Off-axis cavity ringdown spectroscopy: application to atmospheric nitrate radical detection

Atmospheric nitrate radicals (NO3) are detected using off-axis cavity ringdown spectroscopy (CRDS) for the first time to our knowledge with a roomtemperature continuous-wave (cw) diode laser operating near 662 nm. A prototype instrument was constructed that achieved a 1 sigma absorption sensitivity of 5*10^-10 cm-1 Hz-1/2, corresponding to a 1.4 part per trillion by volume 2 sigma detection limit in 4.6 s at 80(degree)C. This sensitivity is a significant improvement over a recent implementation of off-axis cavity-enhanced absorption spectroscopy and comparable to that of the most advanced cw CRDS and pulsed CRDS applications for atmospheric detection of NO3. A comparison of measurements of ambient air in Fairbanks, Alaska, recorded with the off-axis CRDS instrument and a previously characterized conventional cw CRDS instrument showed good agreement (R2=0.97).