Traceability of the GAW-PFR reference Precision Filter Radiometers to the SI

Project Description

PMOD/WRC is designated by the World Meteorological Organisation (WMO) to operate the World aerosol Optical depth Research and Calibration Center (WORCC). WORCC is responsible for hosting the world reference for Aerosol Optical Depth (AOD), formed as a set of Precision Filter Radiometers (PFR). Furthermore, to operate the AOD monitoring network established within the frame of the Global Atmospheric Watch (GAW) program (GAW-PFR network). The WORCC mandate also encompasses to provide AOD traceability from its GAW‑PFR reference to other global and regional AOD monitoring networks. This is achieved through the Filter Radiometer comparison campaign held a PMOD/WRC every five years, and by collocating PFR travel standards at other network calibration sites (e.g. ACTRIS CARS). The collaboration between PTB and PMOD/WRC with respect to the SI-traceability of spectral solar irradiance of the PFR started in 2019 and continued in the framework of the EMPIR project 19ENV04 MAPP, “Metrology for aerosol optical properties”. The proposed project aims at providing SI-traceability to the global AOD monitoring community through comparisons with the GAW‑PFR reference, which is made up of a set of PFR instruments, of which one or more members will be annually calibrated at PTB.
The proposed activities are:

• Calibration of the spectral responsivity of the individual filter channels of the reference PFRs making up the WMO AOD reference (GAW‑PFR)
• Calibration of the spectral responsivity of the PFR traveling standards
• Investigation of the long-term stability of the GAW‑PFR reference, by repeating calibrations on a regular schedule

The long-term objective is to replace the current Langley-based calibration strategy with laboratory-based SI-traceable calibrations of the GAW‑PFR reference.

The mismatch of the spectral ranges measured by different networks will be bridged through the synergetic use of SI-traceable Precision Filter Radiometer (PFR) and Precision Spectroradiometers (PSR). For that purpose, the optimisation of the tuneable-laser based calibration of the PSRs is an additional proposed activity.

Progress Report 2025-01-07

Two Precision Filterradiometers (PFR) were calibrated at the TULIP facility of PTB in August 2024.

• The 3^rd calibration of the Sun-PFR (PFR-N01) since 2019 showed a degradation of 1% for the 368 nm and 412 nm channels, while for the 500 nm and 862 nm channels it was less than 0.5% over 3 years. The degradation of the UV channels is compatible with the Langley transfer (relative) calibration however for the other channels it is within the uncertainty of the relative method. The degradation rate will be used as uncertainty in the calibration of the PFRs operated within the GAWPFR network.
   
• The Lunar-PFR was calibrated for the second time at PTB.
  The degradation rate was not possible to be defined due to a necessary service of the instrument. The observed wavelength shift and tilt of the spectral responsivity functions were attributed to displacement of the optical axis during the assembling process. The new calibration is used to verify the Top‑of‑Atmosphere lunar irradiance defined based on the calibration of 2021. The internal amplification gain of the PFR was measured for all 4 channels and the results agreed with the 2021 calibration. The homogeneity of the field of view of the Lunar PFR and the position of laboratory calibration was investigated. The operational position for atmospheric measurements, defined by scanning the lunar disk, verified the compatibility of both procedures.
   

For 2025 a calibration of two additional reference PFRs is planed while the PFR-N01 is planned to be calibrated in 18 months to investigate the linearity of the degradation.