160 Int. J. Environment and Pollution, Vol. 14, Nos. 1–6, 2000 Copyright © 2000 Inderscience Enterprises Ltd. Validation of models AEROFOUR and AEROPOL using the model validation kit established at Mol M. Kaasik Department of Environmental Physics, University of Tartu, Tähe str. 4, 51010 Tartu, Estonia Abstract: The AEROPOL and AEROFOUR dispersion models developed at Tartu Observatory (Estonia) are validated using the Lillestrøm dataset and following the model comparison rules established at the workshop in Mol. Running the AEROFOUR model, the vertical wind and temperature covariations not included into the official dataset were used. The computed and measured arc-wise maximum and crosswind integrated concentrations match fairly. No clear tendency of over- or under-prediction was found. The best performance was achieved by AEROFOUR model with the above-mentioned covariations included into the initial dataset. This demonstrates that the detailed measurement techniques give clear advances for model estimations if the model is designed to use this kind of data. Keywords: AEROFOUR, AEROPOL, atmospheric dispersion models, Lillestrøm experiment, model evaluation, model validation. Reference to this paper should be made as follows: Kaasik, M. (2000) ‘Validation of models AEROFOUR and AEROPOL using the model validation kit established at Mol’, Int. J. Environment and Pollution, Vol. 14, Nos. 1–6, pp. 160–166. 1 Introduction The model validation exercise for the air pollution model harmonization workshop at Mol consisted of three datasets: those from Kincaid, Copenhagen, and Lillestrøm. As the modelling showed, the last of these was most difficult to reproduce (Olesen, 1995). The experiment was carried out in a plain suburban landscape with 6 to 10 m high buildings and trees. The most important peculiarity of the Lillestrøm experiment was stable thermal stratification, which was present during six experimental runs out of eight. The wind velocity at 10 m ranged from 0.5 to 2 m/s. The passive tracer (sulphur hexafluoride) was released at 36 m height. During each experimental run the concentrations were measured at 2 or 3 arcs at distances of 150 to 900 m downwind from the source. The sampling points were placed at 1 m above street level. All five models compared at Mol under predicted the arc-wise maximum and crosswind integrated concentrations. Dispersion modelling in conditions of thermal inversion is especially difficult, because relatively small thermal and mechanical perturbations may cause large relative changes of diffusivities and, therefore, dramatically change the concentration patterns near the surface. In such cases, direct measurements of wind and temperature fluctuations are useful. These measurements were performed during the Lillestrøm experiment, but only the dispersions of wind components were included into the Model Validation Kit