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 productivity. Recent studies highlight that uncertainties in OCS surface fluxes remain high and the need for satellite datasets with better spatial coverage are required. Here OCS profiles are retrieved using measured radiances from Infrared Atmospheric Sounding Interferometer (IASI) instruments onboard the MetOp-A and MetOp-B satellites, and an adapted version of the University of Leicester IASI Retrieval Scheme (ULIRS). We focus on oceanic and inland water regions for the example year 2018, using an optimal estimation approach for selected microwindows in the 2000–2100 cm⁻¹ wavenumber range. ULIRS 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 measurements 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 accurately measure OCS in the troposphere and observe a reasonable seasonal cycle indicative of being driven by photosynthesis. Further data acquisition is recommended to provide insights into inter-annual variability and seasonality of OCS, and for further application in OCS flux estimation.