Acidity is a critical chemical property of atmospheric aerosols that governs key atmospheric processes, such as secondary organic aerosol formation. However, aerosol acidity is challenging to measure due to the non-conservative nature of H<sup>+</sup> ion concentration, which changes with relative humidity and aerosol water content. Current methods used to determine acidity include ion balance, phase partitioning, and thermodynamic modeling, which are all indirect and have limitations. Herein, we discuss the recent development of direct methods to measure aerosol acidity, using vibrational spectroscopy and colorimetric methods. Specifically, the vibrational modes of several organic and inorganic acids and their conjugate bases are distinct (often ~ 50 cm-1 apart) and measurable, despite structurally differing by only a proton. Using Raman microspectroscopy, the pH of individual particles (&gt; 2 µm) has been determined for a range of chemical equilibrium systems at pH values near the pKa, including bisulfate-sulfate, bicarbonate-carbonate, nitric acid-nitrate, bioxalate-oxalate, and acetic acid-acetate, as well as a mixture of bisulfate-sulfate and bioxalate-oxalate. A second method utilizing image processing of pH paper after aerosol impaction is shown to be effective for aqueous aerosols, but limited for lower water content aerosols. Combined these methods are enabling a more detailed understanding of aerosol acidity, including size-dependence and gas-particle partitioning.The development of direct methods for measuring aerosol acidity is needed to mechanistically understand multiphase chemistry in a range of important atmospheric systems.

Direct measurements of aerosol acidity: Spectroscopic and colorimetric methods

255th National Meeting (2018)

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