Sentinel-5P TROPOMI NO2 reprocessing v2.4.0: Impact of the surface albedo

The Sentinel-5P TROPOMI instrument provides unique observations of atmospheric composition at a high spatial resolution of about 5 km with near-daily global coverage. A new mission reprocessing of all official data products for the full operational phase (30 April 2018 until the present) is currently being generated, co-ordinated by ESA, and is expected to become available in March/April 2023. 

For NO2 this reprocessing will result in a uniform dataseries for the full mission based on processor version 2.4.0 and on version 2 level-1B input data. The reprocessing is replacing the currently available data record for NO2, based on processor versions 1.2 x, 1.3.x, 1.4.0, 2.2.0 and 2.3.1. The upgrades to v1.4 and v2.2 involved major upgrades. Validation activities indicated that the older versions 1.2 and 1.3 had a low bias, a problem which was (at least partly) resolved by introducing v1.4 and v2.y. However, this also implies that trend studies are severely hampered by the jumps in the data series resulting from this sequence of updates. In order to support COVID-19 lockdown air pollution studies an intermediate "PAL" reprocessing of NO2 was made available in December 2021 based on the v2.3.1 NO2 processor and v1 L-1B data (https://data-portal.s5p-pal.com/products/no2.html).  The new official reprocessing with v2.4.0 and v2 L-1B will replace this PAL dataset. The new official reprocessing is expected to be of great use for studying the inter-annual variability in NO2 in recent years.  

The major change in v2.4.0 compared to v2.3.1 is the replacement of the surface albedo datasets with a directional (viewing-angle dependent) Lambertian equivalent reflectivity (DLER) database derived from TROPOMI observations. Before the TROPOMI NO2 processor made use of the OMI (for the NO2 fitting window) and GOME-2 (for the O-2A band spectral region used for the cloud retrieval) LER. The use of the TROPOMI DLER is especially important for the cloud fraction and cloud pressure retrievals, because the clear-sky reflectivity in the NIR is very sensitive to the (viewing) geometry. 

In our contribution we will present the new reprocessed TROPOMI NO2 v2.4 dataset and quantify the impact of the use of the TROPOMI DLER on the cloud properties, air-mass factor and tropospheric NO2 column.