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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 May 2019

Research article | 10 May 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

A New Instrument for Time Resolved Measurement of HO2 Radicals

Thomas H. Speak1, Mark A. Blitz1,2, Daniel Stone1, and Paul W. Seakins1,2 Thomas H. Speak et al.
  • 1School of Chemistry, University of Leeds, Leeds, LS2 9JT, UK
  • 2National Centre for Atmospheric Science, Leeds, LS2 9JT, UK

Abstract. OH and HO2 radicals are closely coupled in the atmospheric oxidation and combustion of volatile organic compounds (VOCs). Simultaneous measurement of HO2 yields and OH kinetics can provide the ability to assign site specific rate coefficients that are important for understanding the oxidation mechanisms of VOCs. By coupling a FAGE LIF detection system for OH and HO2 with a high pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-first-order loss processes up to ~ 100 000 s−1 and to determine HO2 yields via time resolved measurements. This time resolution allows discrimination between primary HO2 from the target reaction and secondary production from side reactions. The apparatus was characterized by measuring yields from the reactions of OH with H2O2 (1 : 1 link between OH and HO2), with C2H4/O2 (where secondary chemistry can generate HO2), with C2H6/O2 (where there should be zero HO2 yield) and with CH3OH/O2 (where there is a well-defined HO2 yield).

As an application of the new instrument, the reaction of OH with n-butanol has been studied at 293 and 616 K. The bimolecular rate coefficient at 293 K, (9.24 ± 0.21) × 10–12 cm3 molecule−1 s−1, is in good agreement with recent literature, verifying that this instrument can both measure HO2 yields and accurate OH kinetics. At 616 K the regeneration of OH in the absence of O2, from the decomposition of the β-hydroxy radical, was observed, which allowed the determination of the fraction of OH reacting at the β site (0.23 ± 0.04). Direct observation of the HO2 product in the presence of oxygen has allowed the assignment of the α-branching fractions (0.57 ± 0.06) at 293 K and (0.54 ± 0.04) at 616 K); branching ratios are key to modelling the ignition delay times of this potential ‘drop-in’ biofuel.

Thomas H. Speak et al.
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Thomas H. Speak et al.
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Short summary
OH and HO2 radicals are important trace constituents of the atmosphere that are closely coupled via several types of reaction. This paper describes a new laboratory method to simultaneously determine OH kinetics and HO2 yields from chemical processes. The instrument also provides some time resolution on HO2 detection allowing one to separate HO2 produced from the target reaction from HO2 arising from secondary chemistry. Examples of applications are presented.
OH and HO2 radicals are important trace constituents of the atmosphere that are closely coupled...