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. Simultaneous measurements of CH3O2 radical concentrations havebeen performed using two different methods in the Leeds HIRAC (HighlyInstrumented Reactor for Atmospheric Chemistry) chamber at 295 K and in80 mbar of a mixture of 3:1 He∕O2 and 100 or 1000 mbar of syntheticair. The first detection method consisted of the indirect detection ofCH3O2 using the conversion of CH3O2into CH3O byexcess NO with subsequent detection of CH3O by fluorescence assay bygas expansion (FAGE). The FAGE instrument was calibrated for CH3O2in two ways. In the first method, a known concentration of CH3O2was generated using the 185 nm photolysis of water vapour in synthetic airat atmospheric pressure followed by the conversion of the generated OHradicals to CH3O2 by reaction with CH4∕O2. Thiscalibration can be used for experiments performed in HIRAC at 1000 mbar inair. In the second method, calibration was achieved by generating a nearsteady state of CH3O2 and then switching off the photolysis lampswithin HIRAC and monitoring the subsequent decay of CH3O2, whichwas controlled via its self-reaction, and analysing the decay using second-order kinetics. This calibration could be used for experiments performed atall pressures. In the second detection method, CH3O2 wasmeasured directly using cavity ring-down spectroscopy (CRDS) using theabsorption at 7487.98 cm−1 in the A←X (ν12) bandwith the optical path along the ∼1.4 m chamber diameter.Analysis of the second-order kinetic decays of CH3O2 byself-reaction monitored by CRDS has been used for the determination of theCH3O2 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 spectrumof CH3O2 between 7486 and 7491 cm−1 did not change shape whenthe total pressure was increased to 1000 mbar, from which we determined thatσCH3O2 is independent of pressure over the pressure range100–1000 mbar in air. CH3O2 was generated in HIRAC using eitherthe 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 agradient of 1.09±0.06. At 100 mbar of synthetic air the FAGE–CRDS correlation plot had a gradient of 0.95±0.024, and at 80 mbar of3:1 He∕O2 mixture the correlation plot gradient was 1.03±0.05.These results provide a validation of the FAGE method to determineconcentrations of CH3O2.

atmospheric measurement techniques

An intercomparison of CH&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)