01 Sep 2023
 | 01 Sep 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

First evaluation of the GEMS formaldehyde retrieval algorithm against TROPOMI and ground-based column measurements during the in-orbit test period

Gitaek T. Lee, Rokjin J. Park, Hyeong-Ahn Kwon, Eunjo S. Ha, Sieun D. Lee, Seunga Shin, Myoung-Hwan Ahn, Mina Kang, Yong-Sang Choi, Gyuyeon Kim, Dong-Won Lee, Deok-Rae Kim, Hyunkee Hong, Bavo Langerock, Corinne Vigouroux, Christophe Lerot, Francois Hendrick, Gaia Pinardi, Isabelle De Smedt, Michel Van Roozendael, Pucai Wang, Heesung Chong, Yeseul Cho, and Jhoon Kim

Abstract. The Geostationary Environment Monitoring Spectrometer (GEMS) onboard GEO-KOMPSAT 2B was successfully launched in February 2020 and has monitored Asia. We present the first evaluation of the operational GEMS formaldehyde (HCHO) vertical column densities (VCDs) during the in-orbit test period (IOT) (August–October 2020) and onward by comparing them with the products from Tropospheric Monitoring Instrument (TROPOMI), Fourier-Transform Infrared (FTIR), and Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) instruments. During the in-orbit test period, the GEMS HCHO VCDs reproduced the observed spatial pattern of TROPOMI VCDs over the whole domain (r=0.62) with high biases (10–16 %). In the afternoon, GEMS VCDs were too high over the west side of the tropics. We corrected this issue by adding polarization sensitivity vectors of the GEMS instrument as an additional fitting parameter in the retrieval algorithm. Using observed radiances from clear-sky pixels as the reference spectrum in the spectral fitting significantly contributed to reducing artifacts in radiance references, resulting in 10–40 % lower HCHO VCDs over the latitude including cloudy areas in the updated GEMS product. We find that the agreement between the two is much higher in Northeast Asia (r=0.90), including the Korean peninsula and East China. GEMS HCHO VCDs well captured the seasonal variation of HCHO mainly driven by biogenic emissions and photochemical activities but showed larger variations than those of TROPOMI over coastal regions (Kuala Lumpur, Singapore, Shanghai, and Busan). In addition, GEMS HCHO VCDs showed consistent hourly variations with MAX-DOAS (r=0.79) and FTIR (r=0.85) but were lower by 30–40 % relative to the ground-based observations. Different vertical sensitivities between GEMS and ground-based instruments caused these systematic biases. The use of averaging kernel smoothing method reduces the low biases by about 10 to 15 % (NMB: -48.5 % to -32.4 %, -39.1 % to -27.3 % for MAX-DOAS and FTIR, respectively). The remaining discrepancies are due to multiple factors, including spatial colocation and different instrumental sensitivities, which need further investigation using inter-comparable datasets.

Gitaek T. Lee et al.

Status: open (until 25 Oct 2023)

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Gitaek T. Lee et al.


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Short summary
This study evaluates the GEMS HCHO retrieval algorithm by comparing the GEMS HCHO VCDs with those of TROPOMI and ground-based observations (MAX-DOAS, FTIR). Based on a few sensitivity tests, obtaining radiance references under clear-sky conditions significantly improves the HCHO retrieval quality. GEMS HCHO VCDs well capture seasonal and diurnal variations during the first one-year observation, showing consistent variability with TROPOMI and ground-based observation products.