Determining the pH of 15–60 μm radius water droplets: a direct fluorescence probe

We demonstrate that the fluorescence spectrum from water droplets containing the dye acridine can be used to accurately determine the pH of droplets in the radius range 15–60 μm and in the pH range 3–10. Using a laser pump wavelength of 355 nm and probing the fluorescence with a 0.5 m spectrograph and CCD detector, the particle size parameters are sufficiently large in this radius range that the fluorescence spectrum conforms to that of the bulk phase, permitting a direct unambiguous determination of droplet pH. This could provide a sensitive method for studying directly the uptake of acidic or basic reagents on aerosol droplets.