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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 partitioning of medium-chain fatty acid surfactants such as nonanoic acid (NA) between the bulk phase and the air/water interface is of interest to a number of fields including marine and atmospheric chemistry. However, questions remain about the behavior of these molecules, the contributions of various relevant chemical equilibria, and the impact of pH, salt and bulk surfactant concentrations. In this study, the surface adsorption of nonanoic acid and its conjugate base is quantitatively investigated at various pH values, surfactant concentrations and the presence of salts. Surface concentrations of protonated and deprotonated species are dictated by surface-bulk equilibria which can be calculated from thermodynamic considerations. Notably we conclude that the surface dissociation constant of soluble surfactants cannot be directly obtained from these experimental measurements, however, we show that molecular dynamics (MD) simulation methods, such as free energy perturbation (FEP), can be used to calculate the surface acid dissociation constant relative to that in the bulk. These simulations show that nonanoic acid is less acidic at the surface compared to in the bulk solution with a p K a shift of 1.1 ± 0.6, yielding a predicted surface p K a of 5.9 ± 0.6. A thermodynamic cycle for nonanoic acid and its conjugate base between the air/water interface and the bulk phase can therefore be established. Furthermore, the effect of salts, namely NaCl, on the surface activity of protonated and deprotonated forms of nonanoic acid is also examined. Interestingly, salts cause both a decrease in the bulk p K a of nonanoic acid and a stabilization of both the protonated and deprotonated forms at the surface. Overall, these results suggest that the deprotonated medium-chain fatty acids under ocean conditions can also be present within the sea surface microlayer (SSML) present at the ocean/atmosphere interface due to the stabilization effect of the salts in the ocean. This allows the transfer of these species into sea spray aerosols (SSAs). More generally, we present a framework with which the behavior of partially soluble species at the air/water interface can be predicted from surface adsorption models and the surface p K a can be predicted from MD simulations. 

Aerosols impact climate, human health, and the chemistry of the atmosphere, and aerosol pH plays a major role in the physicochemical properties of the aerosol. However, there remains uncertainty as to whether aerosols are acidic, neutral, or basic. In this research, we show that the pH of freshly emitted (nascent) sea spray aerosols is significantly lower than that of sea water (approximately four pH units, with pH being a log scale value) and that smaller aerosol particles below 1 μm in diameter have pH values that are even lower. These measurements of nascent sea spray aerosol pH, performed in a unique ocean−atmosphere facility, provide convincing data to show that acidification occurs “across the interface” within minutes, when aerosols formed from ocean surface waters become airborne. We also show there is a correlation between aerosol acidity and dissolved carbon dioxide but no correlation with marine biology within the seawater. We discuss the mechanisms and contributing factors to this acidity and its implications on atmospheric chemistry.