The team at PMOD/WRC have had a busy month in Davos with the 6th Filter Radiometer Comparison campaign for Aerosol Optical Depth (AOD) measurements, which was held from 20 September to 10 October 2025. This campaign was organized by the PMOD/WRC and supported by the World Meteorological Organization. It was conducted in parallel with the International Pyrheliometer Comparison—which hosted 157 visiting instruments—and the Pyrgeometer Comparison involving 35 instruments.
For AOD measurements, a total of 33 sun-photometers and spectroradiometers from 15 institutes participated. These included reference instruments representing several major global initiatives, such as AERONET, GAW-PFR, and Skynet networks, and also national aerosol monitoring networks. The main objective of the campaign was the harmonization and traceability of ground-based aerosol optical depth measurements to the WMO AOD reference scale, as maintained by the PMOD/WRC reference Precision Filter Radiometers (PFRs).
In addition to the PMOD/WRC reference instruments, several ACTRIS-CARS sun-photometer units—operated by the University of Lille, AEMET, and the University of Valladolid—participated under the framework of the ACTRIS collaboration. This effort supports both long-term intercomparisons and focused campaign-based common measurements to strengthen consistency across European and global aerosol observation networks.
