Isotopic characterization of nitrate, ammonium and sulfate in stack PM2.5 emissions in the Athabasca Oil Sands Region, Alberta, Canada

Stable isotope techniques may be a suitable tool for tracing industrial emissions in the atmosphere and the environment provided that the isotopic compositions of industrial emissions are distinct. We determined the isotopic compositions of nitrate, ammonium and sulfate in PM_2.5 emitted from two industrial stacks at a large upgrader site in the Athabasca oil sands region (AOSR), northeastern Alberta, Canada, and compared them to the nitrogen and sulfur isotopic compositions of source materials and upgrading by-products. We found distinct isotopic compositions of nitrate and ammonium in PM_2.5 compared to those reported for atmospheric nitrate and ammonium in the literature. Nitrate in PM_2.5 had δ¹⁵N values of 9.4‰ (Stack A) and 16.1 ± 1.2‰ (Stack B) that were significantly enriched in ¹⁵N compared to the feedstock materials (∼2.5‰), by-products of upgrading (−0.3–1.3‰), and atmospheric N₂ (0‰). δ¹⁵N of ammonium in PM_2.5 showed a large range with values between − 4.5 to +20.1‰ (Stack B). We report the first measurements of the triple oxygen isotopic composition of industrial emitted nitrate. Nitrate emitted as PM_2.5 is not mass-independently enriched in ¹⁷O resulting in Δ¹⁷O = 0.5 ± 0.9‰ (Stack B) and is therefore distinct from atmospheric nitrate, constituting an excellent indicator of industrial derived nitrate. δ¹⁸O values of nitrate in PM_2.5 (36.0 and 17.6 ± 1.8‰ for Stack A and B, respectively) were also significantly lower than δ¹⁸O values of atmospheric nitrates and hence isotopically distinct. δ³⁴S values of sulfate in PM_2.5 were with 7.3 ± 0.3‰ (Stack A) and 9.4 ± 2.0‰ (Stack B) slightly enriched in ³⁴S compared to δ³⁴S in bitumen (4.3 ± 0.3‰) and coke (3.9 ± 0.2‰). δ¹⁸O values of sulfate in PM_2.5 were 18.9 ± 2.9‰ and 14.2 ± 2.8‰ for Stack A and Stack B, respectively. The isotopic composition of sulfate in PM_2.5 was not sufficiently different from δ³⁴S and δ¹⁸O values of sulfate in long-range atmospheric deposition in industrial countries to serve as a quantitative indicator for industrial emitted PM_2.5. We conclude that δ¹⁸O and Δ¹⁷O values of nitrate in stack-emitted PM_2.5 are excellent, and δ¹⁵N values of nitrate and ammonium are suitable indicators for identifying and tracing of PM_2.5 nitrate and ammonium emitted from two stacks in the AOSR in the surrounding terrestrial and aquatic ecosystems.