Abstract
Abstract. The gas-phase mechanism of the formation of highly oxidized multifunctional organic compounds (HOMs) from the ozonolysis of β-caryophyllene was investigated in a free-jet flow system at atmospheric pressure and a temperature of 295 ± 2 K. Reaction products, mainly highly oxidized RO2 radicals containing up to 14 oxygen atoms, were detected using chemical ionization – atmospheric pressure interface – time-of-flight mass spectrometry with nitrate and acetate ionization. These highly oxidized RO2 radicals react with NO, NO2, HO2 and other RO2 radicals under atmospheric conditions forming the first-generation HOM closed-shell products. Mechanistic information on the formation of the highly oxidized RO2 radicals is based on results obtained with isotopically labelled ozone (18O3) in the ozonolysis reaction and from hydrogen/deuterium (H/D) exchange experiments of acidic H atoms in the products. The experimental findings indicate that HOM formation in this reaction system is considerably influenced by the presence of a double bond in the RO2 radicals primarily formed from the β-caryophyllene ozonolysis. Three different reaction types for HOM formation can be proposed, allowing for an explanation of the detected main products: (i) the simple autoxidation, corresponding to the repetitive reaction sequence of intramolecular H-abstraction of a RO2 radical, RO2 → QOOH, and subsequent O2 addition, next forming a peroxy radical, QOOH + O2 → R′O2; (ii) an extended autoxidation mechanism additionally involving the internal reaction of a RO2 radical with a double bond forming most likely an endoperoxide and (iii) an extended autoxidation mechanism including CO2 elimination. The individual reaction steps of the reaction types (ii) and (iii) are uncertain at the moment. From the product analysis it can be followed that the simple autoxidation mechanism accounts only for about one-third of the formed HOMs. Time-dependent measurements showed that the HOM formation proceeds at a timescale of 3 s or less under the concentration regime applied here. The new reaction pathways represent an extension of the mechanistic understanding of HOM formation via autoxidation in the atmosphere, as recently discovered from laboratory investigations on monoterpene ozonolysis.
Highlights
The emission of biogenic volatile organic compounds (BVOCs) from vegetation to the troposphere and their oxidation in the gas phase is the subject of intense research (Calvert et al, 2000; Guenther et al, 2012; Ziemann and Atkinson, 2012)
Other experimental work on highly oxidized multifunctional organic compounds (HOMs) formation from the ozonolysis of monoterpenes (Jokinen et al, 2014; Mentel et al, 2015) and model substances, such as cyclohexene (Berndt et al, 2015b; Mentel et al, 2015; Rissanen et al, 2014), led to the development of an autoxidation mechanism based on RO2 radical chemistry
RO2 radicals were detected as reaction products because the RO2 radical concentrations did not exceed 9×106 molecules cm−3 and bimolecular reactions of the formed RO2 radicals were less efficient for a reaction time of 3.0–7.9 s in these experiments
Summary
The emission of biogenic volatile organic compounds (BVOCs) from vegetation to the troposphere and their oxidation in the gas phase is the subject of intense research (Calvert et al, 2000; Guenther et al, 2012; Ziemann and Atkinson, 2012). Other experimental work on HOM formation from the ozonolysis of monoterpenes (Jokinen et al, 2014; Mentel et al, 2015) and model substances, such as cyclohexene (Berndt et al, 2015b; Mentel et al, 2015; Rissanen et al, 2014), led to the development of an autoxidation mechanism based on RO2 radical chemistry In this process, an RO2 radical internally abstracts an H atom, forming an alkyl radical with a hydroperoxide moiety (RO2 → QOOH). Experiments with heavy water (D2O) and isotopically labelled ozone (18O3) were conducted in order to obtain additional information on elementary reaction pathways needed to explain the observed products This approach allowed for developing an extended mechanism for the HOM formation from the ozonolysis of β-caryophyllene
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