Quinaldine Red as a Fluorescent Probe for Particle Physicochemical Properties

The physicochemical properties of particles affect many important atmospheric processes; yet, they are challenging to measure in situ. Herein, fluorescence aerosol flow tube (F-AFT) spectroscopy is applied to directly probe the ionic strength and pH in model systems of inorganic aerosol particles. The pH-sensitive probe molecule quinaldine red (QR) is incorporated into aerosol particles of sodium chloride, ammonium sulfate, ammonium bisulfate, and their mixtures. Fluorescence spectra are collected for variable particle acidity and ionic strength as mediated by composition and relative humidity. Results show that shifts in fluorescence wavelength are driven by changes in both ionic strength and pH. A two-dimensional regression analysis of the sulfate particle fluorescence line shape as a linear function of both molality and pH results in R-2 = 0.75. For comparison, R-2 values of 0.17 and 0.64 are found for molality and pH considered separately, respectively. The regression model calculated from the sulfate particle system did not fit the sodium chloride particle data well, suggesting that there are chemically specific effects governing the pH and ionic strength interactions with the dye molecule. Overall, these findings indicate the potential of F-AFT spectroscopy for in situ elucidation of the physicochemical properties of submicrometer aerosol particles, contingent upon a detailed understanding of the controlling factors in the fluorescence behavior of the chosen probe molecules in common aerosol particle-phase chemical environments.