Modification of an operational dispersion model for urban applications

An operational multisource, multireceptor Gaussian dispersion model, the Da nish regulatory model Operationelle Meteorologiske Luftkvalitetsmodeller (O ML) has been modified for applications in urban environments. A so-called r oughness sublayer has been introduced into the model to represent the turbu lence characteristics of the lowest part of the surface layer over rough su rfaces like cities. The meteorological preprocessor was enhanced to take in to account an urban energy budget. The performance of the resulting OML-Urb an has been validated for nitrogen oxides and sulfur dioxide for Zurich for 1990. For that year, a detailed emission inventory as well as continuous h ourly measurements at four stations are available. The 24 air pollution mon itoring stations used for validation have been divided into different group s, depending on local influences from nearby roads. In comparison with resu lts from the standard OML using an urban roughness length, the urban modifi cation (roughness sublayer and changes in the meteorological 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 obs erved hourly concentrations, and the simulated yearly averaged concentratio ns show a good correspondence to observations.