Isoprene Peroxy Radical Dynamics

Abstract

Approximately 500 Tg of 2-methyl-1,3-butadiene (isoprene) is emitted by deciduous trees each year. Isoprene oxidation in the atmosphere is initiated primarily by addition of hydroxyl radicals (OH) to C₄ or C₁ in a ratio 0.57 ± 0.03 (1σ) to produce two sets of distinct allylic radicals. Oxygen (O₂) adds to these allylic radicals either δ (Z or E depending on whether the allylic radical is cis or trans) or β to the OH group forming six distinct peroxy radical isomers. Due to the enhanced stability of the allylic radical, however, these peroxy radicals lose O₂ in competition with bimolecular reactions. In addition, the Z-δ hydroxy peroxy radical isomers undergo unimolecular 1,6 H-shift isomerization. Here, we use isomer-resolved measurements of the reaction products of the peroxy radicals to diagnose this complex chemistry. We find that the ratio of δ to β hydroxy peroxy radicals depends on their bimolecular lifetime (τ_bimolecular). At τ_bimolecular ≈ 0.1 s, a transition occurs from a kinetically to a largely thermodynamically controlled distribution at 297 K. Thus, in nature, where τ_bimolecular > 10 s, the distribution of isoprene hydroxy peroxy radicals will be controlled primarily by the difference in the relative stability of the peroxy radical isomers. In this regime, β hydroxy peroxy radical isomers comprise ∼95% of the radical pool, a much higher fraction than in the nascent (kinetic) distribution. Intramolecular 1,6 H-shift isomerization of the Z-δ hydroxy peroxy radical isomers produced from OH addition to C₄ is estimated to be ∼4 s^-1 at 297 K. While the Z-δ isomer is initially produced in low yield, it is continually reformed via decomposition of the β hydroxy peroxy radicals. As a result, unimolecular chemistry from this isomer contributes about half of the atmospheric fate of the entire pool of peroxy radicals formed via addition of OH at C₄ for typical atmospheric conditions (τ_bimolecular = 100 s and T = 25 C). In contrast, unimolecular chemistry following OH addition at C₁ is slower and less important.