A study of pollutant concentration variability in an urban street under low wind speeds

The short time-scale variability in pollutant concentrations in an urban street under very low wind speed conditions and short source-receptor distance has been investigated using the inert tracer sulfur hexafluoride (SF₆) as a continuous point-source (release times ≥ 5 min), and fast detection using separation by gas chromatography coupled with a μ-electron capture detector (ECD). The results are complex but can be broadly interpreted in terms of horizontal wind speed and direction coherence. Comparisons with a simple dispersion model suggest that observed time-averaged maximum concentrations approach predicted values, whilst instantaneous maximum concentrations vary greatly and would therefore be difficult to predict.