Yield of the Four-Carbon Stabilized Criegee Intermediates from Isoprene Ozonolysis

Isoprene is the single most abundant nonmethane hydrocarbon emitted into the atmosphere. Despite this, uncertainties in the oxidation chemistry remain. Here, we investigate the yields of Criegee intermediates that are produced from the ozonolysis reaction, where we conduct a series of atmospheric simulation chamber experiments in which the transient stabilized Criegee intermediates (sCIs) are titrated in the gas phase using either biacetyl or acetylpropionyl. This reaction yields a stable ketone-substituted secondary ozonide (SOZ), which was observed directly in the gas phase using a proton-transfer-reaction time-of-flight mass spectrometer operated in NH₄ ⁺ mode. Both C₁ and C₄ sCIs were observed in this way, with the mass of the NH₄ ⁺ adduct shifting according to the mass of the sCI and its diketone titrant. The relative abundance of the C₄ sCI was constrained against C₁ assuming a similar sensitivity for the two SOZ derivatives. This was supported by quantum chemical calculations that demonstrated very similar binding energies between NH₄ ⁺ and the C₁ and C₄ SOZ adducts. Our results demonstrate an overall yield of ∼11% for the long-lived C₄ sCIs, which may survive long enough to participate in various bimolecular reactions in the atmosphere.