Characterization of atmospheric water-soluble brown carbon in the Athabasca Oil Sands Region, Canada

Abstract. 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 reaction chemistry and planetary albedo. However, the current understanding of WS-BrC throughout 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 throughout the AOSR to collect total suspended particulate for the purpose of evaluating WS-BrC. Ultraviolet – visible spectroscopy and fluorescence excitation-emission matrix (EEM) spectroscopy, complimented by Parallel Factor Analysis (PARAFAC) modelling, were employed for sample characterization. Combined EEM-PARAFAC analysis identified three fluorescent components (fluorophores), including one humic-like (C1) and two protein-like (C2, C3) substances. Sites near oil sands facilities and sample exposures receiving atmospheric transport from local industry (indicated via back-trajectory analysis) displayed increased C1 and C3 fluorescence; moreover, both fluorophores were positively correlated with particulate elements (i.e., vanadium, sulphur) and gaseous pollutants (i.e., nitrogen dioxide, total reduced sulphur) indicative of oil sands emissions. Fluorophore C2 presented high emission intensity at near-field sites and during severe wildfire smoke events, while positive correlations with industry indicator variables suggested C2 was likely reflective of both wildfire-generated and anthropogenic WS-BrC. These results demonstrated that combined EEM-PARAFAC analysis is an accessible and cost-effective tool that can be applied to monitor industrial WS-BrC in the ASOR.