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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 10, issue 12 | Copyright
Atmos. Meas. Tech., 10, 4877-4894, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Dec 2017

Research article | 14 Dec 2017

An intercomparison of HO2 measurements by fluorescence assay by gas expansion and cavity ring-down spectroscopy within HIRAC (Highly Instrumented Reactor for Atmospheric Chemistry)

Lavinia Onel1, Alexander Brennan1, Michele Gianella2, Grace Ronnie1, Ana Lawry Aguila2, Gus Hancock2, Lisa Whalley1,3, Paul W. Seakins1,3, Grant A. D. Ritchie2, and Dwayne E. Heard1,3 Lavinia Onel et al.
  • 1School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
  • 2Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, Oxford, OX1 3QZ, UK
  • 3National Centre for Atmospheric Science, University of Leeds, Leeds, LS2 9JT, UK

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 1000mbar of synthetic air. In the first method, HO2 was measured indirectly following sampling through a pinhole expansion to 3mbar when sampling from 1000mbar and to 1mbar when sampling from 150mbar. 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 185nm photolysis of water vapour in synthetic air with a limit of detection at 1000mbar of 1.6× 106moleculecm−3 for an averaging time of 30s. 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 ∼ 1.4m width of the chamber, with excitation of the first O-H overtone at 1506.43nm using a diode laser and with a sensitivity determined from Allan deviation plots of 3.0× 108 and 1.5×109moleculecm−3 at 150 and 1000mbar respectively, for an averaging period of 30s. 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 ∼ 5–10min periods with the lamps on and also during the HO2 decay after the lamps were switched off. At 1000mbar 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 ∼4–100×109moleculecm−3, while at 150mbar total pressure the corresponding gradient was 0.90±0.12 on average for HO2 concentrations in the range ∼6–750 × 108moleculecm−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 1000mbar total pressure. This enabled a calibration of the FAGE method at 150mbar, an independent measurement of the FAGE calibration at 1000mbar and an independent determination of the HO2 cross section at 1506.43nm, 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 1000mbar respectively. For the FAGE method a very good agreement (difference within 8%) has been obtained at 1000mbar 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.

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Short summary
Hydroperoxy (HO2) radicals are key intermediates participating in a rapid chemical cycling at the centre of the tropospheric oxidation. Fluorescence assay by gas expansion (FAGE) technique is the most commonly used for the HO2 measurements in the atmosphere. However, FAGE is an indirect technique, requiring calibration. This work finds a good agreement between the indirect FAGE method and the direct cavity ring-down spectroscopy method and hence validates FAGE and the FAGE calibration method.
Hydroperoxy (HO2) radicals are key intermediates participating in a rapid chemical cycling at...