Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H₂SO₄+MgSO₄) based on sulfate and bisulfate concentrations determined from ν_s(SO₄^2-) and ν_s(HSO₄^-), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of ν_s(SO₄^2-) and ν_s(HSO₄^-) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.