Estimation of rate coefficients and branching ratios for gas-phase reactions of OH with aliphatic organic compounds for use in automated mechanism construction

Abstract

Abstract. Reaction with the hydroxyl (OH) radical is the dominant removal process for volatile organic compounds (VOCs) in the atmosphere. Rate coefficients for reactions of OH with VOCs are therefore essential parameters for chemical mechanisms used in chemistry transport models, and are required more generally for impact assessments involving the estimation of atmospheric lifetimes or oxidation rates for VOCs. Updated and extended structure–activity relationship (SAR) methods are presented for the reactions of OH with aliphatic organic compounds, with the reactions of aromatic organic compounds considered in a companion paper. The methods are optimized using a preferred set of data including reactions of OH with 489 aliphatic hydrocarbons and oxygenated organic compounds. In each case, the rate coefficient is defined in terms of a summation of partial rate coefficients for H abstraction or OH addition at each relevant site in the given organic compound, so that the attack distribution is defined. The information can therefore guide the representation of the OH reactions in the next generation of explicit detailed chemical mechanisms. Rules governing the representation of the subsequent reactions of the product radicals under tropospheric conditions are also summarized, specifically their reactions with O2 and competing processes.

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

“…Lightfoot et al, 1992;Jenkin and Clemitshaw, 2000;Tyndall et al, 2001;Archibald et al, 2009;Orlando and Tyndall, 2012;Ehn et al, 2017), and rigorous representation of their chemistry is therefore essential for chemical mechanisms used in chemistry-transport models. As discussed in the preceding papers in this series (Jenkin et al, 2018a, b), they are formed rapidly and exclusively from the reactions of O 2 with the majority of carboncentred organic radicals (R) (Reaction R1), these in turn being produced from the reactions that initiate VOC degradation (e.g. reaction with OH radicals), or from other routes such as decomposition of larger oxy radicals (M denotes a third body, most commonly N 2 or O 2 under atmospheric conditions):…”

“…The first entry relates to the β-hydroxy cyclohexadienylperoxy radicals formed from the addition of O 2 to OH-aromatic hydrocarbon adducts. As discussed in the companion paper on the OH-initiated oxidation of aromatic VOCs (Jenkin et al, 2018b), these peroxy radicals are represented to undergo rapid and exclusive ring closure to produce a hydroxy-dioxa-bicyclo or "peroxidebicyclic" radical. This reaction has been calculated to dominate over alternative bimolecular reactions of the peroxy radicals under atmospheric conditions (see Table 14), although evidence for competitive loss via bimolecular reactions has been characterized in experimental studies using high concentrations of NO and/or RO 2 (e.g.…”

“…The rate coefficients assigned to the 1,4-hydroxyl Hshift reactions of (thermalized) α-hydroxy peroxy radicals are based on those estimated for secondary, tertiary and cyclic peroxy radicals in the theoretical study of Hermans et al (2005). As discussed in the companion paper on the OHinitiated oxidation of aliphatic VOCs (Jenkin et al, 2018a), thermalized α-hydroxy peroxy radicals are represented to be increasingly formed from the reactions of O 2 with larger αhydroxy organic radicals (i.e. those with n CON > 5).…”

“…At the assigned rates, the 1,4-hydroxyl H-shift reaction is likely to be the major fate of the majority of thermalized α-hydroxy peroxy radicals under atmospheric conditions, and therefore indistinguishable from that of the chemically activated αhydroxy peroxy radical adducts that are formed predominantly from the reactions of O 2 with small α-hydroxy organic radicals (see Sect. 6.2 of Jenkin et al, 2018a). However, the rates of the 1,4-hydroxyl H-shift reactions are formalized in the present work to allow for the representation of competing rapid isomerization reactions for specific structurally complex peroxy radicals (e.g.…”

“…Similarly, the rate coefficient for the rapid 1,6-enol H-shift reaction is the geometric mean of those calculated for (Z)-HOCH=C(CH 3 )CH(O 2 )CH 2 OH and (Z)-HOCH=CHC(CH 3 )(O 2 )CH 2 OH by Peeters and Olivella et al (2009) for the dominant conformer of the example peroxy radical, formed during the oxidation of benzene. Based on these data, and data for other aromatic systems, analogous ring-closure reactions are assumed to be the exclusive fates of corresponding peroxy radicals formed during the oxidation of aromatic hydrocarbons (Jenkin et al, 2018b). c Denotes substitution of product radical.…”