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Traceability of AERONET-EUROPE to the GAWPFR WMO reference for Aerosol Optical Depth

03 Feb 2021

The World Optical Depth Research and Calibration Center (WORCC), a section within the World Radiation Center at the Physikalisch-Meteorologisches Observatorium Davos (PMOD/WRC), in Davos, Switzerland, is recognised as the primary World Meteorological Organization (WMO) reference centre for Aerosol Optical Depth (AOD) measurements. Within the IDEAS-QA4EO ESA project a precision filter radiometer (PFR) travelling standard was installed at the European calibration site of AERONET to provide continuous traceability of aerosol optical depth measurements to the World reference maintained at Davos through a PFR Triad. The objective is to homogenise the two global monitoring networks of passive remote sensing of aerosol optical properties, which serve as fiducial references for satellite-derived aerosol properties.

The PFR traveling standard was installed in July 2020 at the Observatoire de Haute Provence  in southern France on a solar tracker provided by the Laboratory of Atmospheric Optics (LOA). During clear sky periods, measurements of spectral solar irradiance are used to retrieve aerosol optical depth from the master instrument of ACTRIS/AERONET-Europe and the PFR. A web page displays the real-time comparison between the two instruments, and provides a continuous quality control of the measurements.

Platform at the Observatoire de Haute Provence(OHP), France, with four CIMEL sunphotometers and the PFR traveling standard, on the right on its own solar tracker.


The comparison between the two networks over the period August 2020 to January 2021 has been excellent, with AOD differences of less than 0.01, and satisfying the WMO criteria for AOD traceability for 3 out of the 4 channels.

AOD difference
AOD difference master AERONET-Europe sunphotometer (CIMEL #1141) - PFR traveling standard at 500 nm for the period 21 July 2020 - 19 January 2021 Left panel. The shaded area represents the WMO limit for the AOD agreement between two instruments. The AOD difference (dots) is colored based on the probability density function shown in the color bars on the right panel. Right panel: Normalized probability density functions of the AOD differences for the four wavelengths, approximated by double Gaussian distribution functions.


Comparison of aerosol optical depth measurements between the master AERONET-Europe sunphotometer (CIMEL #1141) and the PFR traveling standard. The WMO limits are defined as measurements being within 0.01/m±0.005, where m is the path length through the atmosphere (airmass). The columns marked with * were extrapolated to a common wavelength using the Angstrøm law. The data set consists of 2359 coincident datapoints within a 1-minute window in the period 21 July 2020 - 19 January 2021.
PFR/CIMEL Wavelength (nm)AOD difference% in WMO limits
Median5th percentile95th percentile


It is planned to extend the measurement campaign to at least the end of 2021, with the expectation to keep a permanent PFR system deployed at OHP to provide a continuous link between AERONET-Europe and the WMO PFR reference maintained at Davos.

In parallel to this activity, the project 19ENV04 MAPP (Metrology for aerosol optical properties), funded jointly by EURAMET and the European Commission through EMPIR (European Metrology for Innovation and Research), aims at extending the traceability to the International System of Units (SI) through the characterisation and calibration of sunphotometers from these networks. This collaboration between research institutes and the European metrology community will establish a consistent framework providing calibrations of sunphotometers with traceability to the SI as well as comprehensive uncertainty budgets that will be an integral part of the data provided to the users and stakeholders of these networks.

Kazadzis, S., Kouremeti, N., Nyeki, S., Gröbner, J., and Wehrli, C.: The World Optical Depth Research and Calibration Center (WORCC) quality assurance and quality control of GAW-PFR AOD measurements, Geosci. Instrum. Method. Data Syst., 7, 39-53,, 2018.