Scaling the turbulent transport of chemical compounds in the surface layer under neutral and stratified conditions

A model of the vertical transport of chemically reactive species is presented which is applicable to all possible stability conditions in the atmospheric surface-layer. The non-dimensional formulation adopted allows any chemical scheme to be included. An equation hierarchy is presented for a fully second-order closure description of the process. The model describes comprehensively the turbulence/chemistry process by calculating the mean concentrations, the turbulent fluxes and concentration covariances of the chemical species. It explicitly takes into account the chemical transformation in first- and second-order variable equations. The model is applied to the NO-NO2-O-3 cycle and to the so-called 'night-time' chemical scheme of nitrogen oxides. An analysis of the flux budget indicates that the relative importance of the chemistry term in the flux equation varies with distance from the surface. For atmospheric neutral conditions, it is suggested that the chemistry term in the flux equation can be used to make an a priori evaluation of the possible effect of chemistry upon the nux of a chemical species. The model also shows that under stable conditions, despite the greatly reduced activity of turbulence, differences can occur between the turbulent transport of an inert tracer and that of a chemically reactive species. Under any stratification and for typical surface-layer fluxes of nitrogen oxides, the intensity of segregation is very low and can in general be neglected in model calculations.