PNNL

Abstract

A series of instantaneous releases of sulfur hexafluoride tracer were carried out as part of the Joint Urban 2003 field campaign conducted in Oklahoma City. Data from a set of ten fast-response tracer samplers were used to examine the crosswind and along-wind spread of the tracer, the decay of tracer concentrations, and the retention of tracer in an urban environment within approximately 1km of the release locations. The time variation of the median values of the tracer concentrations, normalized by the peak value observed at a given sampler, for a set of 3 or 4 puff releases during a study period could be described by an exponential decay with characteristic decay times on the order of one to two minutes. The longer times were found for early morning releases and the shorter times were associated with later morning or afternoon releases, suggesting that atmospheric stability may affect puff dispersion even in urban environments. The median retention times, i.e., the times required for 99% of the exposure to be realized at a given location, were found to be reasonably well correlated with the median decay times. These characteristic time scales should be regarded as lower limits for concentration decay in an urban area because the analysis excluded a number of anomalous cases in which the concentrations from one release could not be readily distinguished from contributions from a previous release. In general, the variation of the along-wind dispersion parameter, sx, did not appear to be well described by linear dependencies on either the effective advection speeds of the puffs or the distance from the release point. Lateral puff dispersion as measured by sy, was roughly 70% of the along-wind dispersion at distances of 0.5 to 1 km from the release point.