Characteristics of Turbulence and Dispersion of Pollutants Near Major Highways

Abstract

The primary objective of this study is to assess the effect of traffic on the turbulence structure and to infer the time and space scales of the eddies generated by the traffic. To this end, time series of wind and temperature were obtained by a three-component sonic anemometer and by copper-constantan thermo-couples adjacent to the Long Island Expressway in New York State. Eddy fluxes of heat and momentum were computed under different atmospheric conditions. Spectral distributions of these parameters were obtained using the fast Fourier transform technique. The flow characteristics in the surface layer are inferred from the wind profiles adjacent to the highway. Results show a distinct bulge in the high-frequency range of the wind spectrum. This bulge appears only during moderate to heavy traffic conditions and with wind across the highway. This traffic-induced turbulent energy appears to be dominant at mean frequencies to 0.1?1.0 Hz corresponding to eddy sizes of the order of a few meters. Even under quite stable atmospheric conditions, no organized convection due to vehicle exhaust heat can be distinguished in the spectral structure. The aerodynamic drag due to the moving vehicles on the highway is manifested by a pronounced acceleration of wind in the lowest 8 m, especially in the cases of wind directions nearly parallel to the highway. The impact of traffic-induced turbulence on the near-roadway dispersion of air pollutants is also discussed.