Trubulent mixing and the chemical breakdown of isoprene in the atmospheric boundary layer

In this paper we study the effect of turbulence on the oxidation rate of isoprene and its reaction products in 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 include 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 data from 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 that is used to assess the impact of higher-order chemistry terms on turbulent mixing and chemical transformations. We focus on covariances of NO with (peroxy-) radicals and covariances of OH with stable intermediate products. We find only small effects on the effective reaction rates due to the OH covariances. A significant effect is found of the covariances of NO, inhibiting the effective reaction rates with the peroxy radicals 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. Higher-order chemistry terms do have an effect on 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.