Global carbon monoxide retrieval from the hyperspectral infrared atmospheric sounder-II onboard FengYun-3E in a dawn-dusk sun-synchronous orbit

Abstract

This study presents an information content analysis of the retrieval of global carbon monoxide (CO) from the 2nd generation of Hyperspectral Infrared Atmospheric Sounder on board Fengyun-3E (FY-3E/HIRAS-II), which is the world's first dawn-dusk orbiting meteorological satellite for civilian use with equatorial overpass times at about 5:30am/pm. Using the distinctive absorption features of CO around 2145 cm-1 in the infrared band, a retrieval algorithm based on optimal estimation is first developed to retrieve the CO profiles from FY-3E/HIRAS-II. The thermal contrast, defined as the temperature difference between the surface and the lower atmosphere, is then examined for the FY-3E/HIRAS-II observations to investigate the potential sensitivity of the observations. Retrievals of CO columns on four representative days over four seasons reveal strong enhancements from global wildfire emissions. Furthermore, the information content from the spectra, quantified by the degree of freedom for signal (DOFS), is assessed using the retrievals. The retrievals are compared with model simulations, which show good agreement. Finally, synthetic experiments are performed to investigate the impacts of thermal contrast and the vertical structure of wildfire-induced CO enhancement on the observational sensitivity. The result highlights the effectiveness of observing wildfire-induced CO enhancement during the dawn and dusk hours, even when thermal contrast conditions are not favorable. This study demonstrates the capability of FY-3E/HIRAS-II to monitor global CO, which sheds light on the potential benefits of adding dawn-dusk orbit observations to the current global air quality observations from mid-morning and early-afternoon orbit constellations.