Titration of Aerosol pH through Droplet Coalescence

The pH of aqueous aerosols, as well as cloud and fog droplets, has an important influence on the chemistry that takes place within these unique microenvironments. Utilizing conjugate acid/base pairs to infer pH changes, we investigate, for the first time, changes in aerosol pH upon coalescence. In particular, we show that the pH within individual aqueous aerosols that are ∼8 μm in diameter can be titrated via droplet coalescence in an aerosol optical tweezer. Using sulfate/bisulfate and carbonate/bicarbonate as model systems, the pH of trapped aerosols is determined before and after introduction of smaller aerosols containing a strong acid. The pH change upon coalescence with the smaller, acidic aerosol is calculated using specific ion interaction theory. Furthermore, we show that the pH of an individual aerosol can be altered along a fairly wide range of pH values, paving the way for future studies requiring rigorous pH control of an aqueous aerosol.