The HO 2 radical was monitored simultaneously using two independent techniques in the Leeds HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry) atmospheric simulation chamber at room temperature and total pressures of 150 and 1000 mbar of synthetic air. In the first method, HO 2 was measured indirectly following sampling through a pinhole expansion to 3 mbar when sampling from 1000 mbar and to 1 mbar when sampling from 150 mbar. Subsequent addition of NO converted it to OH, which was detected via laser-induced fluorescence spectroscopy using the FAGE (fluorescence assay by gas expansion) technique. The FAGE method is used widely to measure HO 2 concentrations in the field and was calibrated using the 185 nm photolysis of water vapour in synthetic air with a limit of detection at 1000 mbar of 1.6×10 6 molecule cm -3 for an averaging time of 30 s. In the second method, HO 2 was measured directly and absolutely without the need for calibration using cavity ring-down spectroscopy (CRDS), with the optical path across the entire ∼1.4m width of the chamber, with excitation of the first O-H overtone at 1506.43 nm using a diode laser and with a sensitivity determined from Allan deviation plots of 3.0×10 8 and 1.5×10 9 molecule cm -3 at 150 and 1000 mbar respectively, for an averaging period of 30 s. HO 2 was generated in HIRAC by the photolysis of Cl 2 using black lamps in the presence of methanol in synthetic air and was monitored by FAGE and CRDS for ∼5-10 min periods with the lamps on and also during the HO 2 decay after the lamps were switched off. At 1000 mbar total pressure the correlation plot of [HO 2 ]FAGE versus [HO 2 ]CRDS gave an average gradient of 0.84±0.08 for HO 2 concentrations in the range ∼4-100×10 9 molecule cm -3 , while at 150 mbar total pressure the corresponding gradient was 0.90±0.12 on average for HO 2 concentrations in the range ∼6-750×10 8 molecule cm -3 . For the period after the lamps were switched off, the second-order decay of the HO 2 FAGE signal via its selfreaction was used to calculate the FAGE calibration constant for both 150 and 1000 mbar total pressure. This enabled a calibration of the FAGE method at 150 mbar, an independent measurement of the FAGE calibration at 1000 mbar and an independent determination of the HO 2 cross section at 1506.43 nm, σHO 2 , at both pressures. For CRDS, the HO 2 concentration obtained using σHO 2 , determined using previous reported spectral data for HO 2 , and the kinetic decay of HO 2 method agreed to within 20 and 12% at 150 and 1000 mbar respectively. For the FAGE method a very good agreement (difference within 8 %) has been obtained at 1000 mbar between the water vapour calibration method and the kinetic decay of the HO 2 fluorescence signal method. This is the first intercomparison of HO 2 between the FAGE and CRDS methods, and the good agreement between HO 2 concentrations measured using the indirect FAGE method and the direct CRDS method provides validation for the FAGE method, which is used widely for field measurements of HO 2 in the atmosphere.
