Occurrences of maximum 0.5 h hour ground level measurements of SO2 near a point source are related to the meteorological processes at the time such that a cause analysis of the occurrences is established. Measurements, from a 10 station, continuous air quality monitoring network, range from 0.12 to 0.65 ppm and are taken over a 2 year period, from 1976 to 1978, in the Alberta Oil Sands Environmental Research Program (ASOERP) study area. Point source emissions during this period average 125 tonnes sulphur per day (125 MTS/day). Meteorological data used in the analysis consist of local minisonde temperature profiles, synoptic weather maps, wind and temperature data from a 152 m tower, data from a nine station climatology network, local measurements of incoming solar radiation, and airport weather office and regional radiosonde observation (RAOB) data. Analysis of ambient monitoring measurements indicates that SO2 concentrations can be grouped into case data sets that suggest an influence of specific synoptic weather systems. Results of the analysis show that the soconcentrations are caused by arctic air mass inversions occurring in the spring, fall, or winter seasons. Frontal inversions with moderate winds, and convective mixing have also been identified as contributing factors for other cases, occurring in the spring. Comparison of observed and calculated ground level concentrations is made using Alberta Environment's dispersion program. The two-layer model predicts within a factor of 2 for the trapping cases. The unlimited mixing cases only predict to within a factor of 10 to 100.