Jahresbericht 08 - PMOD/WRC

Aerosol Optical Depth Inter-Comparison Campaigns at European “Super-Sites”
within the EU 6 th Framework EUSAAR Program
Stephan Nyeki, Christoph Wehrli, and Julian Gröbner
This work summarizes WORCC activities within the EU
FP6 EUSAAR (European Super-Sites for Atmospheric
Aerosol Research) program.
The EUSAAR program aims to integrate and harmonize
measurements of aerosol optical properties at European
observation sites, amongst other goals. WORCC is participating
in EUSAAR by conducting audits and aerosol optical
depth (AOD) inter-comparison campaigns at sites with
existing, extensive aerosol measurement programs, socalled
super-sites.
An audit at Mace Head (Ireland) and two AOD inter-comparisons
at Ispra (Italy) and Cabauw (Netherlands) have so
far been conducted. These sites have CIMEL sun-photometers
federated to the AERONET AOD network which
measured alongside a WORCC PFR (precision filter radiometer)
travelling standard. The travelling standard is regularly
calibrated against the WORCC reference standard
consisting of 3 PFRs. PFR and CIMEL AOD multi-wavelength
measurements which occurred to within ± 30 seconds
were used for analysis. A comparison of individual
channels was constrained by wavelength differences
smaller than ±1% of the mean wavelength which limits the
comparison to λ = 500 and 865 nm at Ispra and λ = 865
nm at Cabauw.
The quality of AOD data from the inter-comparison can be
gauged by comparison with WMO criteria (WMO, 2005).
These state that inter-comparisons should be over a sufficient
period of time to ensure that: a) ≥ 1000 coincident
AOD data points are collected, b) a minimum of 5 clear
sunny days occur, and c) AOD at 500 nm should be in the
range 0.04 - 0.20. If these conditions are met, then for
traceability to exist 95 % of CIMEL-PFR points should lie
within limits defined by Equation 1 where m = airmass:
U95 < ± (0.005 + 0.010/m) Eq. 1
Figure 1a-b illustrates the CIMEL-PFR AOD difference versus
airmass at Ispra and Cabauw, respectively. Although
the criterion for the number of coincident measurements
was not met at both sites despite 8-week campaigns,
WMO limits of ~ 95%at Ispra, and ~ 90%at Cabauw were
close to the recommended value of 95 %.
These measurements suggest that the WMO goal of
achieving an AOD uncertainty < 0.02 is achieved in routine
operations at the EUSAAR Cabauw and Ispra super-sites.
Further on-site audits and AOD inter-comparisons will be
conducted at other EUSAAR sites up to January 2011.
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Figure 1. CIMEL-PFR AOD difference versus airmass, illustrating the
WMO criteria for traceability (solid line) at: a) Ispra Sept. 8-26, 2008
(λ = 500 nm), and b) Cabauw June 11-12, 2008 (λ = 865 nm).
References: WMO: 2005, WMO/GAW Experts workshop on a global
surface-based network for long term observations of column aerosol optical
properties, Davos 2004. (U. Baltensperger, L. Barrie, C. Wehrli, Editors)
GAW No. 162, WMO/TD-No. 1287.
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