a dispersion model has been developed to estimate concentrations of inert gaseous pollutants from curved, circular and straight sections of a complex road interchange. three, two or one map references define curves, lines and circles, respectively. other input data required are traffic flows, emission parameters, road heights, wind direction and speed, cloud cover, time and latitude. a continuous point-source or gaussian plume is integrated trapezoidally by stepping along each section of road. a 5 m step length appeared economical and accurate. concentrations of pollutants are shown to be very dependent on the relative heights of road and observer. at small wind angles predictions for a finite road agreed with published calculations for an infinite line source. concentrations decrease rapidly at large wind angles where the wind-observer line leaves the road. concentration curves of nitric oxide taken within a road system resemble those predicted by the model. simulation of plumes suggests that an initial decrease in concentration downwind from ground-level roads is supplemented by plumes from elevated road sections. this causes maxima in ground level concentrations and reduces the expected effect of distance in diluting motorway pollutants. under conditions of neutral atmospheric stability roads 12 m and 24 m high may cause concentrations of pollutant at 1 km downwind which are still half those 140 m from the interchange.(a)

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