Carbon Monoxide Total Column Retrievals from Sentinel-5P Short-Wave Infrared Nadir Measurements

Abstract

We presented a comprehensive assessment of carbon monoxide (CO) total column retrievals from Sentinel-5P (S5P) Short-Wave Infrared (SWIR) observations. The Level1-2 processing is performed using the Beer Infrared Retrieval (BIRRA) which performs a least squares fit of Earth's radiance (essentially transmission) to retrieve the molecular column densities (essentially density scaling factors) along with some auxiliary parameters (reflectivity, etc.) [Gimeno Garcia et al., AMT 2011]. It has been developed at DLR since about 2005 and its computational core modules have been integrated in the operational SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) Level1b-2 processor to retrieve CO from channel 8 (2.3 µm) and methane from channel 6 (1.6 µm) nadir observations. In the framework of DLR's SCIAMACHY activities BIRRA CO and CH4 columns were thoroughly examined in several intercomparisons [e.g. Hochstaffl et al., Remote Sens. 2018]. The latest version of the BIRRA prototype is featuring new enhancements in the GARLIC (Generic Atmospheric Radiation Line-by-line Infrared Code) [Schreier et al., JQSRT 2014] forward model. It utilizes, among other things, additional line shape parameters given in the latest spectroscopic databases such as the SEOM-IAS (Scientific Exploitation of Operational Missions-Improved Atmospheric Spectroscopy), a project that intended the improvement of spectroscopic information in preparation for the S5P mission. An updated framework is providing appropriate auxiliary information (e.g. a priori atmospheric profiles of pressure, temperature, and concentrations of interfering species, cloud information, topography, the latest molecular spectroscopy data, etc.). Finally, the results were assessed by a thorough comparison with other data products. More specifically, an estimation of the quality of our CO product in view of the accuracy requirements defined in the mission preparation phase (i.e. 15% for CO) is given using TCCON (Total Carbon Column Observing Network) ground-based observations.