First oxidation products from the reaction of hydroxyl radicals with isoprene for pristine environmental conditions

Abstract

Isoprene, C 5 H 8 , inserts about half of the non-methane carbon flux of biogenic origin into the atmosphere. Its degradation is primarily initiated by the reaction with hydroxyl radicals. Here we show experimentally the formation of reactive intermediates and corresponding closedshell products from the reaction of hydroxyl radicals with isoprene for low nitric oxide and low hydroperoxy radical conditions. Detailed product analysis is achieved by mass spectrometric techniques. Quantum chemical calculations support the usefulness of applied ionization schemes. Observed peroxy radicals are the isomeric HO-C 5 H 8 O 2 radicals and their isomerization products HO-C 5 H 8 (O 2)O 2 , bearing most likely an additional hydroperoxy group, and in traces HO-C 5 H 8 (O 2) 2 O 2 with two hydroperoxy groups. Main closed-shell products from unimolecular peroxy radical reactions are hydroperoxy aldehydes, C 5 H 8 O 3 , and smaller yield products with the composition C 5 H 8 O 4 and C 4 H 8 O 5. Detected signals of C 10 H 18 O 4 , C 10 H 18 O 6 , and C 5 H 10 O 2 stand for products arising from peroxy radical self-and cross-reactions.

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.…”

“…-M3 is identical to M2 but using a relative yield of HPALD/di-HPCARP-RO 2 of 0.75 : 0.25 adapted from Berndt et al (2019) (Tables 2 and S5).…”

“…One of the predicted (Peeters et al, 2009(Peeters et al, , 2014 and measured (Crounse et al, 2011;Berndt, 2012;Teng et al, 2017;Berndt et al, 2019) products following the 1,6-H shift is HPALD, which can photolyse, producing OH radicals (Peeters and Müller, 2010;Wolfe et al, 2012;Liu et al, Figure 1. Schematic of the equilibrium reactions between OH-isoprene adducts and isoprene-RO 2 conformers, as well as their formation reactions.…”

“…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 .…”