Abstract
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 fluorescence assay by gaseous expansion (FAGE) laser-induced fluorescence (LIF) detection system for OH and HO2 with a high-pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-1st-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 molec.−1 s−1, is in good agreement with recent literature, verifying that this instrument can measure 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, again in good agreement with recent literature; branching ratios are key to modelling the ignition delay times of this potential “drop-in” biofuel.
Highlights
In the atmosphere, HO2 and OH radicals (OH + HO2 = HOx) are closely coupled via several reactions as shown in Scheme 1
We have looked at the dependence of the HO2 yield from both OH/H2O2 and from OH/CH3OH, but we see no significant effects of secondary radical–radical reaction (< 5%) as the calculated [OH]0 is changed from 5 × 1011 to 5 × 1012 molec. cm−3
As mentioned in the introduction, the instrument has similarities to that presented by Nehr et al (2011), where a fluorescence assay by gaseous expansion (FAGE) system for sequential OH and HO2 is coupled to a lifetime instrument and yields of HO2 from OH-initiated reactions are reported
Summary
The current paper describes a significant development on our earlier FAGE-based instrument for time-resolved OH detection (Stone et al, 2016) Sampling into the low-pressure region reduces both the effect of collisional quenching and temperature on the sensitivity of LIF detection, there is a reduction in the number density of the HOx species in the expansion. We report the adaptation of our time-resolved OH FAGE instrument to allow HO2 detection, the characterization of the instrument (including development of a calibration method for HO2 yields of OH initiated reactions), and the investigation of the influence of RO2 species. The instrument has some similarities to that presented by Nehr et al (2011) where a conventional OH lifetime instrument was altered to allow for chemical conversion of HO2 to OH and the sequential determination of OH and HO2
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