Evaluation of GEMS HCHO retrieval during the in-orbit test period

Volatile Organic Compounds (VOCs) are important precursors for ozone and aerosols in the atmosphere, but their emission estimates are highly uncertain especially in Asia. Because of their short lifetimes, sun-synchronous satellite observations of formaldehyde (HCHO), which is produced by the oxidation of VOCs, have been used to provide observational constraints for VOCs emissions globally. The first geostationary observation focusing on East Asia has been initiated by Geostationary Environment Monitoring Spectrometer (GEMS), which was launched on Feb. 19, 2020. In this study, we evaluate the GEMS HCHO retrieval algorithm by comparing GEMS HCHO products with TROPOMI HCHO products and MAX-DOAS observations at Xianghe, China during the in-orbit test period (2020-08 ~ 2020-10). We find that the initial GEMS HCHO products show good agreements (R = 0.70, 0.63) with the TROPOMI and MAX-DOAS observations, respectively. In particular, GEMS shows a very high correlation coefficient (R = 0.87) for East Asia including east China and the Korean peninsula. However, we also find that the GEMS spectral fitting with observed radiances under high cloud fractions overestimates values compared with the TROPOMI HCHO results, especially in the tropics. To resolve this issue, we use the radiance reference of clean pixels (cloud fraction < 0.4) with an extension of the reference sector (120E ~ 150E) in the fitting. As a result, the correlation coefficients are increased against each observation (TROPOMI = 0.70 0.74, MAX-DOAS = 0.63 0.69). In addition, we conduct a few tests to examine the HCHO retrieval sensitivity to key factors, including the selection of radiance reference areas, cloud masking criterion, and a priori profile. Our sensitivity results are presented in this work.