An operational multi-source, multi-receptor Gaussian dispersion model, the Danish regulatory model OML, has been modified for applications in urban environments. A so-called roughness sublayer has been introduced into the model to represent the turbulence characteristics of the lowest part of the surface layer over rough surfaces like cities. The meteorological preprocessor was enhanced to take into account an urban energy budget. The performance of the resulting OML-Urban has been validated for NOx and SO2 for the city of Zurich for the year 1990. For this year, a detailed emission inventory as well as continuous hourly measurements at four stations are available. The air pollution monitoring stations used for validation have been divided into different groups, depending on local influences from nearby roads. The urban modification (roughness sublayer and changes in the met. preprocessor) results in a 25–35% increase of the annual mean surface concentration. OML-Urban shows a good reproduction of the probability density function of predicted concentrations, and the simulated yearly averaged concentrations show a good correspondence to observations.
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