Peroxy radical chemistry and OH radical production during the NO<sub>3</sub>-initiated oxidation of isoprene

Abstract

Abstract. Peroxy radical reactions (RO 2 + RO 2 ) from the NO 3 -initiated oxidation of isoprene are studied with both gas chromatography and a chemical ionization mass spectrometry technique that allows for more specific speciation of products than in previous studies of this system. We find high nitrate yields (∼ 80 %), consistent with other studies. We further see evidence of significant hydroxyl radical (OH) formation in this system, which we propose comes from RO 2 + HO 2 reactions with a yield of ∼ 38-58 %. An additional OH source is the second generation oxidation of the nitrooxyhydroperoxide, which produces OH and a dinitrooxyepoxide with a yield of ∼ 35 %. The branching ratio of the radical propagating, carbonyl-and alcohol-forming, and organic peroxide-forming channels of the RO 2 + RO 2 reaction are found to be ∼ 18-38 %, ∼ 59-77 %, and ∼ 3-4 %, respectively. HO 2 formation in this system is lower than has been previously assumed. Addition of RO 2 to isoprene is suggested as a possible route to the formation of several isoprene C 10 -organic peroxide compounds (ROOR). The nitrooxy, allylic, and C 5 peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.

Overview of the Focused Isoprene eXperiment at the California Institute of Technology (FIXCIT): mechanistic chamber studies on the oxidation of biogenic compounds

“…During nighttime oxidation, tropo-spheric HO 2 mixing ratios often surpass those of NO 3 (Mao et al, 2012), implying HO 2 reaction to be a common fate for NO 3 -derived RO 2 . However, previous studies of this reaction have maintained conditions where minimal HO 2 + RO 2 chemistry occurs and the dominant fate of RO 2 is reaction with NO 3 and RO 2 Perring et al, 2009;Rollins et al, 2009;Kwan et al, 2012). This may be one of the reasons why nitrooxy hydroperoxides (the RO 2 + HO 2 product) are observed with much higher relative abundances in ambient air than in chamber studies.…”

“…These experiments represent the first coupling between direct OH observations from GTHOS, aldehyde/ketone measurements from GCFID and SRI-ToFMS, online formaldehyde measurements from FormLIF, and online hydroperoxide measurements from the various CIMS instruments present to provide the most comprehensive picture thus far on the humidity-dependent ozonolysis of isoprene. Perring et al, 2009;Kwan et al, 2012). The focus of these experiments was the quantification of INP and MTNP with the various CIMS and with TDLIF, and further exploration of their loss channels to OH oxidation (simulating sunrise) or to dry AS seed particles by measuring organic aerosol growth on the ToF-AMS.…”

Observations of total RONO<sub>2</sub> over the boreal forest: NO<sub>x</sub> sinks and HNO<sub>3</sub> sources

“…the alkoxy radical and OH) (Kwan et al, 2012). If we assume that this channel occurs half of the time, which is within the range estimated by Kwan et al (2012), we calculate that the oxidation rate increases by 66 % when we assume the slower isoprene nitrate ozonolysis rate and 26 % when the faster rate is assumed.…”

“…Recent experimental work on the nitrooxy peroxy radicals derived from the reaction of isoprene with NO 3 indicates that the reaction of this peroxy radical with HO 2 likely has a large flux through the channel forming radical products (i.e. the alkoxy radical and OH) (Kwan et al, 2012). If we assume that this channel occurs half of the time, which is within the range estimated by Kwan et al (2012), we calculate that the oxidation rate increases by 66 % when we assume the slower isoprene nitrate ozonolysis rate and 26 % when the faster rate is assumed.…”

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.…”