Direct measurements of OH and other product yields from the HO<sub>2</sub>  + CH<sub>3</sub>C(O)O<sub>2</sub> reaction

Abstract

Abstract.The reaction CH 3 C(O)O 2 + HO 2 → CH 3 C(O)OOH + O 2 (Reaction R5a), CH 3 C(O)OH + O 3 (Reaction R5b), CH 3 + CO 2 + OH + O 2 (Reaction R5c) was studied in a series of experiments conducted at 1000 mbar and (293 ± 2) K in the HIRAC simulation chamber. For the first time, products, (CH 3 C(O)OOH, CH 3 C(O)OH, O 3 and OH) from all three branching pathways of the reaction have been detected directly and simultaneously. Measurements of radical precursors (CH 3 OH, CH 3 CHO), HO 2 and some secondary products HCHO and HCOOH further constrained the system. Fitting a comprehensive model to the experimental data, obtained over a range of conditions, determined the branching ratios α (R5a) = 0.37 ± 0.10, α (R5b) = 0.12 ± 0.04 and α (R5c) = 0.51 ± 0.12 (errors at 2σ level). Improved measurement/model agreement was achieved using k (R5) = (2.4 ± 0.4) × 10 −11 cm 3 molecule −1 s −1 , which is within the large uncertainty of the current IUPAC and JPL recommended rate coefficients for the title reaction. The rate coefficient and branching ratios are in good agreement with a recent study performed by Groß et al. (2014b); taken together, these two studies show that the rate of OH regeneration through Reaction (R5) is more rapid than previously thought. GEOS-Chem has been used to assess the implications of the revised rate coefficients and branching ratios; the modelling shows an enhancement of up to 5 % in OH concentrations in tropical rainforest areas and increases of up to 10 % at altitudes of 6-8 km above the equator, compared to calculations based on the IUPAC recommended rate coefficient and yield. The enhanced rate of acetylperoxy consumption significantly reduces PAN in remote regions (up to 30 %) with commensurate reductions in background NO x .

Estimation of rate coefficients and branching ratios for reactions of organic peroxy radicals for use in automated mechanism construction

“…As discussed previously (Jenkin et al, 1997;Saunders et al, 2003;Boyd et al, 2003a;Orlando and Tyndall, 2012;Wennberg et al, 2018), the 298 K rate coefficients tend to increase with the size of the organic group. Figure 2 shows the data plotted as a function of n CON .…”

“…Comments: a Based on studies of CH 3 O 2 and C 2 H 5 O 2 (as summarized by Orlando and Tyndall, 2012), and also used as a default in all cases other than those covered by comments b−−i . b Based on studies of CH 3 C(O)O 2 (Niki et al, 1985;Horie and Moortgat, 1992;Hasson et al, 2004;Jenkin et al, 2007;Dillon and Crowley, 2008;Groß et al, 2014;Winiberg et al, 2016;Hui et al, 2019); see Sect. S4.…”

“…Vereecken and Peeters, 2004;Peeters et al, 2009;Crounse et al, 2013;Ehn et al, 2014Ehn et al, , 2017Jokinen et al, 2014;Rissanen et al, 2015;Jørgensen et al, 2016;Praske et al, 2017Praske et al, , 2019Otkjaer et al, 2018;Mohammed et al, 2018), and new information is constantly emerging on this important aspect of peroxy radical chemistry (e.g. Bianchi et al, 2019;Xu et al, 2019;Møller et al, 2019). The present section provides a summary of selected classes of isomerization reactions that are currently being considered and represented in ongoing mechanism development work.…”

“…c Based on a rounded mean of the reported 298 K branching ratios for the following cross-reactions: with loss via bimolecular reactions (or other unimolecular reactions discussed below) under atmospheric conditions. It is noted that Xu et al (2019) have also very recently reported information for a series of isomerization reactions (including ring-closure reactions) for the αand β-pinene systems, which are being considered in ongoing work. Table 15 shows selected hydrogen atom migration reactions that are currently considered.…”

“…The remaining reactions in Table 15 are inferred from information reported for specific unsaturated peroxy radicals formed during the OH-initiated oxidation of isoprene, taking particular account of the work of Peeters et al (2009Peeters et al ( , 2014 on the Leuven isoprene mechanism (LIM1), which has been largely verified by experimental study (e.g. Wennberg et al, 2018; and references therein). The rate coefficients for the 1,5-hydroxyl H-shift reactions are those reported by Peeters et al (2014) for the corresponding unsaturated secondary and tertiary β-hydroxy peroxy radicals formed from the sequential addition of OH and O 2 to isoprene, with these also being generally consistent with those reported by da Silva et al (2010).…”