Determination and validation of water droplet size distributions probed by cavity enhanced Raman scattering

Cavity enhanced Raman scattering (CERS) fingerprints are obtained on a single particle basis, providing a spectroscopic signature of the water droplet sizes generated by a vibrating orifice aerosol generator (VOAG). By sampling the aerosol train generated by the VOAG over a period of seconds, the size distribution of water droplets can be accumulated. The objective of this paper is to validate the size distribution measurements made and to quantify the accuracy of the sizes measured, along with providing an examination of the dependence of the CERS fingerprint on illumination geometry and laser pulse energy. The size distribution measurements are verified by elastic light scattering data, confirming both the sizes measured by CERS and the evolving droplet sizes produced by the VOAG during an operating time of 5 min. The experimental limitations imposed on the accuracy of the sizes measured by CERS are discussed and estimated. The estimated uncertainty in the droplet radius increases from 4 nm for a 10 mum radius droplet to 22 nm for a 50 mum radius droplet.