A numerical study of the dispersion and chemical reaction of nitric monoxide released from a ground area source into the atmospheric boundary layer is presented. The idealized reversible reaction between ozone and nitric oxides, NO + O3 e NO2 + OZ. is considered as a representative reaction. Macromixing resulting from mean advection and turbulent diffusion is duly solved by means of Eulerian averaged transport equations in which mean reaction rates are closed owing to a model that is valid at arbitrary Damkiihler number. This model, providing an approximated joint p.d.f. of reactive species, allows to estimate the influence of small-scale nonhomogeneity on chemical reactions. In the case under study, it is found that moderate chemistry prevails close to the ground while chemical equilibrium is approached with increasing altitude. The outstanding result is the considerable lessening of ozone and nitric monoxide consumption induced by turbulence all over a plume-shaped flow region which is about 10 km long downwind and spreads up to 500 m high; it is proved that neglecting the effect of turbulent fluctuations on chemical reaction rates would lead to large error in the prediction of mean concentrations in this zone.