Homogenization activities of ground-based aerosol optical depth measurements within WMO Filterradiometers Campaign (FRC-VI)

Aerosol optical depth (AOD) is the single most comprehensive variable to assess the columnar aerosol load of the atmosphere. Several studies have investigated long-term AOD from ground-based observation using multiple instruments. The accuracy of AOD measurements and trends from ground-based instruments have a particular significance since they are used for satellite validation, climate model validation and modelling assimilation etc.

 

To ensure consistency across different networks, continuous intercomparisons are used to homogenize multi-wavelength AOD measurements. The World Optical depth Research and Calibration Center (WORCC) at PMOD/WRC operates and maintains the World Meteorological Organization (WMO) AOD reference, known as GAWPFR-TRIAD. This reference consists of three Precision Filter Radiometer (PFR) instruments. PMOD/WRC disseminates the GAWPFR-TRIAD scale through Filter Radiometer Campaigns (FRC) organized every five years on behalf of WMO. These campaigns play a crucial role in harmonizing ground-based AOD measurements globally.

 

The most recent Filter Radiometer Campaign (FRC) took place in September-October 2025 in Davos. This campaign brought together 33 instruments, including both filter radiometers and spectroradiometers, representing nine international and national AOD networks. The intercomparison results of spectral AOD will be presented, and the level of agreement between the networks will be assessed and compared to previous FRC campaigns (GAW Report No. 231 and 280).

 An uncertainty analysis based on the information provided by each instrument will be performed and compared to the WMO traceability level of agreement, which represents calibration uncertainties of 1%. Numerous projects (e.g., ACTRIS/CARS, 19ENV04 MAPP) are focused on the optimization, standardization, and harmonization of AOD products, leading to a reduction in AOD differences to better than 0.01 at least at some wavelengths. FRC provides  an  opportunity to access this  assumption in a  global scale.     

 

Furthermore, building on the work of the EMPIR 19ENV04 MAPP project, which focuses on AOD retrievals based on laboratory calibration of spectroradiometers (Gröbner et al., 2023) and filter radiometers (Kouremeti et al., 2022), emphasis is placed on these comparisons since the methodology has been operationally adapted by a few commercial instruments. In addition, the reference spectroradiometer QASUME is used to validate extrapolation techniques for AOD outside the spectral range of 368 nm to 863 nm covered by the GAWPFR-TRIAD reference instruments.

Acknowledgments: The authors would like to thank all the participants for the FRC-VI campaign for their contributions, time and efforts.  This work was supported by GAW program of WMO and by the joint research project EMPIR 19ENV04 MAPP “Metrology for aerosol optical properties”.

 References 

Fourth WMO Filter Radiometer Comparison (FRC-IV), World Meteorological Organization (WMO), GAW Report No. 231, https://library.wmo.int/idurl/4/55417

Fifth WMO Filter Radiometer Comparison (FRC-V), World Meteorological Organization (WMO), GAW Report No. 280, https://library.wmo.int/idurl/4/66263

Gröbner, J., et al. (2023). Atmos. Meas. Tech., 16,4667-4680. 

Kouremeti, N., et al. (2022). Metrologia, 59,044001.