Caitríona Ní Riain, Ben Croxford, John Littler and Alan Penn
How well does a particular geometry of city space affect pollution dispersion? This is a question that assumes some significance in the search for sustainable urban forms. It has been argued that the compact city may reduce travel and hence vehicle emmissions, but there are also suggestions that it may result in overcrowding, traffic congestion and air pollution, affecting the quality of life. It is thus important to understand the relationship between pollution dispersion and urban form. The geometry of a city contributes to the determination of its own climate both on a city-wide and a street scale. The implications for monitoring and modelling wind field and pollution dispersion in the urban environment are such that it is difficult to generalise for pollution movement in cities, or even different spaces of the same city. The varying relief of an urban environment means it is difficult to make meaningful air quality measurements which can help with decision-making, and it provides a highly demanding modelling challenge. Yet the benefits to be gained from a good, well-tested model extend to policy-makers, urban designers, transport planners, environmental engineers and, ultimately, to the city-dwellers and workers.
The complex topographical structure of a city, and the structure of its urban boundary layer (Fig. 1) mean that the dominant forcing terms of flow and diffusion of pollutants in the urban canopy layer are the geometry of the space, the wind profile and stability conditions in the urban boundary layer-all mutually dependent variables. Atmospheric stability governs the interaction between the city and boundary layer and influences the wind profile. The wind profile above, and in, a city is determined by the meso-scale and micro-scale topography of a city, and influences atmospheric stability conditions. The city's activities in terms of heat production and radiation from surfaces also determine atmospheric stability conditions over a city which, in turn, influence the vertical dispersion of pollutants from a city.
In many cases, such as that of Mexico City, the micro-scale geometry of the