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

Abstract

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 PM2.5 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 PM2.5 compared to those reported for atmospheric nitrate and ammonium in the literature. Nitrate in PM2.5 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 PM2.5 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 PM2.5 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 PM2.5 (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 PM2.5 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 PM2.5 were 18.9 ± 2.9‰ and 14.2 ± 2.8‰ for Stack A and Stack B, respectively. The isotopic composition of sulfate in PM2.5 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 PM2.5. We conclude that δ18O and Δ17O values of nitrate in stack-emitted PM2.5 are excellent, and δ15N values of nitrate and ammonium are suitable indicators for identifying and tracing of PM2.5 nitrate and ammonium emitted from two stacks in the AOSR in the surrounding terrestrial and aquatic ecosystems.