Evolution of Nitrogen Oxide Chemistry in the Nocturnal Boundary Layer

Abstract

The nocturnal cycle of nitrogen oxides in the atmospheric boundary layer is studied by means of a one-dimensional model. The model solves the conservation equations of momentum, entropy, total water content, and of five chemical species. The chemical cycle relates to the nighttime conversion of NO, NO2, and O3 into HNO3 via NO3 and N2O5. For simplicity, only homogeneous chemical reactions are considered. The turbulent fluxes of momentum, temperature, and moisture and of the chemical species are determined by means of a second-order closure model. The fluxes of the chemically reactive species are determined by explicitly taking into account the chemical transformation during the transport process. The one-dimensional model simulates a stable boundary layer with typical rural concentrations of the above-mentioned species. To study the effect of heterogeneous mixing due to the strong gradients of temperature and concentrations, the authors compare the one-dimensional model results with the results obtained with a box model. The study demonstrates that the concentration of NO plays a considerable role in the 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 vertical direction and the formation of large concentration gradients. In these conditions, the study of the process by means of a box model can lead to an inaccurate estimate of the concentrations of species like NO and NO3.