# An intercomparison of CH 3O 2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)

Simultaneous measurements of inline-formulaCH3O2 radical concentrations have been performed using two different methods in the Leeds HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) chamber at 295 K and in 80 mbar of a mixture of inline-formula3:1inline-formulaHe∕O2 and 100 or 1000 mbar of synthetic air. The first detection method consisted of the indirect detection of inline-formulaCH3O2 using the conversion of inline-formulaCH3O2 into inline-formulaCH3O by excess NO with subsequent detection of inline-formulaCH3O by fluorescence assay by gas expansion (FAGE). The FAGE instrument was calibrated for inline-formulaCH3O2 in two ways. In the first method, a known concentration of inline-formulaCH3O2 was generated using the 185 nm photolysis of water vapour in synthetic air at atmospheric pressure followed by the conversion of the generated OH radicals to inline-formulaCH3O2 by reaction with inline-formulaCH4∕O2. This calibration can be used for experiments performed in HIRAC at 1000 mbar in air. In the second method, calibration was achieved by generating a near steady state of inline-formulaCH3O2 and then switching off the photolysis lamps within HIRAC and monitoring the subsequent decay of inline-formulaCH3O2, which was controlled via its self-reaction, and analysing the decay using second-order kinetics. This calibration could be used for experiments performed at all pressures. In the second detection method, inline-formulaCH3O2 was measured directly using cavity ring-down spectroscopy (CRDS) using the absorption at 7487.98 cminline-formula−1 in the inline-formulaAX (inline-formulaν12) band with the optical path along the inline-formula∼1.4 m chamber diameter. Analysis of the second-order kinetic decays of inline-formulaCH3O2 by self-reaction monitored by CRDS has been used for the determination of the inline-formulaCH3O2 absorption cross section at 7487.98 cminline-formula−1, both at 100 mbar of air and at 80 mbar of a inline-formula3:1inline-formulaHe∕O2 mixture, from which inline-formula $M26inlinescrollmathml{\mathrm{italic \sigma }}_{chem{\mathrm{normal CH}}_{normal 3}{\mathrm{normal O}}_{normal 2}}=\left(normal 1.49±normal 0.19\right)×{normal 10}^{-normal 20}$ 146pt18ptsvg-formulamathimga0de3b239060922dec1332333490d350 amt-13-2441-2020-ie00001.svg146pt18ptamt-13-2441-2020-ie00001.png  cminline-formula2 moleculeinline-formula−1 was determined for both pressures. The absorption spectrum of inline-formulaCH3O2 between 7486 and 7491 cminline-formula−1 did not change shape when the total pressure was increased to 1000 mbar, from which we determined that inline-formula $M31inlinescrollmathml{\mathrm{italic \sigma }}_{chem{\mathrm{normal CH}}_{normal 3}{\mathrm{normal O}}_{normal 2}}$ 34pt12ptsvg-formulamathimg91f987b38bd7263d5c8018a9bd5e863c amt-13-2441-2020-ie00002.svg34pt12ptamt-13-2441-2020-ie00002.png is independent of pressure over the pressure range 100–1000 mbar in air. inline-formulaCH3O2 was generated in HIRAC using either the photolysis of inline-formulaCl2 with UV black lamps in the presence of inline-formulaCH4 and inline-formulaO2 or the photolysis of acetone at 254 nm in the presence of inline-formulaO2. At 1000 mbar of synthetic air the correlation plot of [inline-formulaCH3O2]inline-formulaFAGE against [inline-formulaCH3O2]inline-formulaCRDS gave a gradient of inline-formula1.09±0.06. At 100 mbar of synthetic air the FAGE–CRDS correlation plot had a gradient of inline-formula0.95±0.024, and at 80 mbar of inline-formula3:1inline-formulaHe∕O2 mixture the correlation plot gradient was inline-formula1.03±0.05. These results provide a validation of the FAGE method to determine concentrations of inline-formulaCH3O2.

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Onel, Lavinia / Brennan, Alexander / Gianella, Michele / et al: An intercomparison of CH3O2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry). 2020. Copernicus Publications.

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