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