The cavity ring-down technique is applied to an industrial-scale radio-frequency (rf) plasma reactor for the measurement of the density and spatial profile of negative ions in pure oxygen and hydrogen rf plasmas, and for the detection of nanometric particles in argon-silane plasmas. The real-time observation of powder formation is demonstrated to be feasible by the cavity ring-down technique. An observed plasma-induced spurious drift of the ring-down time is also studied and related to water desorption from the reactor walls and electrodes which is re-adsorbed on the mirror surfaces.