High fluences inside cavity ring-down spectroscopy optical resonators lend themselves to fluorescence or Raman spectroscopy. An instrument at 488 nm was developed to measure extinction, and fluorescence of aerosols. A detection limit of 6 x 10^-9 cm^-1Hz^-1/2 (0.6 Mm^-1Hz^-1/2) was achieved. The fluorescence spectral power collected from a single fluorescent microsphere was 10 to 20 pW/nm. This power is sufficient to obtain the spectrum of a single microsphere with a resolution of 10 nm and signal-to-noise ratio of ~10. The relative concentrations of two types of fluorescent microspheres were determined from a time-integrated fluorescence measurement of a mixture of both.