Direct measurements of the Criegee intermediate formaldehyde oxide, CH2OO, produced from ozonolysis of ethene in a plug-flow optical cavity.

Carbonyl oxides, known as Criegee intermediates, are transient species produced in the ozonolysis of unsaturated hydrocarbons. These intermediates play a key role in the chemistry of the troposphere, effecting complex reaction pathways that lead to production of OH radicals, peroxy radicals, highly oxygenated molecules, etc. Currently, Criegee intermediates are studied in isolation from the ozonolysis reaction by synthesis of stabilized carbonyl oxides through the photodissociation of a corresponding iodoalkane and a subsequent addition of O₂. This method has permitted the study of various kinetic rate constants and product yields of reactions of Criegee intermediates with compounds of atmospheric interest. However, the ozonolysis reaction is highly exothermic, and the produced Criegee intermediates have a broad energy distribution. Thus, research on Criegee intermediates as a key step of the ozonolysis reaction network, and their effect on the different reaction pathways, requires also the ability of measuring these transient species in the actual ozonolysis reaction.

Here, we report on the direct measurements of formaldehyde oxide produced from ozonolysis of ethene in a flow cell using cavity ring-down spectroscopy. The gas flow and pressure in the optical cavity were carefully controlled to allow the reaction cell to behave as a plug-flow reactor, and high-resolution (0.01nm) ultraviolet spectra were obtained for the ozonolysis of ethene under different reaction conditions. An a priori chemical mechanism was simulated in the plug-flow reactor to determine reaction conditions that would enhance the quasi steady-state production of formaldehyde oxide, which was quantified by fitting the measured ultraviolet spectra with the known cross-section of its B̃(¹A′) ← X̃(¹A′) transition. Average concentrations of CH₂OO were determined in the flow cell under different residence times, and time profiles were obtained corresponding to different steady states. The time profiles serve as constrains to benchmark a posterior reaction mechanism of ethene ozonolysis, allowing for the determination of yields and other kinetic information, as well as providing insights on some yet-unexplored reactions pathways in the ozonolysis reaction network.