Field measurements of the benefits of increased stack height

Analysis of a series of 31 field diffusion experiments permitted direct specification of the benefit of increased stack height. In the experiments, two tracers were released simultaneously from two different elevations on a 122 m tower, and were sampled at ground level on a series of arcs concentric about the release point. The ratio of observed concentrations at any sampling distance provides a measure of stack height benefits which is unobscured by differences in meteorology between experiments. Three combinations of upper/lower release elevations were used: 26 m/2 m, 56 m/26 m and 111 m/56 m. In a plume with minimal initial rise, there is a dilution at stack exit which is directly proportional to the wind speed at the top of the stack. At distances relatively far from the source, the benefits in reduced concentration resulting from increasing source height from 26 m to 56 m were found to plateau at approximately the ratio of wind speeds at the 26 and 56 m levels. Benefits accruing as a result of increasing from 56 m to 111 m exceeded the wind speed ratio at all distances. Conversely, benefits for an increase from 2 m to 26 m were lacking at distances beyond about 4 km.