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Volz-Thomas, Andreas; Arnold, T.; Behmann, T.; Borrell, P.; Borrell, P. M.; Burrows, J. P.; Cantrell, C. A.; Carpenter, L. J.; Clemitshaw, K. C.; Gilge, S.; Heitlinger, M.; Klüpfel, T.; Kramp, F.; Mihelcic, D.; Müsgen, P.; Pätz, H.- W.; Penkett, S. A.; Perner, D.; Schultz, M.; Shetter, R.; Slems, J.; Weissenmayer, M.
Peroxy Radical Inter Comparison Exercise (PRICE)
133 S., 1998

PRICE was organised as a formal comparison of different methods to measure peroxy radicals (HO2 and RO2). It was conducted in summer 1994 at the TOR(*1)-station Schauinsland in South-West Germany as a task of the EU-project OCTA(*2). The comparison involved matrix isolation ESR spectroscopy (MIESR) and five chemical amplifiers (CA). The campaign lasted 5 weeks, with ambient measurements being performed over a time period of 2 weeks. Between and after the ambient measurements, the different calibration procedures used for the chemical amplifiers were compared. Data delivery was performed after a formal data protocol.

At the beginning of the campaign, relatively large differences in the calibration of the Luminol-chemiluminescence instruments used for detection of the NO2 from the chemical amplifiers were found for some groups. A meaningful comparison of the radical measurements thus required harmonisation of the NO2 standards. This was achieved by tying all NO2 measurements to the NO2 calibration of the TOR station and by adopting a common procedure for taking the non-linear response of the Luminol detector into account. Thereafter, the different chemical amplifiers and MIESR agreed within (30% for measurements of HO2 radicals produced artificially by H2O photolysis. Somewhat larger discrepancies were found for a comparison with CH3O2 radicals produced by photolysis of CH3I and for CH3COO2 radicals produced by thermolysis of PAN.

In ambient air, all instruments found similar diurnal variations in the RO2 concentrations, with maximum concentrations around or shortly after noon time. The correlation was best under situations with high wind speeds and low precursor concentrations. Overall, the chemical amplifiers seemed to have less dynamic range than the MIESR, with the highest RO2 concentrations found by MIESR being underestimated by the chemical amplifiers by up to a factor of 2. Possible explanations are a lower conversion efficiency for large organic peroxy radicals and/or a strong inverse dependence of the chain length on the relative humidity that was found very recently in laboratory experiments and which was not accounted for by the calibration procedures during PRICE. The decrease in chainlength with increasing relative humidity would indeed bring the data from the chemical amplifiers into much better agreement with MIESR. Given the magnitude of the effect and its sensitivity to fluctuations in ambient humidity and temperature, correction of the data in retrospect, however, will be extremely difficult if not impossible.

Since high quality measurements of peroxy radicals can provide deep insight in the degradation mechanism of organic compounds and photo-oxidant formation, a vital interest remains in the further development of the CA, including the conversion efficiencies for the higher organic peroxy radicals and, most important, of the influence of humidity on the chain length.


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