The Effect of Shallow Cumulus Convection on the Segregation of Chemical Reactants

Previous studies have shown that the rate of reaction between surface-emitted biogenic hydrocarbons and in-situ produced oxidants may be reduced by 5-50% in the clear convective boundary layer because the reactants are segregated by turbulence. They have not addressed the effect of fair-weather cumulus clouds on chemical reactivity. In this study we use large-eddy simulation (LES) to examine two effects of the fair-weather cumuli, turbulence dynamics and aqueous-phase chemistry.

The LES code is configured for the conditions observed at the Southern Great Plains site of the Atmospheric Radiation Measurement program on 21 June 1997. The chemistry mechanism used in this study is similar to the mechanism used in NCAR HANK model, but is updated following the NCAR global chemical transport model MOZART2.2. The mechanism contains 141 gas-phase reactions and 29 aqueous-phase reactions for which 54 species are involved. Surface emissions of key biogenic hydrocarbons, isoprene and a-pinene, follow a diurnal-cycle profile, while that of nitric oxide is a constant value. The dynamics, cloud physics, and chemistry are integrated over a model domain of 6.4 km x 6.4 km x 4.4 km (with 96 x 96 x 96 grid points) with periodic boundary conditions in x and y. The simulation of the dynamics and physics begins at 0530 LST (local standard time), while the simulation of the chemistry begins at 0830 LST when the turbulent flow is established and cumulus cloud start to form. The whole simulation time with chemistry is 6 hours.

Our study shows that shallow cumulus convection and aqueous-phase chemistry can enhance the segregation of chemical reactants. Simulation results with only gas-phase chemical reactions show that the greatest segregation (up to 30%) of chemical reactants occurs in the cloud layer. When aqueous-phase chemical reactions are included, the segregation of reactants in the cloud layer increases to 50%. Three-dimensional distributions of the correlation between biogenic hydrocarbon species and hydroxyl radical exhibit negative minima within the clouds. An analysis of the covariance budget will be presented to show the relative importance of dynamical and chemical processes on the segregation of species.