Surface modulated dissociation of organic aerosol acids and bases in different atmospheric environments
Creators: name: Sengupta, Gargi
name: Prisle, Nønne L.
Publisher: name: Taylor & Francis
Year: 2024
Subjects: subject: Space Science
subject: Pharmacology
subject: Biotechnology
subject: Evolutionary Biology
subject: FOS: Biological sciences
subject: Environmental Sciences not elsewhere classified
subject: Chemical Sciences not elsewhere classified
subject: Ecology
subject: Astronomical and Space Sciences not elsewhere classified
subject: Sociology
subject: FOS: Sociology
subject: Biological Sciences not elsewhere classified
subject: Plant Biology

description: The protonation state of organic acids and bases in aqueous aerosols has the potential to impact cloud activating properties by altering H<sup>+</sup> concentrations, and consequently the aerosol chemistry and water activity, but is currently overlooked in most atmospheric aerosol models. We investigated the impact of organic aerosol acid–base dissociation on the cloud droplet number concentration and short-wave radiative effect from cloud formation in environments with varying aerosol concentrations, incorporating organic acid–base dissociation into the aerosol–chemistry–climate box model ECHAM6.3–HAM2.3. The degree of dissociation has previously been observed for several atmospheric organics with Brönsted acid or base character to be significantly modified in the aqueous surface, compared to the bulk solution. We introduced an empirical account of this mechanism for both acids and bases to explore the potential further impact on aerosol climate effects. Suwanee River Fulvic acid and decanoic acid were used as examples of acidic atmospheric organic aerosols and tributylamine and <i>n</i>-butylamine as examples of atmospheric organic bases. Our results show that accounting for the protonation equilibria of organic aerosol components, and their possible surface modulation, results in enhanced cloud droplet number concentration and a substantial short-wave radiative effect of clouds, in pristine, clean, as well as polluted environments. The dissociation of organic acids and bases leads to enhanced H<sup>+</sup> concentrations in the aerosol, which further influences aqueous sulfur chemistry to drastically increase sulfate mass in the whole aerosol population. The increased sulfate mass contributes to a rise in the number of particles within larger size ranges, thereby increasing cloud droplet number concentrations. In addition, increased amounts of H<sup>+</sup> ions and the resulting sulfate significantly increase the amount of available solute and water activity, leading to a higher number of cloud condensation nuclei, and consequently cloud droplets. The effect of acid–base dissociation on cloud properties is smaller for the investigated organic bases than for the organic acids in all environments. The surface modulation of the organic acid–base protonation equilibria has the largest effect on cloud properties in clean atmospheric environments. These results emphasize the potential importance of organic acid–base dissociation for aerosol-cloud-climate properties and their consideration in atmospheric models on both regional and global scales.