Observational studies of air quality and air flow over a ridge downwind of a stack

The body of information presented in this paper is directed to those persons in industry and government who are concerned with the effects of elevated ground on plume dispersion. A field program is being conducted around a sour gas processing plant to determine the extent to which the ridge, situated near the stack, affects the air quality and air flow that would otherwise be observed over a flat terrain. Of particular interest is the definition of the weather types leading to the worst pollution episodes. From such studies optimization of stack design for similar stack/topographical configurations can be made. The analysis of routinely collected SO2 and wind data for two locations along the ridge has shown that, for flow towards the ridge, high wind neutral atmospheres at the location nearest the plant and lighter wind stable atmospheres at the location furthest from the plant produce the highest frequency of peak concentrations respectively. This has prompted an investigation of the flow patterns and plume behaviour for these conditions. A flow visualization experiment is being conducted to examine the effect of stable flow towards the ridge. Smoke trails up to 200m above ground are generated and a photogrammetric procedure is used to examine the smoke trajectories at various heights above the terrain. Experiments conducted for stable flows indicate the presence of a ridge induced directional shear providing evidence of a mechanism to explain the high frequency of peak concentrations observed under these conditions.