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 Earth’s atmosphere and an important source of atmospheric acidity. Satellite observations play an indispensable role in improving our understanding of global HCOOH sources and sinks. However, existing polar-orbiting satellites that are sensitive to tropospheric HCOOH provide only two daily overpasses over the same location, one during the day and the other at night. The diurnal changes of tropospheric HCOOH are therefore under-constrained, limiting our ability to monitor its evolution and transport throughout the troposphere. The Geostationary Interferometric Infrared Sounder (GIIRS) onboard China’s FengYun-4 satellite series is the world’s first geostationary hyperspectral infrared sounder. Here, we present the first retrieval of HCOOH from GIIRS onboard FengYun-4B. Results from July 2022 to June 2023 highlight the seasonal variation of the HCOOH distribution in Asia driven by biomass burning emissions and biogenic sources. Sensitivity to HCOOH peaks during daytime and decreases at night, following diurnal changes of thermal contrast between the surface and the atmosphere. FY-4B/GIIRS in the geostationary orbit will offer important information with its high temporal resolution to improve our understanding of the production, evolution, and loss processes of HCOOH in the atmosphere. Furthermore, cross-validation with IASI HCOOH data shows good agreement, indicating that observations from FY-4B/GIIRS have comparable sensitivity to IASI.