Trends and degradation in long-term TROPOMI L1 radiance signal

The TROPOMI instrument on board of Sentinel 5P has been measuring radiance and irradiance data in an operational schedule since April 2018. When we compare the absorbing aerosol index (AAI) derived from TROPOMI data to that of OMI, we notice that TROPOMI shows a downward trend that is not in the OMI signal. We know that the TROPOMI instrument is suffering from degradation in various parts of the lightpath. The degradation has been divided into several contributions that are attributed to different parts of the instrument. Especially for the radiance signal it is challenging to discriminate between a possible instrument-related trend or possible long-term atmospheric changes. Radiance monitor data is used to see if there are patterns in these changes. Using on-ground measurements we can assess the absolute radiance calibration of the TROPOMI instrument. This is part of the on-going effort to monitor and improve the L1b data quality.