Bulk vs. size-resolved representations in modelling mass transfer and aqueous-phase chemistry in cloud droplets in regional air quality models

In current air quality models which deal with PM issues, aqueous-phase chemistry is often included in a bulk form. However, existing studies and observations have shown that both the solute concentration and the chemical composition within cloud droplets can vary with droplet size. Moreover the pH of the cloud droplets may also be size dependent. These are all important parameters in the process of mass transfer (between gas and aqueous phase) and aqueous-phase chemistry involving atmospheric aerosols. It, therefore, may not be appropriate to assume a bulk cloud water composition when modelling aqueous-phase chemistry. On the other hand, computational burden and limited knowledge on size-dependent aqueous-phase oxidation reactions are some of the things to be considered when moving from a bulk to a size-resolved representation. In the present study an existing (bulk) aqueous-phase chemistry module is adapted for use in a size-resolved regional PM model. Size dependency in modelling mass transfer and aqueous-phase chemistry will be examined, and a comparison between size-resolved and the original bulk representation will be carried out for various modelled aerosol properties.