Characterization of atmospheric water-soluble brown carbon in the Athabasca oil sands region, Canada

Extensive industrial operations in the Athabasca oil sands region (AOSR) (Alberta, Canada) are a suspected source of water-soluble brown carbon (WS-BrC), a class of light-absorbing organic aerosols capable of altering atmospheric solar-radiation budgets. However, the current understanding of WS-BrC across the AOSR is limited, and the primary regional sources of these aerosols are unknown. During the summer of 2021, active filter-pack samplers were deployed at five sites across the AOSR to collect total suspended particulate matter for the purpose of evaluating WS-BrC. Ultraviolet-visible spectroscopy and fluorescence excitation-emission matrix (EEM) spectroscopy, complemented by parallel factor analysis (PARAFAC) modelling, were employed for sample characterization. Aerosol absorbance was comparable between near-industry and remote field sites, suggesting that industrial WS-BrC exerted limited influence on regional radiative forcing. The combined EEM-PARAFAC method identified three fluorescent components (fluorophores), including one humic-like substance (C1) and two protein-like substances (C2 and C3). Sites near oil sands facilities and sample exposures receiving atmospheric transport from local industry (as indicated by back-trajectory analysis) displayed increased C1 and C3 fluorescence; moreover, both fluorophores were positively correlated with particulate elements (i.e. vanadium and sulfur) and gaseous pollutants (i.e. nitrogen dioxide and total reduced sulfur), indicative of oil sands emissions. The C2 fluorophore exhibited high emission intensity at near-field sites and during severe wildfire smoke events, while positive correlations with industry indicator variables suggest that C2 likely reflected both wildfire-generated and anthropogenic WS-BrC. These results demonstrate that the combined EEM-PARAFAC method is an accessible and cost-effective tool that can be applied to monitor industrial WS-BrC in the AOSR.