IMPROVING CONCENTRATION MEASURES USED FOR EVALUATING AIR-QUALITY MODELS

An unfortunate difficulty in model evaluation is that the concentratio n measure that most models predict, namely the ensemble mean concentra tion under the plume centerline (or at some location relative to the p lume centerline), cannot be measured directly. The problem can be amel iorated by judicious selection of a concentration measure against whic h 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 conc entrations along the arc (arc maximum) as the measure of choice. In th is paper, the authors have considered two additional candidate measure s, the fitted maximum concentrations and the near-centerline concentra tions, which, intuitively, relate more closely to the ensemble mean co ncentrations. This study shows that the maximum concentrations along t he arc are significantly higher than either the fitted maxima or the n ear-centerline concentrations. In addition, of the latter two measures , the authors conclude that use of the near-centerline concentration i s preferable to the use of fitted maximum for the purposes of evaluati ng model performance.