Internationale AOD-Vergleiche

Atmosphärische TrübungInternationale AOD-Vergleiche

Einleitung

Filterradiometer-Vergleiche (FRCs) werden alle fünf Jahre am PMOD/WRC organisiert. Diese AOD-Vergleiche werden unter verschiedenen atmosphärischen Bedingungen durchgeführt, wodurch verschiedene Radiometer, Kalibrierungen und AOD-Software verglichen werden können. Die Hauptfaktoren, die für mögliche Abweichungen in globalen oder nationalen AOD-Netzwerken verantwortlich sind, können somit bewertet werden.

Filterradiometer-Vergleiche

Das PMOD/WRC begann den ersten FRC im Jahr 2000 mit Unterstützung der WMO. Ziel der Kampagne war es, verschiedene Instrumente unterschiedlicher globaler oder nationaler Netzwerke zu vergleichen, um die Hauptursachen für mögliche Abweichungen zu quantifizieren. Ziel der gesamten Aktivität war es, Maßnahmen zur Homogenisierung der AOD-Messungen auf globaler Ebene zu initiieren. Das Vergleichsprotokoll wurde gemäß den WMO-Empfehlungen formuliert. Die Messungen jedes Instruments wurden mit der Triade des Precision Filter Radiometer (PFR) verglichen.

FRC-V 2021

Der 3. Filterradiometer-Vergleich (IPgC-III) wurde in Zusammenarbeit mit IPC-XIII und FRC-V am PMOD/WRC vom 27. September bis 15. Oktober 2021 organisiert.

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-Webseite →

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