Stable sulfur isotope measurements to trace the fate of SO₂ in the Athabasca oil sands region

Abstract

Concentrations and δ34S values for SO2 and size-segregated sulfate aerosols were determined for air monitoring station 13 (AMS 13) at Fort MacKay in the Athabasca oil sands region, northeastern Alberta, Canada as part of the Joint Canada-Alberta Implementation Plan for Oil Sands Monitoring (JOSM) campaign from 13 August to 5 September 2013. Sulfate aerosols and SO2 were collected on filters using a high-volume sampler, with 12 or 24 h time intervals. Sulfur dioxide (SO2) enriched in 34S was exhausted by a chemical ionization mass spectrometer (CIMS) operated at the measurement site and affected isotope samples for a portion of the sampling period. It was realized that this could be a useful tracer and samples collected were divided into two sets. The first set includes periods when the CIMS was not running (CIMS-OFF) and no 34SO2 was emitted. The second set is for periods when the CIMS was running (CIMS-ON) and 34SO2 was expected to affect SO2 and sulfate high-volume filter samples. δ34S values for sulfate aerosols with diameter D > 0.49 µm during CIMS-OFF periods (no tracer 34SO2 present) indicate the sulfur isotope characteristics of secondary sulfate in the region. Such aerosols had δ34S values that were isotopically lighter (down to −5.3 ‰) than what was expected according to potential sulfur sources in the Athabasca oil sands region (+3.9 to +11.5 ‰). Lighter δ34S values for larger aerosol size fractions are contrary to expectations for primary unrefined sulfur from untreated oil sands (+6.4 ‰) mixed with secondary sulfate from SO2 oxidation and accompanied by isotope fractionation in gas phase reactions with OH or the aqueous phase by H2O2 or O3. Furthermore, analysis of 34S enhancements of sulfate and SO2 during CIMS-ON periods indicated rapid oxidation of SO2 from this local source at ground level on the surface of aerosols before reaching the high-volume sampler or on the collected aerosols on the filters in the high-volume sampler. Anti-correlations between δ34S values of dominantly secondary sulfate aerosols with D < 0.49 µm and the concentrations of Fe and Mn (r = −0.80 and r = −0.76, respectively) were observed, suggesting that SO2 was oxidized by a transition metal ion (TMI) catalyzed pathway involving O2 and Fe3+ and/or Mn2+, an oxidation pathway known to favor lighter sulfur isotopes. Correlations between SO2 to sulfate conversion ratio (F(s)) and the concentrations of α-pinene (r = 0.85), β-pinene (r = 0.87), and limonene (r = 0.82) during daytime suggests that SO2 oxidation by Criegee biradicals may be a potential oxidation pathway in the study region.