We demonstrate the possibility of measuring parts in ${10}^{12}$ by volume concentrations of radicals of high atmospheric interest, such as $\mathrm{IO}$ or $\mathrm{BrO}$ , as needed for monitoring these species in the environment. We apply cavity-enhanced absorption spectroscopy in the near UV range using a frequency-doubled $\mathrm{Ti}:\mathrm{Sa}$ mode-locked femtosecond laser. Efficient broadband injection of a high-finesse cavity is obtained by matching this optical frequency-comb source to the comb of cavity transmission resonances. A grating spectrograph and a detector array disperse and detect the spectrum transmitted by the cavity carrying the absorption features of intracavity molecules. Spectra recorded over $\sim 4\mathrm{nm}$ with $10\mathrm{s}$ averaging display a noise level of $8\times {10}^{-10}∕\mathrm{cm}$ .