A technique for the selective measurement of atmospheric HO 2 and RO 2 using peroxy radical chemical amplification coupled to laser-induced fluorescence NO 2 detection (PERCA-LIF) is demonstrated. By pulling the air through a filled pre-inlet advantage can be taken of the higher heterogeneous loss rate of HO 2 relative to CH 3 O 2 . Pre-inlet conditions have been found where ca. 90% of HO 2 was removed whereas the comparable CH 3 O 2 loss was 15%. The dependence of loss rate on humidity and

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... xy radical concentration has been investigated. When using glass beads as the surface for peroxy radical removal, the influence of the relative humidity on the removal efficiency becomes negligible. It may therefore be possible to apply this technique to the measurement of absolute concentrations of solely RO 2 as well as the sum of HO 2 and RO 2 . The practical utility of the PERCA-LIF coupled to a denuder has been demonstrated with atmospheric measurements.