Next Generation TROPOMI NO2 (NO2NEXT - MSO project)

Develop realistic high-resolution NO2 retrievals from TROPOMI.

TROPOMI on the Sentinel-5P platform is the leading instrument to monitor NO2 from space, providing city-scale measurements of NO2 pollution. The imagery and scientific results from TROPOMI are generating wide attention from academia and the general public. They are used to study the impacts of COVID-19 and to assess the nitrogen crisis in The Netherlands and worldwide.

To reduce uncertainties and to innovate KNMI’s retrievals for the next generation NO2 capabilities with the CO2M and Sentinel-5 missions, we improve the modeling of atmospheric photon transport and implement fine-scale information on the state of the atmosphere to develop realistic high-resolution NO2 retrievals. This will enable new science and applications of the data, maintain KNMI’s scientific infrastructure at top level, and help upcoming European sensors.

Improved TROPOMI tropospheric NO2 column
Left: improved TROPOMI tropospheric NO2 column for winter 2019. Right: Impact of improved FRESCO+ cloud pressures on the retrieved TROPOMI tropospheric NO2 column. The improved cloud pressures partly correct the (low) bias. From Riess et al., 2022.
Probability distribution of co-located effective cloud pressures from TROPOMI
Probability distribution of co-located effective cloud pressures from different sources. Only pixels with cloud fraction values between 0.05 and 0.20 have been selected as these are relevant for valid TROPOMI NO2 retrievals. From Riess et al., 2022.


The TROPOMI NO2 data product has been extensively validated, tested, and used. This research confirmed that two of the three main ingredients of the NO2 retrieval, the NO2 slant column density (SCD) and the stratospheric column estimate, are of sufficient quality to fulfil the TROPOMI Mission Requirements [ESA, 2017]. The third component of the retrieval however, the air mass factor (AMF), a measure for the vertical sensitivity of the measurement is likely biased high. AMFs are flawed because of too little physical realism in the radiative transfer model calculations, and because the input data are too coarse or suffer from structural uncertainty (Lorente et al. [2017]).


The overall goal of NO2NEXT is to fundamentally improve KNMI’s TROPOMI NO2 retrievals through a better understanding of

  1. 3-D surface reflectivity and cloud effects, and
  2. the spatio-temporal distribution of NO2 profiles,

and how these affect the retrieval and interpretation of the satellite data.


Our work in this project will be guided by 3 objectives:

  1. Improve the physical realism of the TROPOMI cloud and tropospheric NO2 retrievals;
  2. Improve TROPOMI tropospheric NO2 retrievals through better understanding the spatio-temporal distribution of NO2 profiles at pixel-resolution;
  3. Application of the improvements into a new TROPOMI retrieval, and guidance and exchange with key stakeholders.


Folkert Boersma

Henk Eskes

Gijs Tilstra

John Douros

Christoph Riess


Riess, T. C. V. W., Boersma, K. F., van Vliet, J., Peters, W., Sneep, M., Eskes, H., and van Geffen, J.: Improved monitoring of shipping NO2 with TROPOMI: decreasing NOx emissions in European seas during the COVID-19 pandemic, Atmos. Meas. Tech., 15, 1415–1438,, 2022.

van Geffen, J., Eskes, H., Compernolle, S., Pinardi, G., Verhoelst, T., Lambert, J.-C., Sneep, M., ter Linden, M., Ludewig, A., Boersma, K. F., and Veefkind, J. P.: Sentinel-5P TROPOMI NO2 retrieval: impact of version v2.2 improvements and comparisons with OMI and ground-based data, Atmos. Meas. Tech., 15, 2037–2060,, 2022.

Douros, J., Eskes, H., van Geffen, J., Boersma, K. F., Compernolle, S., Pinardi, G., Blechschmidt, A.-M., Peuch, V.-H., Colette, A., and  Veefkind, J. P.: Comparing Sentinel-5P TROPOMI NO2 column observations with the CAMS-regional air quality ensemble, submitted to Geoscientific Model Development, egusphere-2022-365, 2022.