Isoprene NO₃ Oxidation Products from the RO₂ + HO₂ Pathway

Abstract

S1.0 KINETIC MECHANISM DEVELOPMENT.A kinetic mechanism is formulated to simulate the reaction conditions of these experiments.The reactions included are listed in the Appendix (Table SA2, SA3, and SA4). Table SA5 contains a list of the abbreviations used. Rate constants for most of the reactions included in the mechanism are based on recommendations from JPL 1 , IUPAC 2-3 , or MCM v3.2 4 . However, some rate constants and branching ratios are not known. For these, we use our best judgement based on available data; explanations of the assumptions on which these estimates are based are included in this section. Some branching ratios and rate constants are estimated based on the experimental results presented here. Many of these branching ratios depend on the fraction of δ-and β-isomers that form (Table 3 and 5), which will likely depend on the lifetime of the RO 2 radical (Section 4.1). Thus, the reaction products and rates presented here are most consistent with the experimental results for this study in which the overall RO 2 lifetime was ~ 30 s. The kinetic mechanism developed here represents our current level of understanding, and deviations from the experimental results highlight areas for future study. S1.1. Basic Reactions in Kinetic Mechanism. HO 2 was constrained in the kinetic mechanism by the measured H 2 O 2 production rate. Prior to photooxidation, H 2 O 2 is predominantly formed from HO 2 + HO 2 reactions. To match the observed H 2 O 2 production rate in experiments 5, 6, and 8, we arbitrarily increased the reaction rate constant for CH 2 O + NO 3 by a factor of 2.5-3 in the kinetic mechanism above that recommended by IUPAC. Although not perfect when correcting for the missing HO 2 in this manner, the H 2 O 2 curves for the kinetic mechanism and the experimental results were fairly consistent. Under-prediction of HO 2 could be caused by other S3 missing chemistry including unaccounted for surface chemistry, later generation chemistry not incorporated into the kinetic mechanism, or many other possibilities. Here, we are confident that there is a missing source of HO 2 , but are agnostic about the mechanism responsible.Because the predominant loss of isoprene is due to reaction with NO 3 , the measured isoprene decay rate was used to constrain the amount of NO 3 present. Cantrell et al. 5 proposed that N 2 O 5would react with water present on the wall surface to form nitric acid even under dry conditions.We included a wall loss rate for N 2 O 5 (i.e., NO 3 loss rate) such that the isoprene decay in the kinetic mechanism matched with experimental results. This rate constant is chamber/experiment specific. For experiment 5 (24 m 3 , 2.2 ppm CH 2 O), 6 (24 m 3 , 4.7 ppm CH 2 O), 7 (1 m 3 , 2 ppm CH 2 O) and 8 (1 m 3 , 4 ppm CH 2 O), N 2 O 5 wall loss rate constants that best fit experimental conditions were 1.5 x 10 -4 , 12 x 10 -4 , 6 x 10 -4 and 12 x 10 -4 s -1 , respectively. We observe that the N 2 O 5 loss rate appears to be sensitive to both the mixing ratio of CH 2 O and the chamber.However, it s...

Decadal changes in summertime reactive oxidized nitrogen and surface ozone over the Southeast United States

“…In particular, MCM v3.2 suggests significant production of propanone nitrate (PROPNN) from the photooxidation of the C 5 carbonyl nitrate, which is consistent with recent laboratory experiments (Schwantes et al, 2015). We also update the products of the reaction of the nitrooxy alkylperoxy radical (INO 2 ), the peroxy radical from isoprene oxidation by NO 3 , with HO 2 to reflect a lower molar yield (0.77) of C 5 nitrooxy hydroperoxide (INPN; Schwantes et al, 2015). The differences between MCM v3.2 and the most updated version, MCM v3.3.1, in isoprene nighttime chemistry appears to be small (Jenkin et al, 2015).…”

“…Instead of following Mao et al (2013b), we revise the nighttime oxidation of isoprene largely based on the Leeds Master Chemical Mechanism v3.2 (MCM v3.2), allowing for a more complete description of isoprene oxidation by NO 3 . In particular, MCM v3.2 suggests significant production of propanone nitrate (PROPNN) from the photooxidation of the C 5 carbonyl nitrate, which is consistent with recent laboratory experiments (Schwantes et al, 2015). We also update the products of the reaction of the nitrooxy alkylperoxy radical (INO 2 ), the peroxy radical from isoprene oxidation by NO 3 , with HO 2 to reflect a lower molar yield (0.77) of C 5 nitrooxy hydroperoxide (INPN; Schwantes et al, 2015).…”

“…A full list of the reactions can be found in Table S1. This mechanism is based on Mao et al (2013b), but has been significantly revised to incorporate recent laboratory updates on isoprene oxidation by OH, ozone, and NO 3 (Schwantes et al, 2015;Bates et al, 2014Bates et al, , 2016St. Clair et al, 2016;Praske et al, 2015;Müller et al, 2014;Lee et al, 2014;Crounse et al, 2011).…”

Secondary organic aerosol from chlorine-initiated oxidation of isoprene

“…Other RO 2 + RO 2 reaction pathways also produce HO x radicals. Evidence of secondary OH radical production has been reported for NO 3 oxidation of isoprene (Kwan et al, 2012;Schwantes et al, 2015), for chlorine-initiated oxidation of methylnaphthalenes and naphthalene under low-NO x conditions Wang et al, 2005), and for chlorineinitiated oxidation of toluene under high NO x (Huang et al, 2012). Figure 6b shows the accumulation of HOCl observed using (H 2 O) n I − CIMS during the SOA growth period, where HO x radicals produced from chlorine-isoprene oxidation could react with excess chlorine.…”

The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

“…The CPOT has been constructed as a complement to the Caltech 24 m 3 batch chambers (Bates et al, , 2016Schilling et al, 2015;Hodas et al, 2015;Loza et al, 2013Loza et al, , 2014McVay et al, 2014McVay et al, , 2016Nguyen et al, 2014Nguyen et al, , 2015Schwantes et al, 2015;Yee et al, 2013;Zhang et al, 2014; in carrying out studies of SOA formation resulting from the oxidation of volatile organic compounds (VOCs) by oxidants OH, O 3 and NO 3 over timescales not accessible in a batch chamber. Due to its steady-state operation, the CPOT also affords the capability to collect sufficient quantities of SOA generated in the reactor for comprehensive composition determination by offline mass spectrometry.…”