Simultaneous measurements of CH3O2 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 3 : 1 He : O2 and 100 mbar or 1000 mbar of synthetic air. The first detection method consisted of the indirect detection of CH3O2 using the conversion of CH3O2 into CH3O by excess NO with subsequent detection of CH3O by fluorescence assay by gas expansion (FAGE). The FAGE instrument was calibrated for CH3O2 in two ways. In the first method, a known concentration of CH3O2 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 CH3O2 by reaction with CH4 / 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 CH3O2 and then switching off the photolysis lamps within HIRAC and monitoring the subsequent decay of CH3O2 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, CH3O2 has been measured directly using Cavity Ring-Down Spectroscopy (CRDS) using the absorption at 7487.98 cm-1 in the A <– X (ν12) band with the optical path along the ~1.4 m chamber diameter. Analysis of the second-order kinetic decays of CH3O2 by self-reaction monitored by CRDS has been used for the determination of the CH3O2 absorption cross section at 7487.98 cm-1, both at 100 mbar of air and at 80 mbar of a 3 : 1 He : O2 mixture, from which σCH3O2 = (1.49 ± 0.19) × 10–20 cm2 molecule-1 was determined for both pressures. The absorption spectrum of CH3O2 between 7486 and 7491 cm-1 did not change shape when the total pressure was increased to 1000 mbar, from which we determined that σCH3O2 is independent of pressure over the pressure range 100–1000 mbar in air. CH3O2 was generated in HIRAC using either the photolysis of Cl2 with UV black lamps in the presence of CH4 and O2 or the photolysis of acetone at 254 nm in the presence of O2. At 1000 mbar of synthetic air the correlation plot of [CH3O2]FAGE against [CH3O2]CRDS gave a gradient of 1.10 ± 0.02. At 100 mbar of synthetic air the gradient of the FAGE – CRDS correlation plot had a gradient of 1.06 ± 0.01 and at 80 mbar of 3 : 1 He : O2 mixture the correlation plot gradient was 0.91 ± 0.02. These results provide a validation of the FAGE method to determine concentrations of CH3O2.
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