Isomeric Product Detection in the Heterogeneous Reaction of Hydroxyl Radicals with Aerosol Composed of Branched and Linear Unsaturated Organic Molecules

Abstract

The influence of molecular structure (branched vs linear) on product formation in the heterogeneous oxidation of unsaturated organic aerosol is investigated. Particle phase product isomers formed from the reaction of squalene (C30H50, a branched alkene with six C═C double bonds) and linolenic acid (C18H30O2, a linear carboxylic acid with three C═C double bonds) with OH radicals are identified and quantified using two-dimensional gas chromatography-mass spectrometry. The reactions are measured at low and high [O2] (∼1% vs 10% [O2]) to understand the roles of hydroxyalkyl and hydroxyperoxy radical intermediates in product formation. A key reaction step is OH addition to a C═C double bond to form a hydroxyalkyl radical. In addition, allylic alkyl radicals, formed from H atom abstraction reactions by hydroxyalkyl or OH radicals play important roles in the chemistry of product formation. Functionalization products dominate the squalene reaction at ∼1% [O2], with the total abundance of observed functionalization products being approximately equal to the fragmentation products at 10% [O2]. The large abundance of squalene fragmentation products at 10% [O2] is attributed to the formation and dissociation of tertiary hydroxyalkoxy radical intermediates. For linolenic acid aerosol, the formation of functionalization products dominates the reaction at both ∼1% and 10% [O2], suggesting that the formation and dissociation of secondary hydroxyalkoxy radicals are minor reaction channels for linear molecules. The distribution of linolenic acid functionalization products depends upon [O2], indicating that O2 controls the reaction pathways of the secondary hydroxyalkyl radical. For both reactions, alcohols are formed in favor of carbonyl functional groups, suggesting that there are some key differences between heterogeneous reactions involving allylic radical intermediates and those reactions of OH radicals with simple saturated hydrocarbons.

Heterogeneous chemistry and reaction dynamics of the atmospheric oxidants, O3, NO3, and OH, on organic surfaces

“…116 Recently, flow-tube studies by Wilson and coworkers have added insight into the role of molecular structure and O 2 concentration on the mechanism of organic aerosol oxidation by OH. 137,138 Using mass-spectrometric data, this group compared the reaction mechanisms for both branched squalene and linear linonenic acid (Fig. 11).…”

“…The authors found that there is an increase in the surface oxygen-to-carbon ratio over the course of a heterogeneous OH reaction that is controlled by the formation of oxygenated functional groups as well as through carbon loss, as described in prior work. 20,116,138 To help explain the increase in oxygen-to-carbon surface ratio with reaction time, Mysak et al…”

Fundamental Heterogeneous Reaction Chemistry Related to Secondary Organic Aerosols (SOA) in the Atmosphere

“…39(c)) have been studied in some detail in flow tube reactor experiments (Nah et al, , 2014a. The effective uptake coefficients of squalene, oleic, linoleic and linolenic acid are found to be larger than unity, 2.92 ± 0.18 (Nah et al, 2014b), 1.72 ± 0.08, 3.75 ± 0.18 and 5.73 ± 0.14 , respectively, which suggests that secondary reactions take place to consume these species after the addition of OH to the C=C double bond.…”

“…These peaks are separated by 16 Da (amu) and shown as Sqe, SqeO, SqeO 2 , and SqeO 3 denoting the number of oxygenated functional groups (alcohols and ketones). A detailed identification of these product species has been made by Nah et al (2014b) who reported that the functionalization products are composed of C 30 H 49 OH, C 30 H 51 OH and C 30 H 50 (OH) 2 isomers. In addition to these functionalization products, fragmentation products with a carbon number less than C 30 are also formed in the reaction of OH and squalene.…”

“…In addition to these functionalization products, fragmentation products with a carbon number less than C 30 are also formed in the reaction of OH and squalene. From the evidence, Nah et al (2014b) proposed a reac- tion mechanism for the OH-initiated oxidation of squalene aerosol as shown in Fig. 41.…”