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

Abstract

The C 5 -hydroperoxyenals (C 5 -HPALDs) are a newly-recognized class of multi-functional hydrocarbons produced during the hydroxyl radical (OH)-initiated oxidation of isoprene. Recent theoretical calculations suggest that fast photolysis of these compounds may be an important OH source in high-isoprene, low-NO regions. We report experimental constraints for key parameters of photolysis, OH reaction and ozone reaction of these compounds as derived from a closelyrelated, custom-synthesized C 6 -HPALD. The photolysis quantum yield is 1.0 AE 0.4 over the range 300-400 nm, assuming an absorption cross section equal to the average of those measured for several analogous enals. The yield of OH from photolysis was determined as 1.0 AE 0.8. The OH reaction rate constant is (5.1 AE 1.8) Â 10 À11 cm 3 molecule À1 s À1 at 296 K. The ozone reaction rate constant is (1.2 AE 0.2) Â 10 À18 cm 3 molecule À1 s À1 at 296 K. These results are consistent with previous first-principles estimates, though the nature and fate of secondary oxidation products remains uncertain. Incorporation of C 5 -HPALD chemistry with the above parameters in a 0-D box model, along with experimentally-constrained rates for C 5 -HPALD production from isomerization of first-generation isoprene hydroxyperoxy radicals, is found to enhance modeled OH concentrations by 5-16% relative to the traditional isoprene oxidation mechanism for the chemical regimes of recent observational studies in rural and remote regions. This enhancement in OH will increase if C 5 -HPALD photo-oxidation products also photolyze to yield additional OH or if the C 5 -HPALD production rate is faster than has been observed.

Evaluation of HO<sub>x</sub> sources and cycling using measurement-constrained model calculations in a 2-methyl-3-butene-2-ol (MBO) and monoterpene (MT) dominated ecosystem

“…Unspeciated monoterpenes were treated similar to the unspeciated sesquiterpenes described in Wolfe and Thornton (2011) but with rate constants taken from the β-pinene mechanism. Sensitivity tests indicate that newly-proposed isoprene peroxy radical isomerization chemistry (Crounse et al, 2011;Peeters and Müller, 2010;Peeters et al, 2009;Wolfe et al, 2012) influences modeled HO x concentrations by less than 10 % for the conditions of this study. Given the comprehensive suite of constraints, emission and deposition were not considered explicitly, but an additional lifetime of 24 hours was given to all species to avoid the buildup of long-lived products.…”

Impact of isoprene and HONO chemistry on ozone and OVOC formation in a semirural South Korean forest

“…This is an interesting evolution of thoughts considering that Paulson and Seinfeld (1992), one of the pioneering works describing isoprene oxidation, clearly claimed that 22 % of first-generation isoprene oxidation products from the reaction with OH was not identified and likely included multifunctional C5 compounds. Recent advances in analytical techniques (Kim et al, 2013a) have shown that indeed significant C5-hydroxy-carbonyl (e.g., isoprene hydroperoxyaldehydes, HPALD) and peroxide compounds are produced as first-generation isoprene oxidation products (Crounse et al, 2011;Paulot et al, 2009;Wolfe et al, 2012;Zhao and Zhang, 2004). The product yields appeared to be a strong function of NO concentrations (Peeters and Muller, 2010).…”

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

“…A number of mechanisms have been postulated to explain these higher-than-expected observed OH concentrations under low NO x conditions, including the formation and subsequent photolysis of hydroperoxy-aldehyde (HPALD) species (Peeters and Muller, 2010;Peeters et al, 2009;Taraborrelli et al, 2012;Wolfe et al, 2012) and epoxide formation (Paulot et al, 2009). The OH yield from Reaction (R4) for substituted RO 2 radicals has been put forward as a potential explanation for the shortfall in the [OH] prediction under these conditions (Taraborrelli et al, 2009(Taraborrelli et al, , 2012Lelieveld et al, 2008), although at best it merely conserves total HO x concentrations.…”