A three-dimensional numerical model based on the Eulerian conservation equations for momentum, heat, water vapor, salinity, and air pollutants was used to examine the transport and diffusion processes in the planetary boundary layer. Horizontal diffusion was incorporated through a strongly and implicitly diffusive finite-difference scheme for the horizontal advection terms, viz, upwind differencing. However, terms representing other transport and diffusion processes were explicitly included in the differential equations. It is feasible to simulate the temporal variation of meteorological and pollutant variables, on a three-dimensional array containing several thousand grid points, within practical limits on a computer. Simulated urban-rural low level temperature differences in winter are qualitatively realistic. The simulated daytime vertical profile of pollutants show a well-mixed surface layer with quasi-constant concentrations. The nighttime profiles show definite peaks of concentration near the source height. Three-dimensional fields of meteorological and pollutant variables were simulated using Connecticut source inventory data and typical (hypothetical) winter meteorological conditions. (Author)