Jahresbericht 08 - PMOD/WRC
Jahresbericht 08 - PMOD/WRC
Jahresbericht 08 - PMOD/WRC
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Aerosol Optical Depth Inter-Comparison Campaigns at European “Super-Sites”<br />
within the EU 6 th Framework EUSAAR Program<br />
Stephan Nyeki, Christoph Wehrli, and Julian Gröbner<br />
This work summarizes WORCC activities within the EU<br />
FP6 EUSAAR (European Super-Sites for Atmospheric<br />
Aerosol Research) program.<br />
The EUSAAR program aims to integrate and harmonize<br />
measurements of aerosol optical properties at European<br />
observation sites, amongst other goals. WORCC is participating<br />
in EUSAAR by conducting audits and aerosol optical<br />
depth (AOD) inter-comparison campaigns at sites with<br />
existing, extensive aerosol measurement programs, socalled<br />
super-sites.<br />
An audit at Mace Head (Ireland) and two AOD inter-comparisons<br />
at Ispra (Italy) and Cabauw (Netherlands) have so<br />
far been conducted. These sites have CIMEL sun-photometers<br />
federated to the AERONET AOD network which<br />
measured alongside a WORCC PFR (precision filter radiometer)<br />
travelling standard. The travelling standard is regularly<br />
calibrated against the WORCC reference standard<br />
consisting of 3 PFRs. PFR and CIMEL AOD multi-wavelength<br />
measurements which occurred to within ± 30 seconds<br />
were used for analysis. A comparison of individual<br />
channels was constrained by wavelength differences<br />
smaller than ±1% of the mean wavelength which limits the<br />
comparison to λ = 500 and 865 nm at Ispra and λ = 865<br />
nm at Cabauw.<br />
The quality of AOD data from the inter-comparison can be<br />
gauged by comparison with WMO criteria (WMO, 2005).<br />
These state that inter-comparisons should be over a sufficient<br />
period of time to ensure that: a) ≥ 1000 coincident<br />
AOD data points are collected, b) a minimum of 5 clear<br />
sunny days occur, and c) AOD at 500 nm should be in the<br />
range 0.04 - 0.20. If these conditions are met, then for<br />
traceability to exist 95 % of CIMEL-PFR points should lie<br />
within limits defined by Equation 1 where m = airmass:<br />
U95 < ± (0.005 + 0.010/m) Eq. 1<br />
Figure 1a-b illustrates the CIMEL-PFR AOD difference versus<br />
airmass at Ispra and Cabauw, respectively. Although<br />
the criterion for the number of coincident measurements<br />
was not met at both sites despite 8-week campaigns,<br />
WMO limits of ~ 95%at Ispra, and ~ 90%at Cabauw were<br />
close to the recommended value of 95 %.<br />
These measurements suggest that the WMO goal of<br />
achieving an AOD uncertainty < 0.02 is achieved in routine<br />
operations at the EUSAAR Cabauw and Ispra super-sites.<br />
Further on-site audits and AOD inter-comparisons will be<br />
conducted at other EUSAAR sites up to January 2011.<br />
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Figure 1. CIMEL-PFR AOD difference versus airmass, illustrating the<br />
WMO criteria for traceability (solid line) at: a) Ispra Sept. 8-26, 20<strong>08</strong><br />
(λ = 500 nm), and b) Cabauw June 11-12, 20<strong>08</strong> (λ = 865 nm).<br />
References: WMO: 2005, WMO/GAW Experts workshop on a global<br />
surface-based network for long term observations of column aerosol optical<br />
properties, Davos 2004. (U. Baltensperger, L. Barrie, C. Wehrli, Editors)<br />
GAW No. 162, WMO/TD-No. 1287.<br />
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