In-situ measurement of atmospheric NO3 and N2O5 via cavity ring-down spectroscopy

We report the application of cavity ring-down spectroscopy (CaRDS), a high- sensitivity absorption technique, to the in-situ detection of both NO, and N2O5 in ambient air. The detection limit for NO,, measuring absorption in i ts 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, wh ose detection gives the ambient concentration of N2O5. The instrument was s uccessfully field tested in March-April, 2001 at a site in the tropospheric boundary layer in Boulder, Colorado. This study is the first fast-response (5 s - I min), in-situ detection of NO3. It is also the first in-situ dete ction of N2O5 and the first observation of this species in the troposphere. Both NO, and N2O5 showed considerable temporal variability, highlighting t he need for a fast-response instrument.