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.

The oxidation of S­(IV) is a critical step in the fate of sulfur dioxide emissions that determines the amount of sulfate aerosol in the atmosphere. Herein, we measured accelerated S­(IV) oxidation rates in micron-sized aqueous aerosols compared to bulk solutions. We have investigated both buffered and unbuffered systems across a range of pH values in the presence of atmospherically relevant transition-metal ions and salts and consistently found the oxidation rate to be accelerated by ca. 1–2 orders of magnitude in the aerosol. This enhancement is greater than can be explained by the enrichment of species in the aerosol compared to the bulk and indicates that surface effects and potentially aerosol pH gradients play important roles in the S­(IV) oxidation process in the aqueous aerosol. In addition, our experiments were performed with dissolved S­(IV) ions (SO3 2–/HSO3 –), allowing us to demonstrate that acceleration occurs in the condensed phase showing that enhanced sulfate formation is not exclusively due to gas-aerosol partitioning or interfacial SO2 oxidation. Our findings are an important step forward in understanding larger than expected sulfate concentrations observed in the atmosphere and show that inorganic oxidation processes can be accelerated in micron-sized aqueous droplets compared to the bulk solution.

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.

The surface partitioning of a medium chain fatty acid and its conjugate base has been investigated through a combined experimental and theoretical approach of the multi-equilibria involved in the bulk phase and at the air/water interface.