Source Contributions to Brown Carbon Absorption in Toronto Air during the Wildfire Season

With the impacts of wildfires on urban air quality becoming increasingly prevalent, an important question is the degree to which wildfires affect urban brown carbon (BrC), a class of light-absorbing organic aerosols (OA). We deployed real-time aerosol mass spectrometry (AMS) to investigate OA composition in Toronto for 1 month during the summer of 2023 as part of the Toronto Halogens, Emissions, Contaminants, and Inorganics eXperiment (THE CIX), a season marked by extensive Canadian wildfire influence. Source apportionment using positive matrix factorization (AMS-PMF solution) revealed that biomass burning was a dominant contributor (at least 58%) to OA mass, with smaller mass contributions from local OA factors including one with a high nitrogen content (NOA). When aerosol absorption measurements were incorporated directly into the PMF model (aethalometer-PMF solution), we found that biomass burning-related OA (BBOA) had a lower BrC absorption contribution (24-33% of the total BrC absorption) than that from local factors (37-60%), averaged from 375 to 625 nm. The mass absorption coefficients of the local OA factors (at 375 nm) were above 1 m²/g, whereas those for BBOA were below 0.5 m²/g. This is likely because the BBOA had experienced photochemical whitening over the few days of transport from its source to Toronto. Our findings emphasize that sources with lower mass contributions to OA, such as local emissions, can play a disproportionate role in urban aerosol light absorption even in the presence of large mass contributions from aged wildfire smoke.