Meteorological pre-processing and atmospheric dispersion modelling of urban air quality and applications in the Helsinki metropolitan area

Abstract

This theses describes the boundary-layer parameterisation model, which is applied as a meteorological pre-processor by Finnish regulatory dispersion models. The parametrisation scheme is based on the energy-flux method, which evaluates the turbulent heat and momentum fluxes in the boundary layer.

A comparison of the model predictions with those of the corresponding scheme of Berkowicz and Brahm, using the same synoptic input data is presented. The comparison shows the basic physical differences between these two models. Some numerical results calculated by these two pre-processors differ substantially, while the results for net radiation are statistically nearly identical.

Meteorological profile data from Finnish sounding stations is compiled and the inversions (the temperature of the atmosphere increases with altitude) are classified according to the total depth of the inversion and the stability of the boundary layer, which was estimated directly from the temperature gradient in the layer from the ground up to 100 meters. The persistence of the inversions and the influence of cloudiness and wind speed on the temperature gradient are also addressed. The specific problems of an urban area is tackled, using measurements from a meteorological mast situated in the Helsinki metropolitan area by comparing them to those from the radiosonde profiles at the rural site of Jokioinen. Several common schemes for the height of the stable boundary layer are compared to results from our own method developed at Finnish Meteorological Institute (FMI). Based on this analysis a modification to the FMI method is suggested.

An integrated modelling system for evaluating the traffic volumes, emissions from stationary and vehicular sources, and atmospheric dispersion of pollution in an urban area is described. The dispersion modelling is based on combined application of the Urban Dispersion Modelling system for stationary sources (UDM-FMI) and the road network dispersion model (CAR-FMI). The system includes also a meteorological pre-processing model (MPP-FMI) and a statistical and graphical analysis of the computed time series of concentrations. The modelling system contains a method, which allows for the chemical interaction of pollutants, originating from a large number of urban sources.

An application of the integrated modelling system for estimating the NOx and NO2 concentrations in the Helsinki metropolitan area in 1993 is presented. The thesis finally presents a comparison of model predictions with the results of an urban measurement network.