UC Riverside

The critical intermediate produced in the ozonolysis of unsaturated volatile organic compounds (VOCs), Criegee intermediates (CIs), plays an important role in the production of hydroxyl radicals and secondary organic aerosols (SOA) in the troposphere. Due to the transient nature of CIs, decades of efforts on direct measurements of CIs in ozonolysis have proven its difficulty. This dissertation presents a comprehensive study on the kinetics and mechanisms of CIs produced in ozonolysis of a series of alkenes using cavity ringdown spectroscopy and a fast flow reactor. Chapters 2 to 5 of this dissertation investigated the nascent yield of stabilized Criegee intermediates (sCIs) produced in ozonolysis of alkenes using chemical titration with an excess amount of SO2 scavenger. These studies specifically examined ethene, propene, 1-butene, 1-pentene, 1-hexene, 1-heptene, 2-methylpropene, 2-methyl-1-butene, and 3-methyl-1-butene. The findings reveal a chain length dependence behavior of nascent sCI yields that increases with the addition of carbon atoms and eventually reached a plateau at around 31% for longer chain 1-alkenes. In particular, the fraction of stabilized CH2OO reached a plateau from propene, indicating that CH2OO was produced with nearly the same internal energy distribution from propene to 1-heptene and the increased chain in 1-alkenes was not effective in taking away the excess energy. The comparison between the experiments and RRKM calculations suggests that the dissociation of primary ozonide (POZ) of O3 + ethene and propene can be treated by statistical theory, while that of O3 + 1-butene to 1-heptene is non-statistical and intramolecular vibrational redistribution (IVR) of the initial energy on the 1,2,3-trioxolane of POZ throughout the molecule was incomplete on the dissociation time scale. Chapters 6 to 11 of this dissertation focus on the direct measurements and kinetic studies of the simplest CI (CH2OO) in ozonolysis of a series of terminal alkenes ranging from ethene to isoprene. Strong absorption features of CH2OO in near UV region corresponding to the B̃(1A′) ← X̃(1A′) transition were utilized to characterize and quantify the concentrations of CH2OO produced in ozonolysis of ethene, isoprene, propene, 1-butene, 1-pentene, and isobutene. Short residence times were used to generate sufficiently high concentrations of CH2OO. Kinetic studies based on experimental data and reaction network modeling allow the determination of sCI yields and the measurements on the rate coefficients of reactions between CH2OO and various species in ozonolysis, such as alkenes, O3, and aldehydes. Chapter 12 focuses on the direct measurements of CH3CHOO produced in ozonolysis of cis- and trans-2-butene. Branching ratios of syn- and anti-CH3CHOO in both systems were obtained from their spectra features.