Unimolecular β-Hydroxyperoxy Radical Decomposition with OH Recycling in the Photochemical Oxidation of Isoprene

Abstract

A novel process in the photochemical oxidation of isoprene that recycles hydroxyl (OH) radicals has been identified using first-principles computational chemistry. Isoprene is the dominant biogenic volatile organic compound (VOC), and its oxidation controls chemistry in the forest boundary layer and is also thought to contribute to cloud formation in marine environments. The mechanism described here involves rapid unimolecular decomposition of the two major peroxy radicals (beta-hydroxyperoxy radicals) produced by OH-initiated isoprene oxidation. Peroxy radicals are well-known as key intermediates in VOC oxidation, but up to now were only thought to be destroyed in bimolecular reactions. The process described here leads to OH recycling with up to around 60% efficiency in environments with low levels of peroxy radicals and NO(x). In forested environments reaction of the beta-hydroxyperoxy radicals with HO2 is expected to dominate, with a small contribution from the mechanism described here. Peroxy radical decomposition will be more important in the unpolluted marine boundary layer, where lower levels of NO and HO2 are encountered.

Detection of HO<sub>2</sub> by laser-induced fluorescence: calibration and interferences from RO<sub>2</sub> radicals

“…The difference of 0.08 can be regarded as an upper limit for potential prompt HO 2 formation (Eq. 11) in the radical source as suggested by Peeters et al (2009) and da Silva et al (2010) within the time between formation and detection of RO 2 radicals (20 ms). This upper limit would be consistent with the limit of the rate constant for the 1,6-H-shift given by Peeters et al (2009), which converts to a lifetime of 120 ms, but does not fit to results obtained by Nehr et al (2011), who investigated prompt HO 2 formation on a time scale of one second.…”

“…This prompt HO 2 formation may be of importance under low NO conditions. Another theoretical study by da Silva et al (2010) suggests a high yield of HO 2 from unimolecular decomposition of the hydroxyalkyl peroxy radical to HO 2 in the absence of NO. No NO is present in the radical source, so that these reaction pathways would change the ratio of RO 2 to HO 2 towards HO 2 (Eq.…”

Peroxy radical isomerization in the oxidation of isoprene

“…Peeters et al 7 and da Silva et al 8 also calculated rates for 1,5-Hshift isomerization/decomposition reactions, where an alkoxy radical is formed as the alcohol hydrogen is abstracted by the peroxy group in the dominant b-hydroxyperoxy radicals formed from isoprene oxidation by OH (1-OH-2-OO and 4-OH-3-OO). The alkoxy radicals are expected to rapidly decompose, forming OH + HCHO + C 4 -carbonyl (MVK or MACR).…”

“…In particular, motivated by the inability to explain measured concentrations of OH in regions with elevated levels of biogenically-derived hydrocarbons, 2-6 investigators have hypothesized that the peroxy radicals of the dominant VOC, isoprene, may not follow the traditional reactive pathways. Most recently, Peeters et al 7 and da Silva et al 8 have estimated from quantum mechanical calculations that intramolecular hydrogen transfer reactions of the isoprene peroxy radicals (R4) may diminish the impact of isoprene oxidation on HO x concentrations.Here, we report product yields in the oxidation of isoprene and its fully deuterated analog by OH in the presence of NO and HO 2 concentrations similar to those encountered in the environment. We report temperature dependent formation rates for unsaturated C 5 -hydroperoxyaldehydes (HPALDs) 7 generated through 1,6-H-shift isomerization of two of the six peroxy radical isomers, Z-1-OH-4-OO (R5) and Z-4-OH-1-OO (R6).…”

Photolysis, OH reactivity and ozone reactivity of a proxy for isoprene-derived hydroperoxyenals (HPALDs)

“…[5][6][7][8][9][10][11][12] Modelmeasurement discrepancies tend to be most severe in high-VOC regions dominated by isoprene (2-methyl-1,3-butadiene), a biogenic hydrocarbon with an annual global emission rate of B500 Tg per year. 13 In attempts to better understand these issues, renewed theoretical [14][15][16][17] and experimental 5,6,9,18,19 work on isoprene oxidation have revealed a rich chemistry previously unknown, including several mechanisms which effectively recycle HO x (= OH + HO 2 + RO 2 ). One such mechanism, originally proposed by Peeters et al, 17 involves the isomerization and decomposition of isoprene hydroxyperoxy radicals (ISO 2 ) to release HO 2 and a new class of products known as C 5 -hydroperoxyenals (C 5 -HPALDs).…”