Criegee intermediates are formed by the gas-phase reactions of alkenes and ozone in the Earths atmosphere.  Ozone adds across the carbon–carbon double bond of the alkene to form a molozonide, which then decomposes to produce a carbonyl (RRCO) and a carbonyl oxide. The latter is known as the Criegee intermediate.
The alkene ozonolysis reaction is extremely exothermic, releasing about  of excess energy.  Therefore, the Criegee intermediates are formed with a large amount of internal energy.