An Incoherent Broadband Cavity-Enhanced Absorption Spectrometer For Measurements Of Atmospheric Glyoxal

Glyoxal(CHOCHO)is a typical intermediate for most volatile organic compounds(VOCs)oxidations in the atmosphere.It plays an important role in quantifying VOCs emissions,understanding VOCs oxidation mechanisms,and further understanding the formation of O3 and secondary organic aerosol(SOA).Glyoxal has a short lifetime during the day due to photolysis and reaction with OH radicals,only about 2 hours.The heterogeneous reaction of glyoxal at night is also a main removal route.Secondary organic aerosols(SOA)account for an important proportion in organic aerosols.Glyoxal can enter the liquid or particulate phase through heterogeneous reactions,which has a potentially significant contribution to SOA.Glyoxal mainly comes from oxidation and has a short lifetime,making it a good indicator of photochemical reaction.The ratio of glyoxal to formaldehyde can be used to determine the type of VOCs,but there is a divergence between satellite observations and ground-based measurements.In view of the important role of glyoxal in the atmosphere,we conducted incoherent broadband cavity-enhanced absorption spectrometer(IBBCEAS)technology for the measurement of glyoxal.IBBCEAS technology features high sensitivity,real-time online rapid measurement,and simultaneous measurement of multiple components and can achieve high sensitivity and high time resolution measurement of glyoxal.Based on the research of our group,the IBBCEAS system based on blue LED was built,and participated in two field observation experiments in Beijing and Taizhou,and the observation results were analyzed.The main findings are as follows:1)The IBBCEAS system based on blue LED was built.The use of an off-axis parabolic mirror instead of an achromatic lens effectively improves the optical coupling efficiency.The system achieves a high signal-to-noise ratio by selecting high-power blue LEDs,matched high-reflectivity mirrors,bandpass filters,and fiber optic spectrometers.The mirror reflectivity was calibrated using two methods and the calibration results were in good agreement.The detection limits of the developed system were evaluated to be 23 parts per trillion by volume(pptv,2σ)for CHOCHO and 29 pptv(2a)for NO2 with a 30-s acquisition time,respectively.2)The spectral]retrieval of Glyoxal and interference deduction were studied.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.At the same time,the effects of different concentrations of NO2 on glyoxal,methylglyoxal and water vapor were analyzed.3)The indication effect of RGF on precursor VOCs was studied,and the lifetime and nighttime loss rate of glyoxal were calculated.The RGF of Beijing was calculated,with an average of 2.2%.The low RGF value reflects the emission of anthropogenic source precursors.At the same time,the high value(greater than 8%)of RGF appearing during the measurement was analyzed,and it was found that the control types of different precursors existed.The lifetime of glyoxal in the summer atmosphere of Beijing was studied and the average lifetime at noon was calculated to be 1.96 hours.The heterogeneous loss rate of nighttime glyoxal was studied to be 1.68×10-5 s-1.