An intercomparison of HO₂ measurements by Fluorescence Assay by Gas Expansion and Cavity Ring–Down Spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)

The HO2 radical was monitored simultaneously using two independent techniques in the Leeds HIRAC atmospheric simulation chamber at room temperature and total pressures of 150 mbar and 1000 mbar of synthetic air. In the first method, HO2 was measured indirectly following sampling through a pinhole expansion to 3 mbar when sampling from 1000 mbar and 1 mbar when sampling from 150 mbar, with subsequent addition of NO to convert 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 HO2 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 × 106 molecule cm−3 for an averaging time of 30 s. In the second method, HO2 was measured directly and absolutely without the need for a calibration using Cavity Ring Down Spectroscopy (CRDS) with the optical path across the entire ~ 1.4 m 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 an Allan deviation plot of 3.0 × 108 and 1.5 x 109 molecule cm−3 at 150 mbar and 1000 mbar, respectively, for an averaging period of 30 s. HO2 was generated in HIRAC by the photolysis of Cl2 using black lamps in the presence of methanol in synthetic air and was monitored by FAGE and CRDS for ~ 5–10 minute periods with the lamps on and also during the HO2 decay after the lamps were switched off. At 1000 mbar total pressure the correlation plot of [HO2]FAGE versus [HO2]CRDS gave a gradient of 0.836 ± 0.004 for HO2 concentrations in the range ~ 4–100 × 109 molecule cm−3 while at 150 mbar total pressure the corresponding gradient was 0.903 ± 0.002 for HO2 concentrations in the range ~ 6–750 × 108 molecule cm−3. For the period after the lamps were switched off, the second-order decay of the HO2 FAGE signal via its self-reaction 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 HO2 cross section at 1506.43 nm, σHO2, at both pressures. For CRDS, the HO2 concentration obtained using σHO2 determined using previous reported spectral data for HO2 and the kinetic decay of HO2 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 HO2 fluorescence signal method. This is the first intercomparison for HO2 between FAGE and CRDS methods, and the good agreement between HO2 concentrations measured using the indirect FAGE method and the direct CRDS method provides a validation for the FAGE method, which is used widely for field measurements of HO2 in the atmosphere.