Comparison of an Analytical Urban Puff-Dispersion Model with Tracer Observations From the Joint Urban 2003 Field Campaign

Abstract

Puffs of SF6 tracer gas were released at street level in a built-up urban centre (Oklahoma City) during the Joint Urban 2003 (JU2003) field experiment. Although the continuous (20-min) releases during JU2003 have received much attention, the instantaneous “puff” releases have seldom been studied. Ten real-time SF6 samplers (with 0.5-s resolution) produced useful concentration observations at downwind distances, x, from approximately 100 to 1000 m during 36 puff releases over ten days (each day is a numbered intensive operating period). Here we address the maximum 0.5-s concentration, Cmax, for each sampler and puff; the dosage for each sampler and puff; and the maximum concentration over all samplers for each puff. We propose, as an urban dispersion model, a simple analytical Gaussian puff formulation, with the dispersion parameter, σ, assumed equal for all three components. The formulation, σ = σo+ σturb, is used, where σo represents the initial mixing (30 m) due to local obstructions, and σturb is the turbulent dispersion component (0.17x). The model is best used for distances beyond the first row or two of buildings. The simple urban puff model is able to predict Cmax and dosage with low relative mean bias (less than about 30%) but with a scatter of about a factor of two. This simple urban puff model is part of a comprehensive urban dispersion model comparison using the JU2003 puff data and is intended to provide a baseline against which more detailed models can demonstrate statistically significant improvements.