Preliminary Studies on Comparing the Effect of Cloud Chemistry on Tropospheric Oxidants from Modeling a Polydispersed Cloud Drop Population to Modeling a Monodispersed Cloud Drop Population

Cloud chemistry, that is, both aqueous phase chemistry and the modification of gas-phase chemistry due to separation of reactants, can affect the production of important tropospheric oxidants. The influence of cloud chemistry on tropospheric oxidants is dependent on the pH of the drops, the liquid water content, and the chemical environment. Because pH varies across the cloud drop spectrum, the degree that cloud chemistry affects tropospheric oxidants is not obvious. For this study, a cloud parcel model that represents cloud drop activation, condensation and coalescence is coupled with a gas-aqueous chemistry module that represents O₃-NO_x-NMHC chemistry appropriate for the remote troposphere. Results of simulations in which cloud drops activate on a polydispersed CCN distribution are compared to results of a simulation in which a single-sized cloud drop population is assumed. The following hypothesis is tested and discussed. The hypothesis is that the total concentration of species such as O₃, CH₂O, and peroxides will be smaller when a polydispersed cloud drop population is represented than when a single-sized cloud drop population is assumed because previous studies have shown that these oxidants are depleted more or produced less at high pH compared to low pH. This hypothesis assumes that the high pH drops play an important role in the aqueous chemistry as they have previously been shown to do for aqueous sulfur chemistry.