This work describes the diurnal evolution of carbon monoxide, continuously emitted by an area source located at the surface, over a flat terrain region in the city of São Paulo, Brazil. The carbon monoxide was simulated numerically using a large eddy simulation model developed by Moeng for a typical winter daytime period in a subtropical urban area, when the turbulence is dominated by buoyancy but mechanical production is important. The numerical results indicated that the diurnal evolution of the carbon monoxide in the city of São Paulo is characterized by a decreasing in the mixed layer concentration after noontime, that are consistent with the observation carried out at the surface. The entrainment of clean air combined with the bimodal traffic of vehicle is responsible by most of the diurnal evolution of carbon monoxide in the city of São Paulo.