GAW-PFR: Overview of AOD Network

The World Meteorological Organization (WMO) instigated the Global Atmosphere Watch (GAW) programme in 1989 as a successor to the Background Air Pollution Monitoring Network (BAPMoN).

During a 1993 workshop it was recommended that Aerosol Optical Depth (AOD) measurements, conducted previously under BAPMoN, should be discontinued until new instruments, methods and protocols were established to collect AOD data of known and assured quality.

Based on a recommendation by GAW experts in 1995, the World Optical Depth Research and Calibration Center (WORCC) was established in 1996 at the PMOD/WRC as one of the Swiss conributions to GAW. WORCC has since been advised by the GAW Scientific Advisory Group for Aerosols (SAG). Twelve existing GAW stations were chosen for the deployment and operation of 12 "N" type PFRs, provided by the Swiss Government. WORCC was assigned the following tasks:

  • Development of a radiometric reference for spectral radiometry to determine AOD.

  • Development of procedures to ensure worldwide homogeneity of AOD observations.

  • Development of new instrumentation and methods for AOD.

  • Implementation of a pilot network for AOD at GAW Global observatories including quality control and quality assurance of data, called GAW-PFR.

  • Training operators to use and maintain AOD instruments.


  • Homogenisation of global AOD through provision of traceability to the World Standard Group (WSG) of spectral radiometers for contributing networks at co-located sites and/or periodic international filter radiometer comparisons, and further standardisation of evaluation algorithms.

  • Maintenance of the WSG suitable for contributing AOD networks at 4 PFR wavelengths.

  • Long-term operation of an AOD network (GAW-PFR network) and associated stations, including submission of quality controlled data to the World Data Center for Aerosols (WDCA), and maintenance of its internal calibration hierarchy.

  • Development of a central facility with web-based user-interface for unified processing, archiving and presentation of PFR measurements from GAW-PFR and associated stations (automisation of routine data submission and processing protocols).

  • Development of support equipment and procedures to enable online data collection from GAW-PFR and provision of preliminary results in near real-time.

Overview of

  • Classic extinction measurements at the recommended 4 WMO wavelengths 368, 415, 500 and 862 nm using Precision Filter Radiometers (PFRs).

  • PFRs are manufactured by PMOD/WRC, and have optical filters from the same batch.

  • Continuous sampling at a 1- minute frequency by automated systems.

  • Data are transferred on a monthly basis to WORCC which acts as a central processing hub for data evaluation.

  • Quality control flags cover solar pointing, housing temperature, and cloud screening.

  • Continuous in-situ and/or (annual) centralized calibration based on high altitude references, and linear interpolation in time.

  • Data products: AOD and the Angström coefficients alpha and beta (no inversions).

  • Hourly mean AOD archived at the World Data Center for Aerosols (WDCA) at NILU. Data with a 1-minute resolution are available from WORCC upon request.

Overview of



GAW-PFR and Associate stations currently in operation around the world

Overview of
WORCC has developed a central facility with a web-based user interface for unified processing, archiving and presentation of PFR measurements within GAW-PFR.

In addition, WORCC is participating in EUSAAR (European Supersites for Atmospheric Aerosol Research), which is part of the EU I3 Programme (Integrated Infrastructures Initiatives). The objective of EUSAAR is the integration of measurements of atmospheric aerosol properties performed in a distributed network of 20 high quality European ground-based stations.

This integration contributes to a sustainable reliable operational service in support of policy issues on air quality, long-range transport of pollutants and climate change.

Contact   Stelios Kazadzis, Senior Scientist

WORCC, PMOD/WRC, Dorfstrasse 33, Davos Dorf
CH-7260, Switzerland, Tel. +41 58 467 5137


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