资源环境科技发展态势分析平台

Concentrations and delta S-34 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 S-34 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 (SO2)-S-34 was emitted. The second set is for periods when the CIMS was running (CIMS-ON) and (SO2)-S-34 was expected to affect SO2 and sulfate high-volume filter samples.

delta S-34 values for sulfate aerosols with diameter D > 0.49 mu m during CIMS-OFF periods (no tracer (SO2)-S-34 present) indicate the sulfur isotope characteristics of secondary sulfate in the region. Such aerosols had delta S-34 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 delta S-34 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 ac-companied by isotope fractionation in gas phase reactions with OH or the aqueous phase by H2O2 or O-3. Furthermore, analysis of S-34 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 delta S-34 values of dominantly secondary sulfate aerosols with D < 0.49 mu m 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 O-2 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 alpha-pinene (r = 0.85), beta-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.