Representing effects of aqueous-phase reactions in shallow cumuli in global models

Moist convection plays a number of roles in atmospheric chemistry including vertical transport and turbulent mixing of chemical species, photochemistry (by changing the radiation field), lightning production of NOx, and aqueous-phase reactions. Aqueous-phase reactions, in particular, are the dominant pathways for the oxidation of sulfur dioxide. In current global chemical transport models (CTM), aqueous-phase reactions are titrated over CTM time step within the cloudy volume, thus they are independent to the reaction constants and dependent on the CTM time step. In this study, using Large-Eddy Simulations (LES) with reactive tracers mimicking the oxidation of sulfur dioxide, we show that the Eddy-diffusion Mass-flux (EDMF) approach is a valuable alternative. The EDMF approach (with a bulk plume) can represent the transport and aqueous-phase reactions in shallow cumuli quite well when entrainment/detrainment rates and eddy diffusivity are diagnosed using conservative tracers. The reason is that a typical aqueous reaction is slow compared to the eddy mixing time scale in shallow cumuli, so that the surface released  chemical tracers is well correlated with surface released conservative tracers. We have explored the performance of the bulk plume model in a series of cases with changing reaction rate constants and the relative abundance of SO2 and H2O2. There are two error sources when using the bulk plume model: the tracer dependence of effective entrainment/detrainment rates and the segregation error due to heterogeneity within cloudy updrafts. When the reaction rate constant increases, the two errors partly cancel each other so that the total error in the bulk plume model remains relatively small. When H2O2 dominates, the errors are larger due to larger segregation errors. In addition to its use as a convective parameterization, the EDMF with bulk plume approach can potentially be used in super parameterized GCMs to represent effects of aqueous-phase reactions in shallow cumuli.