Kinetic Study on Gas-Phase Oligomer Formation from Stabilized Criegee Intermediate, CH₂OO, with Formic Acid

The stabilized Criegee intermediate, CH₂OO, produced from the ozonolysis of unsaturated organic compounds such as ethene, exhibits high reactivity toward carboxylic acids, yielding hydroperoxymethyl carboxylates as products. Laboratory experimental studies and quantum chemical calculations have shown that CH₂OO can sequentially add to the hydroperoxymethyl carboxylate in the gas phase to form oligomeric hydroperoxides, which may contribute to secondary organic aerosol (SOA) formation owing to their low volatility. However, there is little kinetic information on the oligomerization reactions of CH₂OO except for quantum chemical calculations. In this study, laboratory experiments using a flow-tube reactor coupled with a chemical ionization mass spectrometer are performed to obtain kinetic information on the reaction between CH₂OO and a dimeric hydroperoxide (hydroperoxymethyl formate) composed of CH₂OO and a singly deuterated formic acid, DCOOH. Careful kinetic analysis yields (1.15 ± 0.08) × 10^-12 cm³ molecule^-1 s^-1 as the bimolecular rate coefficient for the reaction between CH₂OO and the dimer leading to the formation of a trimeric hydroperoxide. In addition, the yield of stabilized CH₂OO generated from the ethene ozonolysis was found to be 0.61 ± 0.03, which is higher than the IUPAC recommended value of 0.42 ± 0.10. In order to fill the large gap between recent field observations of oligomeric hydroperoxides and model simulations based on the current understanding of oligomer formation processes, further experimental and theoretical studies on oligomer formation kinetics as well as identification of unknown oligomer formation processes are needed.