Low-pressure and nascent yields of stabilized Criegee intermediates CH₂OO and CH₃CHOO in ozonolysis of propene

The yields of stabilized Criegee intermediates (sCIs), both CH₂OO and CH₃CHOO, produced from ozonolysis of propene at low pressures (7-16 Torr) were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO₂). The method of monitoring the consumption of SO₂ as a scavenger and the production of secondary formaldehyde (HCHO) allows characterization of the total sCI and the stabilized CH₂OO yields at low pressure and in a short residence time. Both the total sCI and the stabilized CH₂OO yields in the propene ozonolysis were found to decrease with decreasing pressure. By extrapolating the 7-16 Torr measurements to the zero-pressure limit, the nascent yield of the total sCIs was determined to be 25 ± 2%. The ranges of nascent yields of stabilized CH₂OO and stabilized CH₃CHOO were estimated to be 20-25% and 0-5%, respectively. The branching ratios of the stabilized and high-energy CH₂OO* and CH₃CHOO* were also determined.