The reaction networks that describe the intricate chemical processes involved in atmospheric and combustion systems are typically dominated by reactive intermediates that usually occur in low concentrations. The identification and quantification of these reactive species within complex reactive mixtures are crucial for developing a fundamental, chemically accurate description of complex atmospheric and combustion systems. Criegee intermediates (CIs), carbonyl oxide reactive intermediates formed during the ozonolysis of unsaturated organic compounds, possess the unique ability to add both carbon and oxygen mass to the co-reactant through either 1,3-dipolar cycloaddition or insertion mechanisms. These mechanisms can lead to the production of high-molecular-weight, low-volatility products, which have been associated with the formation of secondary organic aerosols (SOA) in just a few reaction steps. Their unimolecular decomposition is recognized as a significant source of OH in the atmosphere.
The reaction networks of C1 to C7 CIs formed in the ozone-assisted oxidation reaction of a series of C3 to C7 acyclic and endocyclic alkenes were investigated in an atmospheric pressure jet-stirred reactor to elucidate the influence of CIs carbon number and reactive environment on the distribution of products and the formation of high-molecular-weight, low-volatility compounds. Molecular beam high-resolution mass spectrometry, tunable synchrotron single-photon ionization, and ab initio threshold energy calculations were employed for the detection and identification of gas-phase reactive intermediates and final products. Temperature-dependent analysis employing DART ionization-Orbitrap mass spectrometry was utilized to facilitate the detection of high-molecular-weight, low-volatility reaction products.
The ozone-assisted oxidation of acyclic C3 to C7 alkenes was observed to lead to complex CIs reaction networks that encompass both unimolecular and bimolecular reactions. For instance, formaldehyde oxide CIs were predominantly observed in the ozone-assisted oxidation of 1-propene and 1-butene, resulting in the formation of products corresponding to sequential CIs additions to co-products such as water, alkenes, aldehydes, alcohols, and carboxylic acids. However, oligomerization did not appear to be favored by increasing the CIs carbon number. Notably, no evidence for the formation of bimolecular products was observed in the case of long-chain CIs formed in the ozone-assisted oxidation of endocyclic alkenes, specifically C5 to C7 alkenes. Instead, long-chain CIs were observed to undergo unimolecular decomposition followed by autooxidation.
The outcomes of our research represent a significant advancement in understanding the CIs reactive behavior in complex reactive environments where multiple co-reactants are present. This information provides valuable insights for future kinetic modeling endeavors that can contribute to a more comprehensive understanding of the role and reactivity of CIs in the gas-phase to heterogeneous environments.