International AOD Comparisons

Atmospheric TurbidityInternational AOD Comparisons

Introduction

Filter Radiometer Comparisons (FRCs) are organised every five years at PMOD/WRC. These AOD comparisons are conducted under various atmospheric conditions which allow different radiometers, calibrations and AOD software to be compared. The main factors that are responsible for possible deviations in global or national AOD networks can thus assessed.

Filter Radiometer Comparisons

PMOD/WRC began the first FRC in 2000 with the support of the WMO. The objective of the campaign was to compare different instruments belonging to different global or national networks in order to quantify the main factors that are responsible for possible deviations. The aim of the whole activity was to initiate action towards homogenisation of the AOD measurements on a global scale. The comparison protocol was formulated according to the WMO recommendations. Measurements of each instrument were compared to the Precision Filter Radiometer (PFR) triad.

FRC-V 2021

The fifth Filter Radiometer Comparison (FRC-V) was organised in conjunction with IPC-XIII and IPgC-III at PMOD/WRC from 27 September to 15 October 2021.

GAW Report No. 280 FRC-V, WMO-Webseite →

GAW Report No. 280 FRC-V, PMOD/WRC-Webseite →

Proceedings of the Symposium on Radiation Measurement, WMO-Webseite →

FRC-V website →

FRC-IV 2015

The 4th Filter Radiometer Comparison included 30 filter radiometers and spectroradiometers from 12 countries, as well as from several global aerosol networks. The absolute differences of all instruments compared to the reference have been based on the WMO criterion defined as “95% of the measured data has to be within 0.005±0.001/m” (where m is the air mass). Figure 1 shows that at least 24 out of 29 instruments achieved this goal at both 500 and 865 nm, while 12 out of 17 and 13 out of 21 achieved this at 368 and 412 nm, respectively.

While searching for sources of differences among different instruments, it was found that all individual AOD differences linked to Rayleigh scattering, NO2, ozone, water vapor calculations, and related optical depths and air mass calculations were smaller than 0.01 at 500 and 865 nm. Different cloud detecting algorithms were also compared. Ångström exponent calculations showed relatively large differences among different instruments partly because of the sensitivity of this parameter at low AOD conditions.

The overall low deviations of these AOD results and the high accuracy of reference aerosol network instruments demonstrated a promising framework to achieve homogeneity, compatibility and harmonization among the different spectral AOD networks in the near-future.

Figure 1. Summary of results from the FRC-IV Comparison.

FRC-I to -III

Results from FRC-I to -III were not published as the intercomparisons were effectively organised on an ad hoc basis amongst participants of the International Pyrheliometer Comparisons (IPC). FRC-I to -III were held for 2 weeks in September – October of 2000, 2005, 2010, respectively, and gave AOD results derived from simultaneous measurements by each participant according to their standard protocol and evaluated by their preferred algorithms, including cloud-screening. Recommendations by WMO experts (WMO, 2005) were implemented as of FRC-II. A large number of radiometers were present during both FRC-II (14 from 9 countries) and FRC–III (17 from 10 countries).

The main conclusions were:

  • i) Most of the ground-based AOD measuring instruments were able to achieve comparable results to within ≈±0.005.
  • ii) Algorithms used for calibration and evaluation contributed a significant fraction of the observed dispersion in AOD measurements.
  • iii) Measurements of the Ångström exponent for the wavelength pair 500/862 nm were questionable when AOD < 0.1.

Collaborations with Global Aerosol Networks – Collaboration with SKYNET

Measurement traceability and data quality are essential requirements by the WMO for monitoring atmospheric aerosol optical properties by International radiometer networks. The SKYNET network has been recently included as a WMO-GAW contributing network and a program of traceability to CIMO defined standard (PMOD WORCC triad) instruments and methods, together with intercomparison and calibration of ESR/SKYNET master instruments is necessary. See the European Skynet Radiometer homepage for further information.

References

WMO (1986), Summary Report on the Status of the WMO Background Air Pollution Monitoring Network as at 31 December 1985, September 1986, WMO TD No. 136.
WMO (2005), WMO/GAW Experts workshop on a global surface-based network for long term observations of column aerosol optical properties (ed: Baltensperger, U., Barrie, L., and Wehrli, C), GAW Report No. 162, WMO/TD-No 1287, Davos, 2004.
For further information please contact: Dr. Stelios Kazadzis, Dr. Natalia Kouremeti