Selective Scavenger Studies of Stabilized Criegee Intermediate Formation

GREG T. DROZD (1), Neil M. Donahue (1)

(1) Carnegie Mellon University

     Abstract Number: 408
     Last modified: May 12, 2010

Abstract
Atmospheric aerosol evolution is dominated by oxidation of aerosol components by ozone and hydroxyl radical. Both form products of varying volatility and propagate radical chemistry. Ozonolysis proceeds through an alkene-ozone adduct, the primary ozonide (POZ), which decomposes into a carbonyl + Criegee-intermediate (CI) pair. The exothermicity of ozonolysis leads to CI's with an energy distribution peaked at very high internal energy. The time scale for unimolecular reaction (isomerization/decomposition) controls mechanistic branching ratios in both the gas and condensed phases; because this depends on internal energy, CI dynamics are pressure dependent. Only CI's stabilized by collisions (SCI) have lifetimes permitting bimolecular reactions. In spite of the prominence of SCI reactions in many proposed reaction mechanisms, direct observations of SCI formation are almost non existent. We present studies on the pressure dependence of stabilized CI formation utilizing a hexafluoroacetone scavenger. Increasing pressure or carbon number in linear alkenes increases CI stabilization, and the simple cycloalkenes (e.g. cyclohexene) exhibit no SCI formation. By comparing our results of the pressure dependence of SCI formation and both prompt and long-time OH yields, our results indicate that OH formation from ozonolysis proceeds via a long lived intermediate, generally taken to be a vinylhydroperoxide (VHP).