Evaluation and Inter-comparison of Open Road Line Source Models Currently in Use in the Nordic Countries

Abstract The aim of this study is to inter-compare and evaluate operational open road Gaussian line source models currently in use in the Nordic countries Norway, Denmark and Finland. By comparing the models and their results on different datasets, a more robust and objective assessment of model performance and applicability can be made. Four models, HIWAY2-AQ, OML-Highway, CAR-FMI and WORM, are applied to datasets from three measurement campaigns from each of the mentioned countries. A more specific target is to determine the conditions under which the models perform well or poorly in order to focus attention on these aspects in future model development. The various models arc evaluated primarily with regard to wind speed, wind direction and atmospheric stability in order to identify problem areas. Generally, the correlation between model estimates and observations decreases when normalising with emissions, due to the significant positive correlation between observed concentrations and emissions. Furthermore, we found a reduction of bias when normalising the Norwegian and Danish data, caused by overestimation of the dispersion at lower emission values. This occurs because the initial dispersion is too large in all the models. For higher emissions at the Danish site, the relative bias was higher, compared with the relative bias at the Norwegian site, indicating the influence of traffic density and vehicle speed (which arc both largest at the Danish site) on traffic produced turbulence and model performance. OML-Highway, however, performs best in this regard due to its more advanced parameterization of traffic produced turbulence based on production and decay of turbulent kinetic energy. With regard to horizontal profiles, RB for CAR-FMI increased as function of distance from the road, indicating that the Lagrangian time scales are too short.