Turbulent mixing of reactive gases in the convective boundary layer

We study the interactions of chemistry and turbulent mixing of tracers in the convective boundary layer with a second-order closure model, including higher order chemistry terms. In order to limit the number of predictive equations we prescribe the profiles for (w(2)) over bar, Theta, <(w theta)over bar>, <(theta(2))over bar> and the lengthscale l. However, for model validation we treat temperature and humidity as inert tracers, and compare the results with profiles observed during the Air Mass Transformation Experiment, and with similarity expressions for the surface layer. We find good agreement of the mean profiles, but the (co-) variances are slightly underpredicted. Furthermore, the model uses diagnostic equations expressing third moments of concentration in terms of second moments and their vertical derivatives. They are compared with large-eddy model results, showing good agreement and, therefore, the simplifications are justified. The model is applied to the transport of two gases subject to one bimolecular reaction. The importance of concentration correlations on the mean transformation rate is studied. For two gases diffusing in opposite directions we find for moderate and fast chemistry a 50% and 90% decreased transformation rate due to the negatively correlated concentrations. These values are similar to large-eddy results of Schumann and Sykes et al. For two bottom-up tracers we fmd that the covariance of both reactive species is either positive or negative, increasing or reducing the effective transformation rate depending on the Damkohler number (the ratio of the turbulent and the chemistry timescale). A significant direct influence of chemistry on the flux divergence is found in both cases. According to the model the effective transport to mid-levels of the boundary layer is increased when two reactive tracers diffuse in opposite directions, and decreased in the case of two bottom-up tracers.