zn, Se and Pb have source regions that are less sensitive to the morning rush hour period and are usually situated outside the GTHA. Keywords: Heavy metals, Near-road, Non-parametric statistics, PM2.5, sQTB