Impacts of the 2023 Canadian Wildfires on the Oxidative Potential of Particulate Matter

Particulate matter (PM) from wildfire (WF) smoke is increasingly impacting air quality. However, knowledge about its adverse health effects is limited, particularly regarding its oxidative properties relative to other PM types. In 2023, Canada experienced its worst recorded WF season, with smoke strongly impacting air quality across North America. Here we collected ambient PM samples from two urban (Toronto and Champaign) and one rural (East Trout Lake) sites in North America in 2023 and assessed their oxidative potential (OP; mass-normalized) and burden (OB; volume-normalized) using the dithiothreitol (DTT) assay. Our results indicate that OP of urban WF PM was not greater than non-WF PM. At the rural site, WF PM showed slightly higher OP than background PM, which may be related to a lesser extent of atmospheric aging of WF PM, or low OP of background PM in rural areas. During WF-influenced periods in Toronto, mass fractions of OP-active metals were significantly lower, but there was no significant difference in OP, indicating the importance of nonmetallic components to OP. Across all sites, OB was greater during WF periods, driven by elevated PM concentrations. Our findings suggest that while the OP of WF PM is similar to urban PM, higher PM concentrations during WF episodes can increase exposure to potentially harmful DTT-active species.