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.