Newly observed peroxides and the water effect on the formation and removal of hydroxyalkyl hydroperoxides in the ozonolysis of isoprene

Abstract

Abstract. The ozonolysis of alkenes is considered to be an important source of atmospheric peroxides, which serve as oxidants, reservoirs of HOx radicals, and components of secondary organic aerosols (SOAs). Recent laboratory investigations of this reaction identified hydrogen peroxide (H2O2) and hydroxymethyl hydroperoxide (HMHP) in ozonolysis of isoprene. Although larger hydroxyalkyl hydroperoxides (HAHPs) were also expected, their presence is not currently supported by experimental evidence. In the present study, we investigated the formation of peroxides in the gas phase ozonolysis of isoprene at various relative humidities on a time scale of tens of seconds, using a quartz flow tube reactor coupled with the online detection of peroxides. We detected a variety of conventional peroxides, including H2O2, HMHP, methyl hydroperoxide, bis-hydroxymethyl hydroperoxide, and ethyl hydroperoxide, and interestingly found three unknown peroxides. The molar yields of the conventional peroxides fell within the range of values provided in the literature. The three unknown peroxides had a combined molar yield of ~ 30% at 5% relative humidity (RH), which was comparable with that of the conventional peroxides. Unlike H2O2 and HMHP, the molar yields of these three unknown peroxides were inversely related to the RH. On the basis of experimental kinetic and box model analysis, we tentatively assigned these unknown peroxides to C2−C4 HAHPs, which are produced by the reactions of different Criegee intermediates with water. Our study provides experimental evidence for the formation of large HAHPs in the ozonolysis of isoprene (one of the alkenes). These large HAHPs have a sufficiently long lifetime, estimated as tens of minutes, which allows them to become involved in atmospheric chemical processes, e.g., SOA formation and radical recycling.

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

“…This suggestion is supported by observations of high mixing ratios (up to 5 ppbv) of hydroxymethyl hydroperoxide (HMHP), a characteristic product of reactions of the smallest sCI (CH 2 OO) with water (Neeb et al, 1997), over forested regions and in biomass burning plumes (Gäb et al, 1985;Lee et al, 1993Lee et al, , 2000Valverde-Canossa et al, 2006). Although HMHP and other hydroperoxides produced from ozonolysis are important atmospheric compounds, their yield estimates are highly uncertain (Becker et al, 1990;Neeb et al, 1997;Sauer et al, 1999;Hasson et al, 2001;Huang et al, 2013). This may be attributable to the fact that hydroperoxide yields have mainly been determined by offline methods or under conditions with highly elevated hydrocarbon loadings.…”

“…This may be due to the large differences among studies in the hydrocarbon loadings ([ISO] i = 40-10 000 ppbv), ozone-to-isoprene ratios (< 0.5 to > 100), water vapor content (< 10-20 000 ppmv), reaction pressures (4-760 torr), analytical methods used for product analysis (GC, HPLC, FTIR, direct OH vs. scavenging, etc. ), and methods used to generate sCI (CH 2 I 2 + hν vs. gas-phase ozonolysis) (Simonaitis et al, 1991;Neeb et al, 1997;Sauer et al, 1999;Hasson et al, 2001;Kroll et al, 2002;Johnson and Marston, 2008;Drozd and Donahue, 2011;Welz et al, 2012;Huang et al, 2013).…”

Synthesis and characterisation of peroxypinic acids as proxies for highly oxygenated molecules (HOMs) in secondary organic aerosol

“…Under these conditions, the reaction is limited by O 3 , which according to model calculations is consumed within ∼ 20 s under both humid and dry conditions. The lifetime of O 3 and α-pinene in the flow tube was estimated using the AtChem (https://atchem.leeds.ac.uk, last access: 6 March 2018) numerical box model alongside the Master Chemical Mechanism (MCM) v3.3.1 (http://mcm.leeds.ac.uk, last access: 6 March 2018) (Jenkin et al, 1997;Saunders et al, 2003). For the experiments at higher humidity (∼ 85 %), a humidifier was added to the set-up.…”

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

“…[76][77][78][79] HHPs can subsequently decompose to either aldehydes/ketones and H 2 O 2 or acids and H 2 O, 77,78,80 with the decomposition being assisted by the presence of water vapor. 79,80 Therefore, as RH increases more acids will be produced, 75,76,78 thus favoring the formation of peroxyhemiacetals. Fig.…”