Fast Fourier transform analysis in cavity ring-down spectroscopy:: application to an optical detector for atmospheric NO2

A fast Fourier transform (FFT) method for analysis of ring-down decays from a cavity ring-down (CRD) spectrometer has been tested and compared with alternative fitting methods. The ring-down times derived from the FFT method are obtained with a precision close to that of the Levenberg-Marquardt non-linear least-squares method, but the fitting algorithm is similar to 100 times faster, allowing real-time fitting of individual ring-down traces on a personal computer. Advantages of the FFT method are discussed, and the method is demonstrated for the measurement of NO2 partial pressures equivalent to mixing ratios of 150 pptv and above in laboratory air, using a CRD spectrometer based on an external cavity diode laser operating at wavelengths around 410 nm. The absorption by NO2 is distinguished from other cavity losses either by using synthetic (zero) air reference samples, or by diverting the sampled laboratory airflow through an NO2 chemical scrubber consisting of hydroxyapatite on a TiO2 substrate. Typical mixing ratios of NO2 in the laboratory air are similar to 25 ppbv.