AOD Reference PFR Triad

Aerosol optical depth (AOD) is a quantitative measure of the extinction of solar radiation by aerosol scattering and absorption between an observation point and the top of the atmosphere. It is a measure of the integrated columnar aerosol load and the most important parameter for direct radiative forcing studies. AOD is not directly measurable but retrieved from observations of the spectral transmission of the atmosphere. AOD measurements are performed with sun-pointing direct beam instruments, or with simultaneous global and diffuse measurements. The World Meteorological Organization (WMO, 1986) recommends measuring at least at three of the following center wavelengths: 368, 412, 500, 675, 778, 862 nm, with a bandwidth of 5 nm. The field-of-view geometry for direct beam radiometers should correspond to the WMO (1986) specifications of a full opening angle of 2.5° and a slope angle of 1°.

Different instruments from different global and national networks, but also independent instrumentation, measure the direct irradiance and the aerosol related attenuation. In 2006, the Commission for Instruments and Methods of Observations recognised “the need for establishing a primary reference AOD Center to satisfy the need for traceability of Optical Depth measurements, conducting international intercomparisons, guaranteeing data quality needed in climate studies”. It was recommended that the World Optical Depth Research and Calibration Center (WORCC) at PMOD/WRC be designated the primary WMO Reference Center for AOD measurements as part of WRC activities (WMO, 2005).

The World Standard Group of three PFRs (so-called “PFR triad”, Figure 1) was established in 2005 by WORCC in order to fulfil the WMO mandate on: “Homogenization of global AOD through provision of traceability to the WSG of spectral radiometers for contributing networks at co-located sites and/or periodic international filter radiometer comparisons, and further standardization of evaluation algorithms.” Since 2005, five different well-maintained instruments have been used as part of the PFR triad whose calibration history is shown in Figure 2.

According to WMO (WMO, 2005), as traceability is not currently possible based on physical measurement systems, the initial form of traceabilty will be based on difference criteria. That is, at an inter-comparison or co-location, traceability will be established if the AOD difference between networks is within specific limits. The 95% uncertainty limits (U95) for finite field-of-view instruments have been set (WMO, 2005) to 0.005 + 0.01/m optical depths and the acceptable traceability is when 95% of the absolute AODs are within those limits. The first term (0.005) is linked to instrument uncertainties (signal linearity, sun pointing, temperature effects, processing, etc.) and the second term to a calibration uncertainty of 1%.

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