EVOLUTION OF NITROGEN-OXIDE CHEMISTRY IN THE NOCTURNAL BOUNDARY-LAYER

The nocturnal cycle of nitrogen oxides in the atmospheric boundary lay er is studied by means of a one-dimensional model. The model solves th e conservation equations of momentum, entropy, total water content, an d of five chemical species. The chemical cycle relates to the nighttim e conversion of NO, NO2, and O-3 into HNO3 via NO3 and N2O5. For simpl icity, only homogeneous chemical reactions are considered. The turbule nt fluxes of momentum, temperature, and moisture and of the chemical s pecies are determined by means of a second-order closure model. The fl uxes of the chemically reactive species are determined by explicitly t aking into account the chemical transformation during the transport pr ocess. The one-dimensional model simulates a stable boundary layer wit h typical rural concentrations of the above-mentioned species. To stud y the effect of heterogeneous mixing due to the strong gradients of te mperature and concentrations, the authors compare the one-dimensional model results with the results obtained with a box model. The study de monstrates that the concentration of NO plays a considerable role in t he formation of NO3, N2O5, and HNO3. The reduced-activity of turbulent transport shows that the chemical activity in the boundary layer can be decoupled from that of the so-called reservoir layer. The stability conditions induce inhomogeneous distribution of the species in the ve rtical direction and the formation of large concentration gradients. I n these conditions, the study of the process by means of a box model c an lead to an inaccurate estimate of the concentrations of species lik e NO and NO3.