Abstract
Abstract. Peroxy radical reactions (RO2 + RO2) from the NO3-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 RO2 + HO2 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 RO2 + RO2 reaction are found to be ~18–38%, ~59–77%, and ~3–4%, respectively. HO2 formation in this system is lower than has been previously assumed. Addition of RO2 to isoprene is suggested as a possible route to the formation of several isoprene C10-organic peroxide compounds (ROOR). The nitrooxy, allylic, and C5 peroxy radicals present in this system exhibit different behavior than the limited suite of peroxy radicals that have been studied to date.
Highlights
The global emissions of isoprene (440–660 Tg yr−1) (Guenther et al, 2006) are larger than those of any other nonmethane hydrocarbon
Because of its high abundance and reactivity towards atmospheric radicals, isoprene plays a major role in the oxidative chemistry of the troposphere (Chameides et al, 1988; Williams et al, 1997; Roberts et al, 1998; Horowitz et al, 1998; Paulot et al, 2009a) and is an important precursor for secondary organic aerosol (SOA) (Claeys et al, 2004; Kroll et al, 2005, 2006; Surratt et al, 2006, 2010; Carlton et al, 2009)
In a previous study (Ng et al, 2008), we show that the SOA yield from the reaction of isoprene with NO3 radicals is higher when experimental conditions favor RO2 + RO2 reactions over RO2 + NO3 reactions
Summary
The global emissions of isoprene (440–660 Tg yr−1) (Guenther et al, 2006) are larger than those of any other nonmethane hydrocarbon. In a previous study (Ng et al, 2008), we show that the SOA yield from the reaction of isoprene with NO3 radicals is higher when experimental conditions favor RO2 + RO2 reactions over RO2 + NO3 reactions This phenomenon is explained in part by the formation of low vapor pressure C10organic peroxides (ROOR), a product channel that had previously been considered insignificant. The large excess of hydrocarbon with respect to N2O5 maximizes peroxy radical self- and crossreactions and minimizes NO3 reactions with both peroxy radicals and stable first generation products (i.e., species other than isoprene) This excess is magnified by adding the hydrocarbon after the N2O5 is well-mixed in the chamber: within the injected plume, hydrocarbon concentrations will be much greater than 800 ppb. While this theoretical approach compares favorably with experimentally derived sensitivities for many compounds (Garden et al, 2009; Paulot et al, 2009b,a), it represents the largest source of uncertainty (±25 %) for the CIMS data
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