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

George, Midhun; Andrés Hernández, Maria Dolores; Nenakhov, Vladyslav; Liu, Yangzhuoran; Burrows, John Philip

Hydroperoxyl (inline-formulaHO2) and organic peroxy (inline-formulaRO2) radicals have an unpaired spin and are highly reactive free radicals. Measurements of the sum of inline-formulaHO2 and inline-formulaRO2 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 (inline-formulaNO2) with the sensitive detection of inline-formulaNO2 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 inline-formula M7inlinescrollmathml chem normal RO normal 2 23pt14ptsvg-formulamathimg9badf6148d55a8938c35eb9f925ecff6 amt-13-2577-2020-ie00001.svg23pt14ptamt-13-2577-2020-ie00001.png (inline-formula M8inlinescrollmathml chem normal RO normal 2 = normal HO normal 2 + normal RO normal 2 + normal OH + normal RO 162pt14ptsvg-formulamathimg32944cee136d8e757fc232627d5be475 amt-13-2577-2020-ie00002.svg162pt14ptamt-13-2577-2020-ie00002.png , with R being an organic chain) in ambient air. For a 60 s measurement, the inline-formula M9inlinescrollmathml chem normal RO normal 2 23pt14ptsvg-formulamathimg5ad9b05d81fe95b177981272e8e72d4f amt-13-2577-2020-ie00003.svg23pt14ptamt-13-2577-2020-ie00003.png detection limit is < 2 pptv for a minimum (inline-formula2σ) inline-formulaNO2 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.

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George, Midhun / Andrés Hernández, Maria Dolores / Nenakhov, Vladyslav / et al: Airborne measurement of peroxy radicals using chemical amplification coupled with cavity ring-down spectroscopy: the PeRCEAS instrument. 2020. Copernicus Publications.

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