Prediction of short-range maximum NO2 concentrations using scalar PDFS

Significant emissions of NOx (oxides of nitrogen: NO and NO2) are common from sources such as power stations, road tunnel ventilation outputs and freeways. As these species are chemically reactive (NO + O-3 --> O-2 + NO2), their concentrations downwind from the source are influenced by both the atmospheric turbulence (dispersion) and the chemical reactions. As part of an ongoing research project to investigate the effects of mixing on chemical reactions in the atmosphere, we make use of a new modelling technique based on the concentration statistics of a non-reactive (conserved) scalar. A simplified form of the model is an upper (conservative) limit on toxic NO2 concentrations. The novelty of this limit is that it is less conservative than previous methods commonly in use by air quality modellers but has a sound physical basis and can readily be calculated for atmospheric plumes. Model results are presented for NO, O-3 and NO2. When model predictions for NO2 are compared to atmospheric field data it is found that the model limit is a conservative bound on their concentrations. The present model is restricted to the plume centreline. The two main inputs are a parameterisation for the concentration fluctuations (conditional dissipation of scalar concentration fluctuations) and the probability density function (pdf) of the conserved scalar in the plume. Careful choices of these inputs are necessary for correct model behaviour and to ensure internal model consistency. Future work needed to improve present model limitations is described including: use of the pdf transport equation; ways to remove the need for model parameters by appeal to basic physics, and generalisation of the present centreline restriction. (C) 2003 Elsevier Ltd. All rights reserved.