A numerical model with detailed microphysics that simulates the evolution of the raindrop spectra by activation, condensation, coalescence, breakup and the scavenging process of sulfur dioxide (SO2), hydrogen peroxide (H2O2), ozone (O3), formaldehyde (HCHO) and formic acid (HCOOH) has been developed. Three distribution functions were defined in the cloud model a) for CCN, with a maximum of 64 categories from 0.0041 to 5.93 microns, b) for smaller drops up to 1micron, and c) for drops up to 2503 microns. A modified form of the Köhler equation is introduced, that takes into account the influence of dicarboxylic acids in the initial CCN.

The diffusion of gas-phase species (SO2, H2O2 ,O3, HCHO, HCOOH ) was calculated by means of a quasi analytical solution of the diffusion equation. Aqueous phase kinetics is implemented in order to calculate the generation of sulfuric acid (S(IV) to S(VI) conversion) and the production of formic acid from hydrated formaldehyde, that is oxidized by the hydroxyl radical in the aqueous phase (OH)aq.

The cloud microphysical and chemical codes are linked to a one dimensional dynamical framework, that calculates the dynamic evolution of the distribution functions of droplets, dissolved gaseous species (S(IV), S(VI), H2O2 ,O3, HCHO and HCOOH ) and CCN mass. Simulations were performed in order to test the influence of the aerosol composition in the microphysical structure of the simulated cloud, and the relative importance of the aqueous phase generation of sulfuric and formic acid in the final acidity of droplets.

Some differences are observed between the simulated clouds in the case of pure inorganic aerosol spectra and the mixed case (with dicarboxylic acids), with lower droplet concentrations and an earlier development of precipitation in the mixed case. Although S(IV) to S(VI) conversion appears to be the most important process in generating the final pH of precipitation, a significant reduction because of formic acid production is found.