Deviations from the Henryʹs law equilibrium during cloud events: a numerical approach of the mass transfer between phases and its specific numerical effects

Cloud drops perturb the gaseous-phase concentrations of pollutants through aqueous-phase reactions and also mass transfer between the two phases. Regional models of pollution require an accurate description of the mass transfer from gaseous to aqueous phase or from aqueous to gaseous-phase. We show that the most soluble species deviate from the Henryʹs law equilibrium at the sudden apparition of the aqueous-phase due to increasing phase equilibration times higher than chemical and dynamical time scales. Using Henryʹs law equilibrium for all species result in spurious predictions of the concentrations even for moderate soluble species. Mass transfer between phases must, therefore, be described in a real kinetic form. We also show that simple kinetic solvers (QSSA) with constant timestep are not accurate enough to resolve the increasing stiffness due to the sudden formation of a cloud when the mass transfer is modelled by kinetic equations. Depletion or overestimation of concentrations are artificially generated when the aqueous chemistry and mass transfer are taken into account. As regional models of pollution require simple, rapid chemical solvers, we have preserved the simplicity and low memory storage of the QSSA code by developing a variable timestep version of QSSA code. This modified version was revealed to be more accurate than the basic version of QSSA and about 30% faster than a Gear code.