Modification of an operational dispersion model for urban applications

Abstract

An operational multisource, multireceptor Gaussian dispersion model, the Danish regulatory model Operationelle Meteorologiske Luftkvalitetsmodeller (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 nitrogen oxides and sulfur dioxide for Zurich for 1990. For that year, a detailed emission inventory as well as continuous hourly measurements at four stations are available. The 24 air pollution monitoring stations used for validation have been divided into different groups, depending on local influences from nearby roads. In comparison with results from the standard OML using an urban roughness length, the urban modification (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 observed hourly concentrations, and the simulated yearly averaged concentrations show a good correspondence to observations.