We report the application of cavity ring-down spectroscopy (CaRDS), a high-sensitivity absorption technique, to the in-situ detection of both NO3 and N2O5 in ambient air. The detection limit for NO3, measuring absorption in its strong, 662-nm band, is 0.3 pptv at STP (50 s integration time). Heating the air flow through the inlet thermally dissociates N2O5 to yield NO3, whose detection gives the ambient concentration of N2O5. The instrument was successfully field tested in March--April, 2001 at a site in the tropospheric boundary layer in Boulder, Colorado. This study is the first fast-response (5s--1 min), in-situ detection of NO3. It is also the first in-situ detection of N2O5 and the first observation of this species in the troposphere. Both NO3 and N2O5 showed considerable temporal variability, highlighting the need for a fast-response instrument. ¿ 2001 American Geophysical Union