Detecting pH of Sub-Micrometer Aerosol Particles Using Fluorescent Probes

Acidity is one of the most fundamental properties of atmospheric aerosol particles, influencing both chemical processes and environmental impacts. A couple of methods for measuring the pH of aerosol particles have been developed, yet no approach is currently available for quantifying the pH of suspending submicron particles. We developed an aerosol fluorescence cell (AFC) for quantifying the pH of aerosol particles in a continuous flow. Particles containing fluorescent pH probes (fluorescein and Oregon green 488) were injected into the AFC. Ratiometric analysis of pH for these particles was conducted by alternatively exciting them with two wavelengths of diode lasers (450 and 488 nm). The employment of the two types of fluorescent probes allowed the measurement of pH in the range of 2-7. The pH measurement by the AFC for submicron particles was compared with that measured by using pH indicator papers, providing a reasonably good agreement. Measurement of size-selected particles suggested that the AFC approach is applicable to particles of 80 nm in diameter. In combination with recent developments for analyzing renebulized water-soluble matter samples, we suggest that the AFC method can be applied not only for laboratory standards but also for atmospherically relevant samples in the future.