Validation of TROPOMI NO2 Retrievals Using PANDORA Measurements in Athens, Greece

Nitrogen dioxide (NO₂) is an important trace gases in the Earth's atmosphere, present in both the troposphere and the stratosphere. In order to have global coverage of NO₂ only satellite based measurements can be used. Launched in October 2017, Sentinel-5P is the first Copernicus mission dedicated to observe the atmosphere, including NO₂. To this direction the instrument TROPOMI has been developed from various scientific and construction Netherlands-based bodies and included in the Sentinel-5P as a single payload. TROPOMI NO2 columnar retrieval is based on an adaption of DOMINO-2 and QA4ECV algorithms for OMI satellite and utilizes the bands of the ultraviolet-near infrared spectrometer (405-465 nm). TROPOMI has a pixel size of roughly 7 km x 3 km. This is considerably smaller than all its predecessor satellites and provides the opportunity to study NO2 in cities scale. Evaluation of the retrievals is an on-going aspect that will contribute to the improvement of Tropomi NO₂ post correction algorithm improvements.

PANDORA is a spectrometer that measures direct solar irradiance at spectral range 280-525nm at 0.6nm resolution. Total NO2 is retrieved by calculating relative NO2 slant columns using a reference spectrum and then using direct sun air mass factor. Accuracy of this retrieval has been estimated at 2.7*10¹⁴ molecules cm^-2. Retrievals will soon be available from the PANDONIA network.

In this study, we compare 10 months of TROPOMI and PANDORA retrievals in Athens, Greece. PANDORA retrievals are averaged at ±15 minutes from TROPOMI overpasses, when cloud free conditions are detected from the ground based instrument. Also, the agreement of the satelite and ground based retrievals regarding weekly and seasonal variabality is inspected.