Modeling the Diurnal Cycle of Conserved and Reactive Species in the Convective Boundary Layer

The behavior of trace reactive species in the convective boundary layer (CBL) is of considerable interest for determining the fate of substances emitted by vegetation (or the ground) or entrained into the CBL from the overlying free troposphere (FT). These species may react photochemically or with other species. If their reaction time constants are between about 0.1 and 10 times the mixing time of the CBL, the species mean and flux profiles may be significantly modified from conserved species profiles. To study this, we have developed a simple one-dimensional, second-order closure model for reactive scalar evolution in the CBL with parameterized third-order moment terms. A simple mixed layer model calculates the diurnally-varying entrainment rate across the CBL top (that is then used to calculate CBL height), the mean virtual potential temperature in the CBL, and the temperature difference across the CBL top. These variables are then combined with the second-order model of the turbulence and mean CBL structure for both conserved and chemically reactive species. The species can have surface sources or sinks, and fluxes across the top of the CBL are calculated from the turbulent entrainment of FT air. Previously, the model was applied to a steady-state CBL; here we generalize the model to allow time variability in order to simulate a typical diurnal cycle. The results presented here are for a shear-free CBL and use free-convection surface-layer scaling; however the model can also be used for other boundary layers, such as a CBL with shear. We apply the model to both conserved species with differing surface and entrainment fluxes and to the O₃ –NO – NO₂ triad, and compare the results with large-eddy simulations (LES). The eventual goal is to provide a tool for investigating mean and flux profiles of trace species that is intermediate in complexity between simple box-model/mixed-layer approaches and three-dimensional turbulence-resolving LES, yet resolving the mean and turbulence structure of chemically active species throughout the CBL, as well as the detailed surface-layer structure.