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FRM4DOAS: the first centralized processing system for MAX-DOAS air quality measurements

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In order to ensure that products delivered by air quality satellite sensors meet user requirements in terms of accuracy, precision, and fitness for purpose, it is essential to develop a robust validation strategy relying on well-established and traceable reference measurements. In this context, the ESA FRM4DOAS project coordinated by BIRA-IASB aims at developing the first centralized processing system delivering harmonized MAX-DOAS measurements of pollutants like NO2 and HCHO in a near-real-time mode. The availability of such harmonized ground-based data is critical for the validation of present and future satellite missions like the atmospheric Sentinels (S4, S5, S5P) in Europe, GEMS in Asia, and TEMPO in the US.
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MAX-DOAS air quality measurements

MAX-DOAS (Multiple-AXis Differential Optical Absorption Spectroscopy) is a recognized ground-based remote-sensing technique. It measures the amount of atmospheric trace gases absorbing in the UV-visible wavelength range of the solar spectrum by scanning the sky up and down from the horizon to the zenith and collecting the corresponding scattered sunlight.

Since they provide observations of key tropospheric pollutants like nitrogen dioxide (NO2) and formaldehyde (HCHO) at the same horizontal resolution as nadir backscatter satellite sensors, the number of ground-based MAX-DOAS instruments has grown substantially over the last two decades, in support to satellite validation activities.

The ESA FRM4DOAS project

To harmonise the MAX-DOAS data sets measured all over the world, the Fiducial Reference Measurements for ground-based DOAS Air-Quality Observations (FRM4DOAS) project has been set up as a demonstration project providing a centralized rapid-delivery (24h latency) processing service for a selection of MAX-DOAS instruments.

Further, in the scope of FRM4DOAS, clear guidelines for instrument calibration and operation were defined. This activity, which has been specifically developed to validate the European Space Agency (ESA) Sentinel air quality missions (S-5P, S-4, and S-5), is coordinated by  BIRA-IASB and carried out with the following partners:

In its current state, the FRM4DOAS project is limited to NO2, HCHO, and ozone (O3) from a selection of 10 demonstration stations (see Figure 1).

In brief, the centralized processing service starts with the engagement of the scientists who submit their calibrated spectra to a centralized level-1 database. This is followed by the data processing which is the core of the service and which involves state-of-the-art retrieval algorithms. The final data products – quality-controlled vertical profiles of atmospheric NO2 and tropospheric HCHO, and total O3 columns - are all created with their corresponding error budgets and are destined to be publicly distributed for operational and scientific uses through the NDACC (Network for the Detection of Atmospheric Change) and EVDC (ESA Atmospheric Validation Data Centre) data hosting facilities.


In the future, it is planned to distribute the guidelines and necessary information to interested parties in order to include a larger number of stations (see Figure 1) and build a more comprehensive network. This also means that an improved infrastructure which can handle large amounts of data needs to be build. It is also planned to include further additional air quality products, such as e.g. sulphur dioxide (SO2), tropospheric O3, glyoxal (CHOCHO), and aerosols.

For further information on the FRM4DOAS service website

FRM4DOAS demonstration stations:

  • Ny-Ålesund (79°N, 12°E)
  • Harestua (60°N, 11°E)
  • Bremen (53°N, 9°E)
  • De Bilt (52°N, 5°E)
  • Uccle (51°N, 4°E)
  • Mainz (50°N, 8°E)
  • Heidelberg (49°N, 8°E)
  • Xianghe (40°N, 116°E)
  • Athens (38°N, 24°E)
  • Neumayer (71°S, 8°W)
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Figure 1: Map of FRM4DOAS demonstration stations (green circles) + potential candidate stations (red circles) identified through a questionnaire circulated among to the whole MAX-DOAS community.
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