An new article by the Ault Lab detailing a new method to measure the pH of individual aerosol particles was recently accepted by the Journal of Physical Chemistry A. Co-first author Rebecca Craig worked in collaboration with Dr. Joel Rindelaub (co-first author), a recent graduate from the Shepson Lab at Purdue University, and Lucy Nandy, a graduate student from the Dutcher Lab at the University of Minnesota, on the paper titled "Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity."
Aerosol particle acidity plays on important role in secondary organic aerosol (SOA) formation and other atmospheric processes, but has traditionally been difficult to measure as sulfate and bisulfate are not distinguishable by most methods used to determine aerosol chemical composition. This work used a Raman microspectrometer to probe the sulfate and bisulfate vibrational modes in laboratory generated aerosol particles, which were atomized from solutions of magnesium sulfate with varying concentrations of sulfuric acid. Using calibration curves relating integrated peak area to concentration for sulfate and bisulfate and their relationship with the acid dissociation constant, activity coefficients and pH could be determined for each aerosol particle.
Particles at ambient relative humidity had measured pH between 0.18 and 0.79. Continued probing of the particles as relative humidity was varied showed that aerosol particle pH increases as a function of increasing relative humidity and yielded pH measurements within the range of -0.68 to 1.33. This is consistent with the -0.5 to 3 range recently reported for ambient aerosol pH in the southeast United States from indirect methods. Future work will expand this method to other acid/base equilibrium systems to cover a wider range of pH, as well as work to apply this method to ambient aerosol particles.
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