Modeling of scavenging processes in clouds: some remaining questions about the partitioning of gases among gas and liquid phases

Clouds can play an important role by affecting the chemical composition of the troposphere through modification of photolysis rates, and by redistributing compounds through the vertical transport of species and their removal by wet deposition and finally by aqueous phase chemical reactions within cloud water or precipitation water. Several examples of the effects of clouds on tropospheric chemistry are shown, using a box model or a mesoscale model illustrating the role of clouds on hydrogen peroxide: its partitioning between the gas and aqueous phases, including deviations from Henryʹs law. The main results are that deviations from Henryʹs law exist even for small droplets, which are located on the edges of orographic clouds while equilibrium is attained in the center of the cloud. The partitioning of gases is a function of the cloud development conditions such as the air mass in which the cloud has been formed (continental vs. maritime), the microphysical properties (cloud water content, rainwater content).