An IBBCEAS system for atmospheric measurements of glyoxal and methylglyoxal in the presence of high NO 2 concentrations
Abstract. A system based on incoherent broadband cavity-enhanced
absorption spectroscopy (IBBCEAS) has been developed for simultaneous
measurement of nitrogen dioxide ( NO2 ), glyoxal (GLY), and methylglyoxal
(MGLY). In this system, the measured light absorption at around 460 nm is
spectrally resolved. The concentration of absorbers is determined from a
multicomponent fit. At an integration time of 100 s, the measurement
sensitivity ( 2σ ) for NO2 , GLY, and MGLY is 18, 30, and
100 ppt, respectively. The measurement uncertainty, which mainly originates
from path length calibration, sampling loss, and uncertainty of absorption
cross sections is estimated to be 8 % for NO2 , 8 % for GLY, and
16 % for MGLY. When deploying the instrument during field observations, we
found significant influence of NO2 on the spectra fitting for
retrieving GLY and MGLY concentrations, which is caused by the fact that
 NO2 has a higher absorption cross section and higher ambient
concentration. In order to minimize such an effect, a NO2 photolytic
convertor (NPC), which removes sampled NO2 at an efficiency of 76 %,
was integrated on the IBBCEAS system. Since sampled GLY and MGLY are mostly
( ≥95  %) conserved after passing through the NPC, the quality of the
spectra fitting and the measurement accuracy of ambient GLY and MGLY under
 NO2 -rich environments could be improved.