Buoyant plumes and wakes.The problem of mixing of pollutants induced by vehicular motions is considered as an example of dispersion that is initially unaffected by atmospheric turbulence. Some models are discussed that have proven helpful in understanding certain aspects of the problem of dispersion of air pollutants in the atmosphere. However, the considerable variability of atmospheric flow, as compared with a laboratory flow, makes the application of these models, both experimental and theoretical, of limited scientific usefulness to the prediction of ambient air-pollutant levels.