An investigation of the factors influencing the detection sensitivity of cavity enhanced Raman scattering for probing aqueous binary aerosol droplets

Stimulated Raman scattering (SRS) from single aerosol droplets can be observed at extremely low laser threshold intensities at wavelengths commensurate with whispering gallery modes. Although droplet size can routinely be determined from the ensuing cavity enhanced Raman scattering (CERS) fingerprint, determining droplet composition is a considerably more challenging measurement. We present here an examination of the factors that influence and limit the detection sensitivity of CERS in quantifying the concentrations of sulfate and nitrate in water droplets, 20 - 50 mm in radius. In particular, we consider the variation in nitrate and sulfate SRS signal with variation in species concentration, probe laser intensity and droplet size. We illustrate that the band contour of the OH stretching band can be used as a relative measure of the internal light intensity circulating within the droplet and experimentally investigate how the threshold condition for SRS is achieved.