Development of an incoherent broadband cavity-enhanced absorption spectrometer for measurements of ambient glyoxal and NO₂ in a polluted urban environment

Abstract

Abstract. We report the development of an instrument for simultaneous fast measurements of glyoxal (CHOCHO) and NO2 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 N2 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 N2 mirror purge gas. The detection limits of the developed system were evaluated to be 23 parts per trillion by volume (pptv, 2σ) for CHOCHO and 29 pptv (2σ) for NO2 with a 30 s acquisition time. A potential cross-interference of NO2 absorption on accurate CHOCHO measurements has been investigated in this study, as the absorption of NO2 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 NO2 based on traditional convolution simulation did not match the measurement spectra well enough. In this work, we applied actual NO2 spectral profile measured by the same spectrometer as a reference spectral profile in subsequent atmospheric spectral analysis and retrieval of NO2 and CHOCHO concentrations. This effectively reduced the spectral fitting residuals. The instrument was successfully deployed for 24 d of continuous measurements of CHOCHO and NO2 in the atmosphere in a comprehensive field campaign in Beijing in June 2017.