Case study on the interaction of size dependent multiphase chemistry and detailed microphysics

It is widely accepted that clouds and precipitation systems are very important for the transformation and redistribution of chemical species in the atmosphere. Cloud droplets provide an alternate medium to the gas phase in which chemical reactions may proceed. The spectral composition of the droplets, on the one hand, is controlled by the drop formation via the activation of cloud condensation nuclei (CCN), the uptake of trace-gases and aqueous-phase chemical reactions. On the other hand, the chemical composition of the drops affects the dissolution of trace-gases and the rates and equilibria of the aqueous-phase reactions. A simple closed box model including explicitly spectral microphysics and a representative chemical mechanism is used to study the time evolution of the spectrally chemical composition of cloud droplets and to contrast the effects of dilution by condensation, of mixing by coagulation and break-up, of liquid phase reactions, and of interaction between the phases. The presented study suggest that cloud droplets of different sizes have different solute concentrations determined by the activation of size distributed aerosol particles and affected by condensation, coagulation, and break-up. If a chemically homogeneous aerosol distribution is assumed, the droplets of different sizes will have in this manner different dilutions and different pH but equal ratios between the aerosol related compounds. If also gas liquid interactions and liquid-phase reactions are taken into account, the different pH and mass transfer coefficients in conjunction with microphysical processes will lead to quite different chemical composition over the drop spectrum and to a change of the spectral pH level.