Study of NO2 and HCHO vertical profile measurement based on Fast Synchronous MAX-DOAS

This paper investigates a multi-elevation Fast Synchronous observation MAX-DOAS system(FS MAX-DOAS) that can quickly obtain trace gas profiles. Compared to the traditional sequential scanning of elevation angles by motors, the system employs a grating spectrometer with a two-dimensional array CCD, we also designed telescopes of small field(<1⁰), a high-speed shutter switching module, and a multi-mode multi-core fiber which is divided into twelve beams to achieve multitrack spectroscopy. It greatly improves the time resolution of spectra collection(a elevation cycle within two minutes). The influence of spectral resolution on FS MAX-DOAS detection of trace gases was analyzed, and the optimal resolution range (0.277-0.569nm) was determined to select the grating used in the spectrometer. The selection of actual binning rows takes into account the SNR of each row of pixels to improve the quality of spectral data, and two-step acquisition is used to overcome the influence of difference in light intensity for low elevation angles. The stability of the system was analyzed using Allan variance. The outfield comparison experiment with differential optical absorption spectroscopy was conducted, and the comparison test was conducted with the ground-based MAX-DOAS system for NO₂ and HCHO in the actual atmosphere. The Pearson correlation coefficient of NO₂ reached 0.9, HCHO had a good correlation(Pearson’s R was mostly between 0.65-0.78). In the experiment, it was found that the RMS of FS MAX-DOAS spectral inversion can be stably lower than that of MAX-DOAS system for a long time, and the gas profile obtained by the former can show more details due to the improved time resolution. Compared to the near surface concentration of NO₂ using active Long Path DOAS instrument, the Pearson’s R of FS MAX-DOAS data is higher. New system can quickly and simultaneously obtain vertical distribution profiles of NO₂ and HCHO with high accuracy, which provides a possibility for mobile MAX-DOAS to achieve gas profile inversion.