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The HO2 radical was monitored simultaneously using two independenttechniques in the Leeds HIRAC (HighlyInstrumented Reactor for Atmospheric Chemistry) atmospheric simulation chamber at roomtemperature and total pressures of 150 and 1000 mbar of synthetic air.In the first method, HO2 was measured indirectly following samplingthrough a pinhole expansion to 3 mbar when sampling from 1000 mbar and to 1 mbarwhen sampling from 150 mbar. Subsequent addition of NO converted it toOH, which was detected via laser-induced fluorescence spectroscopy using theFAGE (fluorescence assay by gas expansion) technique. The FAGE method is usedwidely to measure HO2 concentrations in the field and was calibratedusing the 185 nm photolysis of water vapour in synthetic air with a limit ofdetection at 1000 mbar of 1.6 × 106 molecule cm−3 for anaveraging time of 30 s. In the second method, HO2 was measured directlyand absolutely without the need for calibration using cavity ring-downspectroscopy (CRDS), with the optical path across the entire ∼ 1.4 m width of the chamber, with excitation of the first O-H overtone at1506.43 nm using a diode laser and with a sensitivity determined fromAllan deviation plots of 3.0 × 108 and 1.5 × 109 molecule cm−3 at 150 and 1000 mbar respectively, for an averagingperiod of 30 s. HO2 was generated in HIRAC by the photolysis of Cl2using black lamps in the presence of methanol in synthetic air and wasmonitored by FAGE and CRDS for ∼ 5–10 min periods with thelamps on and also during the HO2 decay after the lamps were switchedoff. At 1000 mbar total pressure the correlation plot of [HO2]FAGEversus [HO2]CRDS gave an average gradient of 0.84 ± 0.08 forHO2 concentrations in the range ∼ 4–100 × 109 molecule cm−3, while at 150 mbar total pressure thecorresponding gradient was 0.90 ± 0.12 on average for HO2concentrations in the range ∼ 6–750  ×  108 molecule cm−3.For the period after the lamps were switched off, thesecond-order decay of the HO2 FAGE signal via its self-reaction was usedto calculate the FAGE calibration constant for both 150 and 1000 mbar totalpressure. This enabled a calibration of the FAGE method at 150 mbar, anindependent measurement of the FAGE calibration at 1000 mbar and anindependent determination of the HO2 cross section at 1506.43 nm,σHO2, at both pressures. For CRDS, the HO2 concentrationobtained using σHO2, determined using previous reported spectraldata for HO2, and the kinetic decay of HO2 method agreed to within20 and 12 % at 150 and 1000 mbar respectively. For the FAGE method a verygood agreement (difference within 8 %) has been obtained at 1000 mbarbetween the water vapour calibration method and the kinetic decay of theHO2 fluorescence signal method. This is the first intercomparison ofHO2 between the FAGE and CRDS methods, and the good agreement betweenHO2 concentrations measured using the indirect FAGE method and thedirect CRDS method provides validation for the FAGE method, which is usedwidely for field measurements of HO2 in the atmosphere

atmospheric measurement techniques

An intercomparison of HO&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)