Evaluation and inter-comparison of open road line source models currently in use in the Nordic countries

The aim of this study was to compare and evaluate operational open-road Gaussian line-source models currently in use in Norway, Denmark and Finland. Four models, HIWAY2-AQ, OML-Highway, CAR-FMI and WORM, were applied to datasets from three measurement campaigns from each of the mentioned countries. The results were assessed through analysis with regard to normalisation, wind speed, wind direction, horizontal profiles and stability. Generally, the correlation between model estimates and observations decreased 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. Due to OML-Highway's more advanced parameterisation of traffic-produced turbulence, this model performed best at higher emission values when the influence of traffic density and vehicle speed on traffic produced turbulence was higher. With regard to horizontal profiles, the relative bias for CAR-FMI increased as a function of distance from the road, indicating that the Lagrangian time scales are too short.