Simultaneous retrieval of OCS, and CO2 from the IASI shortwave spectral band: assessment of the accuracy of the retrieval products and validation with in situ observations.

To date, a limited understanding of the source and sink action for CO₂ by vegetation persists because it is impossible to distinguish between the CO₂ captured by photosynthesis processes and that released for operations related to plant respiration. In this context, carbonyl sulfide (OCS) is receiving increasing interest. The same as CO₂, OCS is captured by plants in the chlorophyll photosynthesis phase through catalysis of the same enzyme AC (carbonic anhydrase), but, unlike CO₂ , the reaction is irreversible, i.e., the captured OCS is no longer released into the atmosphere. In this way, OCS could be a powerful potential proxy for photosynthetic capture of CO₂ and, therefore, directly related to Gross Primary Productivity or GPP. The objective of this work consists of a feasibility study for the simultaneous estimation of CO₂ and OCS with hyperspectral infrared observations provided by IASI (Infrared Atmospheric Sounder Interferometer) instrument. The OCS has a relatively intense band at 2060 cm^-1, which is well within the coverage of the IASI. This band has been analyzed with the aim of carrying out a complete assessment of the feasibility of estimating the columnar content of the OCS from satellite, simultaneously with CO₂. The analysis of sensitivity and error of the estimate has been carried out with the level 2 processor for IASI, developed by the Applied Spectroscopy group of the School of Engineering of the University of Basilicata. For the analysis, different (parametric and non-parametric) retrieval strategies were considered. It will be shown that the non-parametric strategy shows better performance. Finally, the results have been validated with in situ observations from the Mauna Loa station in Hawaii. The validation study has covered a period from 2014 to 2019, and we conclude that both OCS and CO₂ can be simultaneously retrieved from IASI band 3 with a very high degree of accuracy. A significant, unexpected, result from our analysis is that OCS concentration in the atmosphere is going down since 2017. We have shown that CO parallels this reduction as well, which could be the effect of world-wide actions to reduce the combustion and use of coal (a source of both OCS and CO) in industry and residential sectors.