Impact of acidity and surface modulated acid dissociation on cloud response to organic aerosol

Abstract. Dissociation of organic acids is currently not included in most atmospheric aerosol models. Organic dissociation in aqueous aerosols could alter the H⁺ concentrations and affect the cloud activating properties. We implemented a simple representation of organic dissociation in a box model version of the aerosol–chemistry–climate model ECHAM-HAMMOZ and investigated the impact on cloud droplet number concentrations and short-wave radiative effect through changes in kinetically driven sulfate concentrations in an aerosol population. Organic dissociation has been observed in X-ray photo-electron spectroscopy measurements to be significantly suppressed in the aqueous surface. We therefore additionally introduced an empirical account of this mechanism to explore the potential further impact on aerosol effects. Malonic acid and decanoic acid were used as proxies for atmospheric organic acid aerosols. Both acids were found to yield sufficient hydrogen ion concentrations from dissociation in an aqueous droplet population to strongly influence the sulfur chemistry, leading to enhanced cloud droplet number concentrations and a cooling short-wave radiative effect. Further considering surface modulated suppressed dissociation, the impact on cloud microphysics was smaller than according to the well-known bulk solution organic dissociation, but still significant. Our results show that organic aerosol acidity can significantly influence predictions of aerosol formation and aerosol-cloud-climate effects. Furthermore, it may be important to also consider the specific influence of surface effects, also in relation to bulk solution phenomena such as organic acid dissociation.