Effect of atmospheric stability on concentration fluctuations and wake retention times for dispersion in the vicinity of an isolated building

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Abstract
This study is concerned with the effect of changes in atmospheric stability on concentration fluctuations near an isolated building. Field experiments were conducted to measure concentration fluctuations at several locations close to a model building, for continuous point sources upwind of the building. Experiments were conducted in flat terrain using a 2 m cube as the model. Concentrations were measured at a frequency of 200 Hz using a detector system with a response time of 1/50 second. Meteorological conditions varied from stable to unstable. Analysis of the time series of concentration produced by these experiments has shown that there are several periods when the gas behaves as if it were switched off at the source (due to shifts in the wind direction). Comparisons were made in such cases of the time taken for the source gas to evacuate the wake recirculation region. It was found that this period was several times longer in stable meteorological conditions than in unstable conditions. This finding has important implications in relation to hazard assessment near buildings.