Concentrations and δ 34 S values for SO 2 and sizesegregated 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 SO 2 were collected on filters using a high-volume sampler, with 12 or 24 h time intervals.Sulfur dioxide (SO 2 ) enriched in 34 S 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 34 SO 2 was emitted. The second set is for periods when the CIMS was running (CIMS-ON) and 34 SO 2 was expected to affect SO 2 and sulfate highvolume filter samples.δ 34 S values for sulfate aerosols with diameter D > 0.49 µm during CIMS-OFF periods (no tracer 34 SO 2 present) indicate the sulfur isotope characteristics of secondary sulfate in the region. Such aerosols had δ 34 S 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 δ 34 S 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 SO 2 oxidation and ac-companied by isotope fractionation in gas phase reactions with OH or the aqueous phase by H 2 O 2 or O 3 . Furthermore, analysis of 34 S enhancements of sulfate and SO 2 during CIMS-ON periods indicated rapid oxidation of SO 2 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. Anticorrelations between δ 34 S 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 SO 2 was oxidized by a transition metal ion (TMI) catalyzed pathway involving O 2 and Fe 3+ and/or Mn 2+ , an oxidation pathway known to favor lighter sulfur isotopes.Correlations between SO 2 to sulfate conversion ratio (F (s)) and the concentrations of α-pinene (r = 0.85), βpinene (r = 0.87), and limonene (r = 0.82) during daytime suggests that SO 2 oxidation by Criegee biradicals may be a potential oxidation pathway in the study region.

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