Global optimal estimation retrievals of atmospheric carbonyl sulfide over water from IASI measurement spectra for 2018

Abstract

Carbonyl sulfide (OCS) is consumed by vegetation during photosynthesis in a one-way hydrolysis reaction, making measuring OCS vegetative uptake a means of inferring and quantifying global gross primary production. Recent studies highlight that uncertainties in OCS surface fluxes remain high and OCS datasets with better spatial coverage are required, particularly from satellite. Here OCS profiles are retrieved using measured radiances from the Infrared Atmospheric Sounding Interferometer (IASI) instruments. We estimate total column amounts over oceanic and inland water regions for the example year 2018, using an optimal estimation scheme, using the University of Leicester IASI retrieval scheme (ULIRS) for selected microwindows in the 2000-2100 cm-1 wavenumber range. Information content exceeds one between ±50° latitude and a peak in vertical sensitivity around 6-10 km (500-300 hPa) in the troposphere. Diurnal variations are limited to ±2 %, showing larger total column amounts at the daytime overpass. The IASI OCS observations show a correlation of at least 0.74 at half the ground-based flask measurement sites compared. Results also agree with the University of Leeds TOMCAT 3-D chemical transport model simulations within ±5 % throughout most tropical regions. This study demonstrates the ability of the IASI instrument to detect OCS in the troposphere and observe a reasonable seasonal cycle indicative of being driven by photosynthesis. Further data acquisition is recommended to gain insight into inter-annual variability in OCS. This novel work will also help improve our understanding of the role of vegetation in the carbon cycle, particularly when utilised in inversion methods.