Vicarious Calibration of the TROPOMI-SWIR module over the Railroad Valley playa

The SWIR module of the TROPOMI instrument on board ESA's Sentinel-5p mission has been very stable during its five years in orbit. Calibration was performed on-ground, complemented by measurements during inflight instrument commissioning. The radiometric response and general performance of the SWIR module are monitored by onboard calibration sources. We show that after five years in orbit, TROPOMI-SWIR has continued to show excellent performance with degradation of at most 0.1 % in transmission and having lost less than 0.3 % of the detector pixels. Independent validation of the instrument calibration, via vicarious calibration, can be done through comparisons with ground-based reflectance data. In this work, measurements at the Railroad Valley Playa are used to perform vicarious calibration of the TROPOMI-SWIR measurements, using both dedicated measurement campaigns, as well as automated reflectance measurements through RADCALNET. As such, TROPOMI-SWIR is an excellent test case to explore the methodology of vicarious calibration applied to infrared spectroscopy. Using methodology developed for the vicarious calibration of the OCO-2 and GOSAT missions, the absolute radiometry of TROPOMI-SWIR performance is independently verified to be stable down to ~ 6–10 % using the Railroad Valley, both on the absolute and, thus, relative radiometric calibration. Differences with the onboard calibration originate from the BRDF effects of the desert surface, the large variety in viewing angles, and the different sizes of footprints of the TROPOMI pixels. However, vicarious calibration is shown to be an additional valuable tool in validating radiance-level performances of infra-red instruments such as TROPOMI-SWIR in the field of atmospheric composition.