Peroxy Radical Inter Comparison Exercise: a formal comparison of methods for ambient measurements of peroxy radicals: final report

PRICE was organised as a formal comparison of different methods to measure peroxy radicals (HO$_{2}$ and RO$_{2}$). lt 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 theLuminol-chemiluminescence instruments used for detection of the NO$_{2}$ from the chemical amplifiers were found for some groups. A meaningful comparison of the radical measurements thus required harmonisation of the NO$_{2}$ standards. This was achieved by tying all NO$_{2}$ measurements to the NO$_{2}$ 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 130% for measurements of HO$_{2}$ radicals produced artificially by H$_{2}$O photolysis. Somewhat larger discrepancies were found for a comparison with CH$_{3}$O$_{2}$ radicals produced by photolysis of CH$_{3}$I and for CH$_{3}$COO$_{2}$ radicals produced by thermolysis of PAN. In ambient air, all instruments found similar diurnal variations in the RO$_{2}$ 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 RO$_{2}$ concentrations found by MIESR being underestimated by the chemical amplifiersby 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 mach 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 ean 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 arid, most important, of the infiuence of humidity on the chain length.