Traffic pollution modelling and emission data

Evaluation of traffic pollution in streets requires basically information on three main factors: traffic emissions, the meteorological conditions and the street surroundings. Dispersion models exist with various degree of sophistication, which are able to properly describe the dispersion conditions, and thus to predict the relationships between emissions and the concentration levels in the street. However, for real-world applications, the model calculations must be based on ‘‘true’’ emission data, and their estimation is not trivial. Significant uncertainty is still connected with emission data. Examining the relationships between model predictions and measurements with respect to the meteorological conditions and inter-relationships between different pollution components allows quantitative evaluation of the traffic emissions. This methodology is illustrated using the Danish Operational Street Pollution Modele OSPM, and time series of traffic related pollutants. Street level concentrations of NOx and CO are calculated using OSPM as the dispersion model and emission data estimated by the widely used COPERT methodology. Comparison with measurements shows significant underestimation of the pollution concentrations and especially the CO/NOx ratio. An alternative set of traffic emission factors, using a more simplified vehicle classification methodology, provides better agreement with the measured concentrations. Evaluation of these results provides some guidance on the necessary modifications of the ‘‘real-world’’ traffic emission factors.