Fate of SO2 and Particulate SO4 Based on Airborne Measurements in the Oil Sands Region of Alberta, Canada

An intensive airborne measurement campaign was undertaken in August to September 2013 to support the objectives of the Joint Oil Sands Monitoring (JOSM) program. The overarching objectives of the study were to characterize air pollutants being emitted, to determine the extent of atmospheric transport and chemical transformation, to support air quality model prediction capabilities, and to compare measurements with satellite column retrievals. Sulphur dioxide (SO2) and particulate sulphate (p-SO4) were among the pollutants studied. SO2 is emitted from elevated stacks within the oil sands facilities and undergoes atmospheric transformation into p-SO4. Deposition of these species from the atmosphere to the surface can lead to impacts on ecosystems downwind of the facilities. The processes of emission, transformation, transport, and deposition of SO2 and p-SO4 were investigated in detail using data collected during aircraft flights that were designed to study pollution transformation. The aircraft was flown at increasing distances downwind of the oil sands facilities, sampling the same plume at different times as it was transported away from the sources. Flight tracks were perpendicular to the wind direction at multiple altitudes to create virtual flight screens that encompassed the entire plume. Fluxes across each of the virtual screens were determined using the wind speed vector normal to the screen and the pollutant concentrations; the flux integration across the two-dimensional plume transect on the screen yielded the pollutant transfer rates at that particular screen location. Transformation of SO2 to p-SO4 between screens was determined based on OH radical levels estimated using concurrently measured concentrations of a suite of hydrocarbons. Based on mass balance between screens using the transfer rates, SO2 oxidation rates and p-SO4 formation rates, the deposition rates of both species are estimated along the plume transport path downwind of the oil sands operations. These observation-derived estimates are compared to corresponding predicted results from a nested air-quality model (GEM-MACH) operating for the same time period.