A new instrument for time-resolved measurement of HO ₂ radicals

inline-formulaOH and inline-formulaHO₂ radicals are closely coupled in the atmospheric oxidation and combustion of volatile organic compounds (VOCs). Simultaneous measurement of inline-formulaHO₂ yields and inline-formulaOH 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 inline-formulaOH and inline-formulaHO₂ with a high-pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-1st-order loss processes up to inline-formula∼100 000 sinline-formula⁻¹ and to determine inline-formulaHO₂ yields via time-resolved measurements. This time resolution allows discrimination between primary inline-formulaHO₂ from the target reaction and secondary production from side reactions. The apparatus was characterized by measuring yields from the reactions of inline-formulaOH with inline-formulaH₂O₂ (inline-formula1:1 link between inline-formulaOH and inline-formulaHO₂), with inline-formulaC₂H₄∕O₂ (where secondary chemistry can generate inline-formulaHO₂), with inline-formulaC₂H₆∕O₂ (where there should be zero inline-formulaHO₂ yield), and with inline-formulaCH₃OH∕O₂ (where there is a well-defined inline-formulaHO₂ yield).

As an application of the new instrument, the reaction of inline-formulaOH with inline-formulan-butanol has been studied at 293 and 616 K. The bimolecular rate coefficient at 293 K, inline-formula $M26inlinescrollmathml(\mathrm{normal\; 9.24}\pm \mathrm{normal\; 0.21})\times {\mathrm{normal\; 10}}^{-\mathrm{normal\; 12}}$ 101pt15ptsvg-formulamathimgb3b8bd64a88c94a7fd4252c2e8579b28 amt-13-839-2020-ie00001.svg101pt15ptamt-13-839-2020-ie00001.png  cminline-formula³ molec.inline-formula⁻¹ sinline-formula⁻¹, is in good agreement with recent literature, verifying that this instrument can measure accurate OH kinetics. At 616 K the regeneration of inline-formulaOH in the absence of inline-formulaO₂, from the decomposition of the inline-formulaβ-hydroxy radical, was observed, which allowed the determination of the fraction of OH reacting at the inline-formulaβ site (inline-formula0.23±0.04). Direct observation of the inline-formulaHO₂ product in the presence of oxygen has allowed the assignment of the inline-formulaα-branching fractions (inline-formula0.57±0.06) at 293 K and (inline-formula0.54±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.