Different pathways of the formation of highly oxidized multifunctional organic compounds (HOMs) from the gas-phase ozonolysis of β-caryophyllene

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.