The Importance of Peroxy Radical Hydrogen-Shift Reactions in Atmospheric Isoprene Oxidation

Abstract

With an annual emission of about 500 Tg, isoprene is an important molecule in the atmosphere. While much of its chemistry is well constrained by either experiment or theory, the rates of many of the unimolecular peroxy radical hydrogen-shift (H-shift) reactions remain speculative. Using a high-level multiconformer transition state theory (MC-TST) approach, we determine recommended temperature dependent reaction rate coefficients for a number of the H-shift reactions in the isoprene oxidation mechanism. We find that most of the (1,4, 1,5, and 1,6) aldehydic and (1,5 and 1,6) α-hydroxy H-shifts have rate constants at 298.15 K in the range 10 −2 to 1 s −1 , which make them competitive with bimolecular reactions in the atmosphere under typical atmospheric conditions. In addition, we find that the rate coefficients of different diastereomers can differ by up to 3 orders of magnitude, illustrating the importance of chirality. Implementation of our calculated reaction rate coefficients into the most recent GEOS-Chem model for isoprene oxidation shows that at least 30% of all isoprene molecules emitted to the atmosphere undergo a minimum of one peroxy radical hydrogen-shift reaction during their complete oxidation to CO 2 and deposited species. This highlights the importance of peroxy radical H-shifts reactions in atmospheric oxidation.

Importance of isomerization reactions for OH radical regeneration from the photo-oxidation of isoprene investigated in the atmospheric simulation chamber SAPHIR

“…A more recent experimental study (Teng et al, 2017) suggests a total yield of HPALD of 0.4, distinguishing between β-(0.25) and δ-HPALD (0.15) with large uncertainties in the assignment of the latter; the remainder, 0.6, is assigned to di-HPCARP-RO 2 LIM0 (Peeters and Müller, 2010) and LIM1 (Peeters et al, 2014); both proposed a yield of ∼ 0.5 for HPALD and di-HPCARP-RO 2 . In contrast, a recent experimental study by Berndt et al (2019) sets a lower limit of 0.75, with a recent addition (Müller et al, 2019) to the theoretical work within the LIM1 also rationalizing a much higher yield of HPALD (0.74) than previously reported in LIM1. In addition to the above products, both experimental (Berndt et al, 2019) and theoretical (Müller et al, 2019) studies suggest the formation of an hydroperoxy-epoxy-carbonyl compound (∼ 0.15).…”

“…In contrast, a recent experimental study by Berndt et al (2019) sets a lower limit of 0.75, with a recent addition (Müller et al, 2019) to the theoretical work within the LIM1 also rationalizing a much higher yield of HPALD (0.74) than previously reported in LIM1. In addition to the above products, both experimental (Berndt et al, 2019) and theoretical (Müller et al, 2019) studies suggest the formation of an hydroperoxy-epoxy-carbonyl compound (∼ 0.15). Both currently available explicit isoprene oxidation mechanisms, i.e.…”

“…istry to the formation of tropospheric pollutant ozone (O 3 ). Because high levels of OH radicals were observed in field experiments in mainly forested environments with large concentrations of isoprene (Tan et al, 2001;Ren et al, 2008;Hofzumahaus et al, 2009;Kubistin et al, 2010;Whalley et al, 2011), a large number of investigations over the last decade focused on OH-initiated isoprene chemistry, including laboratory and chamber studies (Crounse et al, 2011;Berndt, 2012;Wolfe et al, 2012;Fuchs et al, 2013;Teng et al, 2017;Berndt et al, 2019), theoretical calculations (Peeters et al, 2009(Peeters et al, , 2014Da Silva et al, 2010;Peeters and Müller, 2010;Wang et al, 2018;Møller et al, 2019), and global model impact (Lelieveld et al, 2008;Taraborrelli et al, 2012;Bates and Jacob, 2019;Møller et al, 2019;Müller et al, 2019). The observed OH levels could only be explained if an OH radical regeneration mechanism exists independently of NO and thus without the formation of O 3 .…”

A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol

“…Another focus of interest has been the role of isoprene as a sink for NO x (≡ NO + NO 2 ) through organonitrate formation and the subsequent fates of these organonitrates (von Kuhlmann et al, 2004;Horowitz et al, 2007;Wu et al, 2007;Paulot et al, 2012). Differences between models in organonitrate yields and recycling have large effects on simulated ozone Xie et al, 2013;Müller et al, 2014). Recent work has established that hydrolysis and deposition of isoprene-derived organonitrates can be a dominant NO x loss process in some environments Fisher et al, 2016).…”

Supplementary material to "A new model mechanism for atmospheric oxidation of isoprene: global effects on oxidants, nitrogen oxides, organic products, and secondary organic aerosol"

“…• ICN1OO (ISOP1N2OH3OO4CO) and ICN2OO (ISOP1CO2OO3OH4N) are found to undergo rapid H-shifts under almost all ambient conditions (Möller et al, 2019), and are therefore replaced with the H-shift products.…”