Spectroscopic Determination of Aerosol pH from Acid-Base Equilibria in Inorganic, Organic, and Mixed Systems

Atmospheric aerosol acidity impacts key multiphase processes, such as acid-catalyzed reactions leading to secondary organic aerosol formation, which impact climate and human health. However, traditional indirect methods of estimating aerosol pH often disagree with thermodynamic model predictions, resulting in aerosol acidity still being poorly understood in the atmosphere. Herein, a recently developed method coupling Raman microspectroscopy with extended Debye-Hückel activity calculations to directly determine the acidity of individual particles (1-15 μm projected area diameter, average 6 μm) was applied to a range of atmospherically relevant inorganic and organic acid-base equilibria systems (HNO₃/NO₃^-, HC₂O₄^-/C₂O₄^2-, CH₃COOH/CH₃COO^-, and HCO₃^-/CO₃^2-) covering a broad pH range (-1 to 10), as well as an inorganic-organic mixture (sulfate-oxalate). Given the ionic strength of the inorganic solutions, the H⁺ activity, γ(H⁺), yielded lower values (0.68-0.75) than the organic and mixed systems (0.72-0.80). A consistent relationship between increasing peak broadness with decreasing pH was observed for acidic species, but not their conjugate bases. Greater insight into spectroscopic responses to acid-base equilibria for more complicated mixtures is still needed to understand the behavior of atmospheric aerosols.