Organic peroxy radicals (RO_2) are an important intermediate in VOC photooxidation andrnozone production. At the high Nox levels of urban air, existing RO_2 detection methods arernsubject to interferences from peroxynitrate dissociation or formation. We have developed arnmethod that is free of such interferences, and distinguishes total RO_2 from HO_2. In this method,rnRO_2 and HO_2 are converted to OH in an atmospheric-pressure flowtube, and the OH is detectedrnby low-pressure FAGE (fluorescence assay with gas expansion). In the converter, variation ofrnthe added NO reagent concentration allows the determination of HO_2 and total RO_2.rnPeroxynitrate interference is limited by the short residence time of the converter. Thernconverter and injector design, obtained by trying many modifications, achieves a satisfactoryrncompromise among the conflicting goals of quasi-uniform residence time, effective reagentrnmixing, and low wall loss of free radicals.rnWe have calibrated the FAGE analyzer for OH, HO_2, and three RO_2 speciesrn(methylperoxyl, ethylperoxyl, and acetylperoxyl). We have made preliminary measurements ofrnHO_2 and total RO_2 in urban air.
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