Determining the composition of aqueous microdroplets with broad-band cavity enhanced Raman scattering.

We demonstrate that broad-band cavity enhanced Raman scattering (CERS) can be used to determine the composition of binary alcohol-water aerosol droplets over a wide compositional range from 10% v/v to 90% v/v. In contrast to conventional CERS using narrow-band laser excitation, the excitation is provided by a broad-band Nd:YAG pumped dye laser. A change in the spontaneous spectrum resulting from the change of the linewidth of the excitation laser permits tuning of the sensitivity range over which the droplet composition can be determined by CERS. We demonstrate that this change in sensitivity can be estimated using a simulation of the change in the sensitivity to the species in spontaneous bulk phase measurements. We further show that the compositional calibration is independent of droplet radius in the range 33-56 microm. The compositional range over which CERS is sensitive can be controlled and optimised for any particular application by exploiting the dependence of the stimulated Raman scattering on the laser linewidth and wavelength. Thus, quantitative measurements of droplet composition can be made in situ with high accuracy, providing a valuable new tool for analysing aerosol composition.