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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 unique information about the chemical processing in an air mass. This paper describes the experimental features and capabilities of the Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). This is an instrument designed to make measurements on aircraft from the boundary layer to the lower stratosphere. PeRCEAS combines the amplified conversion of peroxy radicals to nitrogen dioxide (NO2) with the sensitive detection of NO2 using cavity ring-down spectroscopy (CRDS) at 408 nm. PeRCEAS is a dual-channel instrument, with two identical reactor–detector lines working out of phase with one another at a constant and defined pressure lower than ambient at the aircraft altitude. The suitability of PeRCEAS for airborne measurements in the free troposphere was evaluated by extensive characterisation and calibration under atmospherically representative conditions in the laboratory. The use of alternating modes of the two instrumental channels successfully captures short-term variations in the sum of peroxy radicals, defined as RO2∗ (RO2∗=HO2+∑RO2+OH+∑RO , with R being an organic chain) in ambient air. For a 60 s measurement, the RO2∗ detection limit is < 2 pptv for a minimum (2σ) NO2 detectable mixing ratio < 60 pptv, under laboratory conditions in the range of atmospheric pressures and temperatures expected in the free troposphere. PeRCEAS has been successfully deployed within the OMO (Oxidation Mechanism Observations) and EMeRGe (Effect of Megacities on the transport and transformation of pollutants on the Regional and Global scales) missions in different airborne campaigns aboard the High Altitude LOng range research aircraft (HALO) for the study of the composition of the free troposphere. [ABSTRACT FROM AUTHOR]

Hydroperoxyl, HO2, and organic peroxy, RO2, radicals are reactive short lived species, which play a key role in tropospheric chemistry. Measurements of the sum of HO2 and RO2, known as RO2*, provide unique information about the chemical processing in an air mass. This paper describes the experimental features and capabilities of Peroxy Radical Chemical Enhancement and Absorption Spectrometer (PeRCEAS). This is an instrument designed for use on aircraft from the boundary layer to the lower stratosphere. PeRCEAS combines the amplified conversion of peroxy radicals to nitrogen dioxide, NO2, with the sensitive detection of NO2 using absorption spectroscopy (CRD) at 408 nm. PeRCEAS is a dual channel instrument, with two identical reactor-detector lines working out of phase with one another at a constant and defined pressure lower than ambient. The suitability of PeRCEAS for the airborne measurement in the free troposphere was evaluated by extensive characterisation and calibration under atmospherically representative conditions in the laboratory. The use of the alternating modes of the two instrumental channels captures successfully short term variations in the composition of RO2* in ambient air. For 60s measurement, the RO2* detection limit for a minimum (2σ) NO2 detectable mixing ratio < 60 pptv, is < 2 pptv under controlled conditions in the range of atmospheric pressures and temperature gradients expected in the free troposphere. PeRCEAS has been successfully deployed in different airborne campaigns onboard the High Altitude LOng range research aircraft (HALO) for the study of the composition of the free troposphere within the missions OMO in 2015 and EMeRGe in 2017 and 2018. [ABSTRACT FROM AUTHOR]

Observations of tropospheric peroxy radicals are a key point for interpretation of theprocessing and transformation of polluted outflows from major populated centers (MPCs). Aseries of European and Asian MPCs are investigated by the project EMeRGe (Effect ofMegacities on the transport and transformation of pollutants on the Regional and Globalscales). With this objective two airborne campaigns using the research platform HALO (HighAltitude and LOng Range aircraft) were carried out over Europe in summer 2017 and in EastAsia in spring 2018. The Institute of Environmental Physics (IUP) in Bremen participated in both EMeRGecampaigns with the airborne measurement of the total sum of peroxy radicals, RO2∗=(HO2 + ∑ RO2), by using the home made PeRCEAS instrument based on the combinationof the PERCA (peroxy radical chemical amplification) and CRDS (cavity ring downspectroscopy) techniques. One of the main purposes of the campaigns was the investigationof MPC outflow at the local and regional scales. Within the EMeRGe targeting MPCs inEurope and Asia, Rome and Manila are selected in this presentation for havingsimilar meteorology and photochemical conditions during the EMeRGe campaigns.Significant RO2∗ were observed ca. 55 pptv around Rome and ca. 75 pptv aroundManila. Peroxy radical mixing ratios observed upwind and downwind Rome and Manila arecompared and interpreted according to the different composition of the air masses sampled. [ABSTRACT FROM AUTHOR]