Radiocarbon-based source apportionment of carbonaceous aerosols over the Athabasca Oil Sands Region in Alberta, Canada

Abstract. The Athabasca Oil Sands Region (AOSR) hosts one of the world's largest unconventional fossil fuel operations, producing a complex carbonaceous aerosol mixture: fossil emissions from oil sands operations, biogenic secondary organic aerosol (SOA) from surrounding boreal landscapes, and episodic wildfire emissions, with consequences for regional air quality and climate. Here, we present a first dual-isotope (¹⁴C/δ¹³C) source apportionment of total carbon (TC) and elemental carbon (EC) in PM_2.5 collected at Fort McKay from May to October 2017. Sampling-day PM_2.5 ranged 0.8–20.3 µg m^-3 (mean 6.9 ± 4.7, n = 29), versus 0.2–38.1 µg m^-3 in the continuous record, indicating that filter sampling did not capture the highest pollution episodes. TC and organic carbon (OC) correlated strongly (Spearman ρ = 0.89) and positively with temperature (ρ = 0.67), consistent with biogenic SOA. EC co-varied with NO₂ (ρ = 0.77) and SO₂ (ρ = 0.51), implicating diesel mining fleets and upgrader stacks. During smoke-free periods, fossil fuel combustion dominated TC (mean: 51 %, range: 26–77 %), with westerly air masses crossing active mining areas more ¹⁴C-depleted (F¹⁴C = 0.58 ± 0.22) than northerly masses (F¹⁴C = 0.63 ± 0.14). During wildfire-impacted periods, biomass burning was the dominant source (f_bb mean: 45 %, range: 9–76 %). The AOSR exhibited substantially lower summertime F¹⁴C values than Arctic and boreal comparison sites during non-fire periods, reflecting persistent local fossil fuel influence. These results provide information that can contribute to an isotopic baseline for emission inventories, atmospheric models, and air quality management in the AOSR.