Dispersion of traffic pollution from street canyons in Edinburgh

The IOM was sponsored by The Scottish Office to look at the effect of the canyon-like topography of streets in Scottish cities on the dispersion of traffic pollution. In particular there was concern that the retardation of the dispersion of traffic pollution by tall buildings in some central city streets may be sufficient to have an adverse effect on the health of those who live or work in these streets.The first stage of the project involved a literature review to assess the likelihood of the canyon-like topography of Scottish city streets having a significant effect on local air quality. Model simulations do suggest that the dispersion of traffic pollution may be retarded by streets with similar aspect ratios to some Edinburgh streets. The greatest build-up of pollutants would be expected in streets perpendicular to the wind with maximum concentrations arising at street level on the sheltered side of the street. Measurements of traffic pollution in other cities have not, however, unequivocally established the existence of a canyon effect. Some reports suggest that street geometry has a smaller effect on local air quality than patterns of traffic flow. Edinburgh street canyons are not particularly deep or continuous and any canyon effect is likely to be smaller than in either the model simulations or other cities, where building heights may exceed twenty storeys. Given that the reduction in pollutant concentrations with height above the street, is in proportion to the height of the canyon, even if the canyon effect does arise in Edinburgh, relatively high concentrations of pollution are only likely to arise at street level. In other cities, the same concentrations might extend to four or five floors of a twenty storey building.The second phase of the project involved measuring indoor and outdoor concentrations of air pollution in two representative street canyons in Edinburgh (Frederick Street and South Bridge). The aims of these measurements were to establish whether the canyon topography of Scottish streets has a detectible effect on the dispersion of pollutants and whether it is likely that pollution concentrations within street canyons exceed national and international guides and limits more frequently than is suggested by routine air quality monitoring data. Further aims were to establish whether the canyon phenomenon has a detectible effect on indoor air quality and to establish whether there is a need to introduce more or less radical changes in traffic control in order to reduce the concentration of pollutants within street canyons.