Development of an incoherent broadband cavity-enhanced absorption spectrometer for measurements of ambient glyoxal and NO 2 in a polluted urban environment

Abstract. We report the development of an instrument for simultaneous fast measurements of glyoxal (CHOCHO) and NO 2 based on incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) in the 438–465 nm wavelength region. The highly reflective cavity mirrors were protected from contamination by N 2 purge gas. The reduction of the effective cavity length was calibrated by measuring collision-induced oxygen absorption at ∼477  nm of pure oxygen gas input with and without the N 2 mirror purge gas. The detection limits of the developed system were evaluated to be 23 parts per trillion by volume (pptv, 2 σ ) for CHOCHO and 29 pptv (2 σ ) for NO 2 with a 30 s acquisition time. A potential cross-interference of NO 2 absorption on accurate CHOCHO measurements has been investigated in this study, as the absorption of NO 2 in the atmosphere could often be several hundred-fold higher than that of glyoxal, especially in contaminated areas. Due to non-linear spectrometer dispersion, simulation spectra of NO 2 based on traditional convolution simulation did not match the measurement spectra well enough. In this work, we applied actual NO 2 spectral profile measured by the same spectrometer as a reference spectral profile in subsequent atmospheric spectral analysis and retrieval of NO 2 and CHOCHO concentrations. This effectively reduced the spectral fitting residuals. The instrument was successfully deployed for 24 d of continuous measurements of CHOCHO and NO 2 in the atmosphere in a comprehensive field campaign in Beijing in June 2017.