Measurement of atmospheric NO2 by pulsed cavity ring-down spectroscopy

[ 1] We have constructed a pulsed cavity ring-down spectrometer ( CARDS) for simultaneous measurements of nitrogen dioxide (NO2), the nitrate radical (NO3), and dinitrogen pentoxide (N2O5) in the atmosphere. In this paper, we describe the development of the instrument to measure NO2 via its absorption at 532 nm. The NO2 detection channel was calibrated against a NIST traceable calibration standard as well as a photolysis-chemiluminescence (P-CL) NO2 detector. The absorption cross section of NO2 at 532 nm was determined to be (1.45 +/- 0.06) x 10(-19) cm(2). The NO2 detection limit (1 sigma) for 1 s data is 40 pptv, and the instrument response is accurate within +/- 4% (1 sigma) under laboratory conditions. The linear dynamic range of the instrument has been verified from the detection limit to above 200 ppbv (r(2) > 99.99%). For field measurements it is necessary to correct the CARDS NO2 signal for absorption by ozone. Under ambient conditions we report 1 s NO2 CARDS data with total uncertainty +/-(4%, 60 pptv + 0.4 x (pptv/ppbv) x O-3) (1 sigma). The instrument was deployed in the field during the New England Air Quality Study - International Transport and Chemical Transformation on board the NOAA research vessel Ronald H. Brown in the summer of 2004 and in Boulder, Colorado, in the winter of 2005. In both campaigns, CARDS and P-CL NO2 measurements were highly correlated (r(2) > 98%), indicating the absence of interfering gas phase absorbers at 532 nm other than ozone and the suitability of CARDS to measure NO2 in the troposphere.