Abstract
Abstract. The HO2 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, HO2 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 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 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 Allan deviation plots of 3.0 × 108 and 1.5 × 109 molecule cm−3 at 150 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 min 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 an average gradient of 0.84 ± 0.08 for HO2 concentrations in the range ∼ 4–100 × 109 molecule cm−3, while at 150 mbar total pressure the corresponding gradient was 0.90 ± 0.12 on average 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 of HO2 between the FAGE and CRDS methods, and the good agreement between HO2 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 HO2 in the atmosphere.
Highlights
The hydroperoxy radical, HO2, plays a central role in the chemistry of the atmosphere (Levy, 1971) and is a significant reactive species in several media such as hydrocarbon combustion (Zador et al, 2011; Blocquet et al, 2013; Djehiche et al, 2014), atmospheric pressure plasmas (Gianella et al, 2016) and dielectric barrier discharges used to remove volatile organic compounds (VOCs) in air (Blin-Simiand et al, 2016)
The σHO2, 150 mbar from the kinetics studies (1.02 ± 0.18) × 10−19 cm2 molecule−1 (Sect. 3.3.1) is within the error ranges of the values derived from two previous studies: (i) σHO2, 150 mbar = (1.25 ± 0.19) × 10−19 cm2 molecule−1 computed using the 50 mbar He measurements of Thiebaud et al (2007), where the line centres and strengths of the HO2 transitions at ∼ 1506.43 nm were extracted from the reported spectra and combined with the pressure broadening in air (Ibrahim et al, 2007) (Sect. 3.2) to give σHO2 at 150 mbar, and (ii) σHO2, 150 mbar = (1.29 ± 0.23) × 10−19 cm2 molecule−1 obtained by extrapolating the cross sections determined by Tang et al (2010) in the range of 27–133 mbar
The fluorescence assay by gas expansion (FAGE) technique is the most commonly used method for the measurement of HO2 in the atmosphere by conversion of HO2 to OH by reaction with added NO followed by OH on-resonance Laser-induced fluorescence (LIF) at 308 nm
Summary
The hydroperoxy radical, HO2, plays a central role in the chemistry of the atmosphere (Levy, 1971) and is a significant reactive species in several media such as hydrocarbon combustion (Zador et al, 2011; Blocquet et al, 2013; Djehiche et al, 2014), atmospheric pressure plasmas (Gianella et al, 2016) and dielectric barrier discharges used to remove volatile organic compounds (VOCs) in air (Blin-Simiand et al, 2016). HO2 is generated directly by the reaction of a OH radical with CO in the presence of O2 (Reaction R1) and indirectly by the oxidation of larger VOCs (Reactions R2–R4) (Lu et al, 2012). With a typical lifetime in clean air of ∼ l–2 min, HO2 radicals participate in rapid chemical cycling at the heart of tropospheric oxidation; HO2 is one of the best targets with which to compare chemical models with field data. Its self-reaction and reaction with RO2 are the major loss pathways for HO2 via Reactions (R5)–(R7): HO2 + HO2 → H2O2 + O2
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