High pollution levels have been often observed in urban street canyons due to the increased traffic emissions and reduced natural ventilation. Microscale dispersion models with different levels of sophistication may be used to assess urban air quality and support decision making for pollution control strategies and traffic planning. Mathematical models calculate pollutant concentrations by solving either analytically a simplified set of parametric equations or numerically a set of differential equations that describe in detail wind flow and pollutant dispersion. Street canyon models, which might also include simplified photochemistry and particle deposition-resuspension algorithms, are often nested within larger-scale urban dispersion codes. Reduced-scale physical models in wind tunnels may also be used for investigating atmospheric processes within urban canyons and validating mathematical models. A range of monitoring techniques is used to measure pollutant concentrations in urban streets. Point measurement methods (continuous monitoring, passive and active pre-concentration sampling, grab sampling) are available for gaseous pollutants. A number of sampling techniques (mainly based on filtration and impaction) can be used to obtain mass concentration, size distribution and chemical composition of particles. A combination of different sampling monitoring techniques is often adopted in experimental studies. Relatively simple mathematical models have usually been used in association with field measurements to obtain and interpret time series of pollutant concentrations at a limited number of receptor locations in street canyons. On the other hand, sophisticated numerical codes have often been applied in combination with wind tunnel and/or field data to simulate small-scale dispersion within the urban canopy.

No abstract

Abstract:The requirements of the framework Directive on air quality assessment and management introduce real practical problems for the meteorological community. Some of the meteorological variables needed in urban air pollution assessments are not routinely measured and in normal circumstances the number of meteorological stations in urban areas is limited to a few sites often just at airports. The European wide project COST 715 on "Meteorology applied to Urban Air Pollution Problems" has been set up to review these problems. This paper describes limitations of current methods. It recognises that the urban boundary layer is a non-equilibrium situation where standard theories applied to rural areas may not be valid. It shows that predictions of concentrations for assessments should always state the uncertainty associated with them. This uncertainty can be large because of the complexity of urban pollution situations, particularly the description of the local urban meteorology. An example is given of how with some additional, local meteorological measurements, the uncertainty can be reduced. The paper concludes by listing some of the key areas where further work is required. Information about the COST 715 project is available from the Web site at

No abstract