Evaluation of GRAL for the pollutant dispersion from a city street tunnel portal at depressed level

A new data set for evaluation of atmospheric dispersion models, designed to treat road tunnel portals, is presented. Further, the performance of GRAL 3.5 (Graz Lagrangian model Version 3.5) using this data is discussed. One main drawback of the model in its current version is that two empirical parameters have to be adjusted for each tunnel site separately in dependence on average traffic volume and construction of the tunnel portal. As the model has been tested for five different tunnel sites by now, which differ significantly in traffic volume, terrain, and construction, model users should be able to choose reasonable values for those empirical parameters for other tunnel sites as well. GRAL has not been tested against field data, where the tunnel air is colder than ambient air. Here, further research is necessary.