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. Hydroperoxyl ( HO2 ) and organic peroxy ( RO2 ) radicals have an unpaired spin and are highly reactive free radicals. Measurements of the sum of HO2 and RO2 provide uniqueinformation about the chemical processing in an air mass. This paperdescribes the experimental features and capabilities of the Peroxy RadicalChemical Enhancement and Absorption Spectrometer (PeRCEAS). This is aninstrument designed to make measurements on aircraft from the boundary layerto the lower stratosphere. PeRCEAS combines the amplified conversion ofperoxy radicals to nitrogen dioxide ( NO2 ) with the sensitive detectionof NO2 using cavity ring-down spectroscopy (CRDS) at 408 nm.PeRCEAS is a dual-channel instrument, with two identical reactor–detectorlines working out of phase with one another at a constant and definedpressure lower than ambient at the aircraft altitude. The suitability ofPeRCEAS for airborne measurements in the free troposphere was evaluated byextensive characterisation and calibration under atmosphericallyrepresentative conditions in the laboratory. The use of alternating modes ofthe two instrumental channels successfully captures short-term variations inthe sum of peroxy radicals, defined as RO 2 ∗ ( RO 2 ∗ = HO 2 + ∑ RO 2 + OH + ∑ RO , with R being anorganic chain) in ambient air. For a 60 s measurement, the RO 2 ∗ detection limit is 2σ ) NO2 detectable mixing ratio < 60 pptv, under laboratory conditions inthe range of atmospheric pressures and temperatures expected in the freetroposphere. PeRCEAS has been successfully deployed within the OMO(Oxidation Mechanism Observations) and EMeRGe (Effect of Megacities on thetransport and transformation of pollutants on the Regional and Globalscales) missions in different airborne campaigns aboard the High Altitude LOng rangeresearch aircraft (HALO) for the study of the composition of the freetroposphere.

Airborne measurement of peroxy radicals using chemical amplification coupled with cavity ring-down spectroscopy: the PeRCEAS instrument