Abstract
Experimental studies have shown that tracers injected over short time intervals can often be measured downwind near the ground for a much longer time than would be expected considering the duration of the release and the wind speed. A two-dimensional advection-diffusion model was used in a numerical study to investigate this effect. The results from the numerical study showed that the rate of decrease in concentrations downwind at a receptor after a source has stopped emitting was more sensitive to magnitude of the wind speed and alongwind mixing coefficient than the rate of increase in concentrations when the source first starts emitting. The lower the wind speed or the greater the mixing, the more slowly the concentration decreased as the region upwind of the receptor continued to contribute tracer until it was depleted by advection and vertical diffusion. These atmospheric processes are able, qualitatively, to account for the slowly decreasing air concentrations.
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