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Insights into Acidity-Driven Surface Chemistry and Kinetics of Aqueous Aerosols


Atmospheric aerosols are generated by the millions globally from the bursting of ocean bubbles, the action of wind on dust, burning processes, and many other sources. Aerosol particles have high surface area to volume ratios and typically contain chemicals in significantly greater concentrations compared to the bulk source. From serving as the seeds for clouds to promoting multiphase chemistry and interacting with cells, aerosols have profound impacts on the climate and human health. Among the many properties of aerosols, one of the most important is pH, a measure of the particle’s acidity. Low pH aerosols have detrimental effects on the lungs, and many atmospheric transformations exhibit pH dependence. Although aerosol pH is critically important, it is challenging to measure due to the very small volumes individual aerosols occupy. As a result, prior to the work presented here, the pH of nascent sea spray aerosol (the largest source of aqueous particles globally) was unknown. Here, a novel method for the determination of aerosol pH and its application to the measurement of sea spray aerosol acidity is presented. It is found that, in just two minutes, these aerosols are acidified to pH levels ranging from 2 to 4 depending on particle size. Following this result, several key causes and effects of aerosol pH are discussed. Acidity is shown to impact the extent of acceleration of S(IV) oxidation in aerosols, an important chemical reaction for the fate of sulfur dioxide emissions and air quality. A unique type of buffering to aerosols is presented next, where the titration of the particle results in a decrease in the organic content of the aerosol, instead of a drop in pH, due to partitioning of acids. Following this, the kinetics of nitrate and chloride depletion for single aerosols is shown for the first time. These processes both modulate aerosol pH and change particle hygroscopicity, which is a central parameter for atmospheric cloud formation. Finally, the link between aerosol acidity and surface chemistry is shown by an exploration of the pH-dependent surface propensity of amino acids in aqueous environments. Together, these studies shed light on the drivers of aerosol pH and the kinetics of critical aerosol-phase reactions. Ultimately, this research improves understanding of the complex ocean-climate system and how invisible particles present health challenges for populations globally.

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