Closures for fluxes and covariances are evaluated with large-eddy simulations (LESs) for various cases with chemistry in an entraining and solid-lid convective atmospheric boundary layer (CABL). The closure for the fluxes is based on the combined effects of gradient mixing and nonlocal convective mixing, and includes the impact of chemistry on the nonlocal part of the flux. The covariances are estimated from top-hat distributions as is common in mass-flux schemes. Two different reaction schemes are studied, covering all LESs of reactive species that are currently in the literature. The first reaction scheme consists of a one-way reaction between bottom-up and top-down diffusing species. In the cases with this simple reaction scheme the fluxes at the surface (and for the solid-lid CABL also at the top) of the CABL are prescribed and no deposition takes place. The second reaction scheme is a realistic photochemical scheme consisting of 10 reactions involving 6 species: O3, NO, NO2, RH (a generic hydrocarbon), OH, and HO2. A steady-state tropical continental background chemistry case is studied with the photochemical scheme; emission fluxes are prescribed and deposition is modelled using a deposition parametrization. It is shown that the combined closures accurately model fluxes and covariances for all cases studied. For the simplest reaction scheme we find that the top-hat formulas for the fluxes and covariances give comparable results for different chemistry cases in the entraining CABL (up to approximately z/zi=0.8) and the solid-lid CABL. With a one-dimensional model we show that the combined closures model improved concentration profiles compared to a local first-order closure for the fluxes and neglecting the covariances. For the photochemical case the most important turbulence-chemistry interaction is in the parametrization of the deposition flux of NO2. It is shown with a high-resolution version of the one-dimensional model that an increased resolution near the surface, as compared to the 16 m vertical resolution of the LES concerned, leads to a more accurate modelling of the deposition of NO2
Symposium on Interdisciplinary Issues in Atmospheric Chemistry