Improving Concentration Measures Used for Evaluating Air Quality Models

An unfortunate difficulty in model evaluation is that the concentration measure that most models predict, namely the ensemble mean concentration under the plume centerline (or at some location relative to the plume centerline), cannot be measured directly. The problem can be ameliorated by judicious selection of a concentration measure against which to compare model predictions. Insufficient attention has been given in the past to the selection of an appropriate measure for use in air quality model evaluation studies, which may have resulted in biases in the results of those studies. Some studies have used the maximum concentrations along the arc (arc maximum) as the measure of choice. In this paper, the authors have considered two additional candidate measures, the fitted maximum concentrations and the near-centerline concentrations, which, intuitively, relate more closely to the ensemble mean concentrations. This study shows that the maximum concentrations along the arc are significantly higher than either the fitted maxima or the near-centerline concentrations. In addition, of the latter two measures, the authors conclude that use of the near-centerline concentration is preferable to the use of fitted maximum for the purposes of evaluating model performance.