The plume from the GCOS power plant chimney near Fort McMurray, Alberta was sampled by helicopter during a total of four weeks in February and June, 1977, with the primary aim of studying the rate of SO2 oxidation to sulfate particulates under various meteorological conditions. A double-filter system was used for the rate studies, and continuous SO2 and O3 analysers were also on board for some of the flights. The atmospheric temperature and relative humidity at plume height varied between −13 and 23°C, and 40 and 95% respectively. The field study also covered relatively large changes in solar intensity.

The SO2 oxidation rate was found to be low during the February and early morning June flights — typically, less than 0.5% h−. Later in the day in June, appreciable oxidation was observed (approx. 1–3% h−). An ozone “bulge” was also noted downwind in the plume during several of these latter flights. The results suggest that photochemical processes play an important role in SO2 oxidation for this power plant plume.