Measurement of glyoxal using an incoherent broadband cavity enhanced absorption spectrometer

Abstract. We describe an instrument for simultaneous measurements of glyoxal (CHOCHO) and nitrogen dioxide (NO₂) using cavity enhanced absorption spectroscopy with a broadband light source. The output of a Xenon arc lamp is coupled into a 1 m optical cavity, and the spectrum of light exiting the cavity is recorded by a grating spectrometer with a charge-coupled device (CCD) array detector. The mirror reflectivity and effective path lengths are determined from the known Rayleigh scattering of He and dry zero air (N₂+O₂). Least-squares fitting, using published reference spectra, allow the simultaneous retrieval of CHOCHO, NO₂, O₄, and H₂O in the 441 to 469 nm spectral range. For a 1-min sampling time, the precision (±1σ) on signal for measurements of CHOCHO and NO₂ is 29 pptv and 20 pptv, respectively. We directly compare measurements made with the incoherent broadband cavity enhanced absorption spectrometer with those from cavity ringdown instruments detecting CHOCHO and NO₂ at 404 and 532 nm, respectively, and find linear agreement over a wide range of concentrations. The instrument has been tested in the laboratory with both synthetic and real air samples, and the demonstrated sensitivity and specificity suggest a strong potential for field measurements of both CHOCHO and NO₂.