Observing a Volatile Organic Compound From a Geostationary Infrared Sounder: HCOOH From FengYun-4B/GIIRS

Formic acid (HCOOH) is one of the most abundant volatile organic compounds (VOCs) in the Earth's atmosphere and an important source of atmospheric acidity. Here we present the first retrieval of HCOOH from a geostationary orbit using observations from the Geostationary Interferometric Infrared Sounder (GIIRS) on board FengYun-4B. The results from July 2022 to June 2023 show the monthly variation of the HCOOH distribution in Asia. In addition, we show that FY-4B/GIIRS effectively tracks HCOOH enhancements from wildfires both during the day and at night. Finally, inter-comparison with IASI HCOOH data shows good agreement, indicating that FY-4B/GIIRS observations have comparable sensitivity to IASI. This study is an important first step toward monitoring VOCs from geostationary infrared sounders, including the existing and future GIIRS on board the FY-4 series and the forthcoming European geostationary infrared sounder (IRS) on board Meteosat Third Generation (MTG). Satellite observations play an indispensable role in improving our understanding of the global sources and sinks of HCOOH, an important volatile organic compound in the atmosphere. However, existing polar-orbiting satellites that are sensitive to tropospheric HCOOH typically provide up to two overpasses per day over the same spot, one during the day and one during the night. The diurnal variations of tropospheric HCOOH are therefore under-constrained to track its evolution in the lower troposphere and transport in the free troposphere. The Geostationary Interferometric Infrared Sounder (GIIRS) onboard China's FengYun-4 satellite series is the world's first geostationary hyperspectral infrared sounder. Using observations from FY-4B/GIIRS, this study presents the first retrieval of HCOOH from a geostationary satellite instrument that captures the HCOOH variation in Asia driven by biomass burning emissions and possible biogenic sources, providing key observations for understanding the complex processes of HCOOH production, evolution, and loss in the atmosphere. This study is an important first step toward monitoring VOCs from geostationary infrared sounders. First retrieval of atmospheric HCOOH from a geostationary sounder is demonstrated HCOOH in fire plumes can be continuously tracked with FY-4B/GIIRS FY-4B/GIIRS HCOOH observations have comparable sensitivity to IASI