Organic acid evaporation kinetics from aqueous aerosols: implications for aerosol buffering capacity in the atmosphere

The acidity of atmospheric aerosols controls their impacts on heterogeneous and multiphase reactions, cloud formation, and human health. Recently, it has been shown that multiphase buffering can shift aerosol pH substantially compared to bulk solutions. Here, we highlight a unique type of multiphase buffering for aerosols that occurs when organic acids partition from aqueous salt aerosols upon acidification with a strong acid. In this case, rather than lowering the pH of the aerosol, titration with strong acids lowers the organic fraction within the aerosol while maintaining constant pH. We investigate evaporation rates for the model system lactic acid as well as other atmospherically-relevant species such as acetic, butyric, and methacrylic acids. We demonstrate that the timescales for evaporation of organic acids from aerosols are on the order of minutes, comparable to acidification rates in the atmosphere. The organic acid evaporation we observe for lactic acid in salt aerosols is enhanced compared to bulk measurements within what is expected based on differences in surface to volume ratios, indicating surface effects are important. In addition, we show that a salting-out effect drives small organic molecules to the surface, where they quickly evaporate, reducing acidity and causing a "superbuffering" effect. Our results can explain why aerosols in the pH range from 2 to 4 are able to resist further acidification by strong acids in the atmosphere. Overall, this work highlights unique properties of concentrated aerosols and demonstrates how inorganic ions and organic compounds together control multiphase buffering in the atmosphere. The acidity of atmospheric aerosols controls their impacts on heterogeneous and multiphase reactions, cloud formation, and human health.