TROPOMI (the TROPOspheric Monitoring Instrument) is the single instrument aboard the ESA’s Copernicus Sentinel-5 Precursor (S-5P) satellite launched on October 13, 2017 . S-5P is dedicated to monitoring our atmosphere over a period of at least 7 years. Through spectral absorption features within reflected sunlight, TROPOMI measures total column densities of a large number of gases relevant for air quality and climate. This is done at unprecedented spatial resolution of approximately 3.5-7x7 km2. At this resolution, air pollution at scales of midsized cities can be distinguished from its surroundings. In addition, it can be used to estimate other air pollutant quantities such as aerosols. Each gas has a unique spectral fingerprint across the TROPOMI channels, which range from the UV to the infrared. The Short Wavelength Infrared channel, or SWIR, measures reflected sunlight between wavelengths of 2305 to 2385 nanometers. This channel has been designed to accurately determine methane (CH4) and carbon monoxide (CO) columns
To obtain scientifically useful values for both gas species, measured spectra require the SWIR channel to be calibrated to a very high precision and accuracy. The requirements are particularly important for the CH4 product to achieve the ambitious S-5P mission goals. A reliable CH4 product will prove invaluable to the atmospheric scientific community in the next decade.
The first six months after the launch of S5-P were used a.o. to execute an in-flight calibration campaign. This campaign was performed as a complement to the on-ground calibration campaign. It was designed to
(a) take calibration measurements that were unable to be done on-ground,
(b) validate on-ground calibration and
(c) prepare for the instrument calibration monitoring during regular operations.
Here, we report the calibration results obtained by the SRON Level 1 team for the TROPOMI-SWIR channel. This includes the correct method and settings for background corrections (both dark flux and offset), determination of the initial pixel quality map, the evolution of the background correction and quality map during the first 10 months of flight, validation of the on-ground calibration of the instrumental spectral response function (ISRF) and stray light correction.