Mixing of Reactive Gases in the Convective Boundary Layer

In this paper we study the effect of turbulence on the oxidation rate of hydrocarbons the atmospheric boundary layer. We use two models of different complexity: a simple model consisting of two well-mixed layers and a one-dimensional off-line second-order closure model. Both models use a prescribed physical characterization of the convective boundary layer, as well as an extensive set of chemical reactions to describe the oxidation of isoprene. A 5-day simulation is performed to compare the simple model output with observations during the the Amazon Boundary Layer Experiment (ABLE-2A). The model is able to represent fairly the basic dynamics and chemistry during this experiment. Subsequently, the simple model provides boundary and initial conditions for a one-dimensional second-order closure model. This model allows us to assess the impact of covariances of reacting gases on the rate of transformation, which is is usually neglected in atmospheric transport and chemistry models. A significant effect is found of the covariances involving nitrogen monoxide (NO), inhibiting the effective reaction rates by a maximum of 10% in the afternoon. The inclusion of covariance terms resulted in an increase of radical concentrations, but the NO concentration profiles remained unchanged. The applicability of (inert) K-theory for reactive species was tested by taking higher-order chemistry terms explicitly into account in the equation for the turbulent flux. We find significant effects on the NO and NO2 fluxes, which change by 5 to 30% in the middle of the boundary layer. Therefore these terms have to be taken into account when flux-gradient relationships or deposition velocities are derived from observations. The present results indicate that the incorporation of higher-order chemistry terms is not essential for a correct representation of the mean profiles of most stable species involved.