Traceability of Aerosol Optical Depth measurements and links with the Aerosols, Clouds and Trace Gases Research Infrastructure (ACTRIS) / Calibration of Aerosol Remote Sensing (CARS)

World Optical Depth Research and Calibration Center (WORCC) is participating in the Aerosol, Clouds and Trace Gases Research Infrastructure (ACTRIS), Switzerland program as part of the  ACTRIS Calibration of Aerosol Remote Sensing (CARS). In the ACTRIS-CARS program, WORCC maintains three precision filter radiometers (PFRs) at Izanã (IZO) in Spain, Observatoire de Haute Provence (OHP) in France and University of Valladolid (VLD) in Spain which are the main calibration facilities of ACTRIS sun photometers. The aim of this collaboration is to link the AOD product from ACTRIS-CH with the AOD scale recognized by the WMO and maintained by WORCC.In this analysis, the AOD measurement comparisons were performed between ACTRIS-CARS PFR and Aerosol Robotic Network (AERONET)-Europe CIMEL sun-photometers at IZO, OHP and VLD for 2021 and 2022. The comparisons are performed in common wavelengths and also PFR AOD is extrapolated to the non-matching CIMEL wavelengths using Ångström law. The annual comparison results of PFR and CIMEL AOD at IZO showed that more than 99% of AOD differences were within the WMO uncertainty limits for all wavelengths above 340 nm. In case of OHP, more than 99% percentage of AOD differences were within the WMO uncertainty limits for all wavelengths longer than 440 nm while it was above 80% for all wavelengths except 340 nm. At VLD, the percentage of AOD differences within the WMO uncertainty limits were mostly above 99% for all wavelengths above 440 nm and above 90% for all wavelengths except 340 nm. In additions, instruments were inter-compared during the fifth filter radiometer campaign (FRC-V) held in 2021 and differences with the Davos reference triad of PFRs were mostly within the WMO limits.Acknowledgment The work has been supported by the  Aerosol, Clouds and Trace Gases Research Infrastructure – Swiss contribution funded by the State Secretariat for Education, Research, and Innovation, Switzerland.