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Intercomparison of actinic flux / photolysis frequency measurements

In the framework of the EU network project ACCENT we carried out a spectral actinic flux/photolysis frequency measurement inter-comparison with an opportunity for calibration of filterradiometers. The campaign was mainly intended to provide a common standard for calibration rather than an assessment of different methods or instruments.

Photolysis frequencies determine the primary production of radicals and are therefore crucial for any quantitative analysis of photochemical field data. Equipment for measurement of photolysis frequencies needs continuous supervision to ensure the quality of the data but calibration devices are expensive and not available to all groups. The situation is most ambiguous for j(O1D) filterradiometers where instrumental properties influence the conversion of output signals to photolysis frequencies in a fairly complex way. These properties may change with time unnoticed. Moreover, recommendations concerning O(1D) quantum yields of ozone photolysis changed significantly within the last 10 years and these changes also influence the conversion. The situation is improved for j(NO2) filterradiometers where usually a single calibration factor is sufficient for conversion of output signals to photolysis frequencies. Nevertheless, regular calibrations are also recommended for these instruments. Spectral actinic flux measurements using spectroradiometers have the advantage that any photolysis frequency can be calculated from the data. Moreover, calibrations can be made by irradiance standards traceable to national standards. However, there may still be problems with linearity, mechanical stability (scanning systems), straylight subtraction (diode array instruments) or software errors which can be revealed by an inter-comparison under real measurement conditions.

To improve comparability we offered an opportunity for parallel measurements with our spectroradiometers at Forschungszentrum Jülich after setting up a suitable platform for a larger number of instruments. The campaign was scheduled from 30 May to 12 June 2005. Preliminary results are available from a poster presented at the ACCENT symposium in Urbino.