Parameterization of the pollutant transport and dispersion in urban street canyons

A two-dimensional, time-dependent flow model has been applied to the evaluation of pollutant dispersion properties in urban street canyons. The processes by which pollutants emitted at street level are transported and diffused from street canyons are examined to determine the exchange rates of pollutants between street canyons and the upper air under variable flow conditions and different aspect ratios (the ratio of the building height to the street width). The time constant, defined as the time required for the average pollutant concentration in the street canyon to be reduced to e−1, is parameterized as a function of canyon cross-sectional area, influx strength, Reynolds number, Peclet number, and aspect ratio. Time constants may be applied directly to urban air quality models for better estimation of emission rates.