Effect of fair-weather cumulus on chemical species in the convective boundary layer

It is generally assumed in large-scale chemistry transport models that each grid box is well mixed. For the boundary layer, this assumption may not be realistic because in this region many physical processes (e.g. large convective eddies, surface barriers, and clouds) can separate chemically reactive species and subsequently retard chemical reaction rates. Furthermore shallow convection atop the convective boundary layer (CBL) is generally parameterized in large-scale models, and chemistry in the shallow convection is not usually accounted for. In this study, the influence of shallow convection (via aqueous chemistry) on the chemistry in the boundary layer is examined through the use of a large eddy simulation (LES) coupled with both cloud and chemistry processes. These simulations allow us to realistically depict the transport of air parcels through the shallow convection and also to depict the influence of the cloud drops via dissolution and aqueous reaction of the species. We simulate the atmospheric and cloud influences on hydrocarbon chemistry for the Southern Great Plains ARM site on June 21, 1997. A comparison between a simulation with aqueous chemistry to a simulation without aqueous chemistry yet with cloud physics indicates the influence of the in-cloud chemistry. Quantitative results from these simulations will be presented.