Photolysis of methylethyl, diethyl and methylvinyl ketones and their role in the atmospheric HOx budget.

Abstract

Quantum yields for acyl (RCO) radical production from ketone photolysis as a function of temperature, pressure and the atmospherically relevant wavelengths (308 and 320 nm) have been determined for methylethyl ketone (MEK), methylvinyl ketone (MVK) and diethyl ketone (DEK) via direct observation of the OH product from the RCO + O2 reaction. The methodology has been applied previously to acetone photolysis. The kinetics and OH yields of the RCO + O2 reactions have been investigated to demonstrate that this technique can be used to monitor the dissociation of higher ketones. These kinetic studies have been used to confirm CH3CO + R as the dominant radical dissociation mechanism in the unsymmetrical ketones MVK and MEK. At 308 nm MEK and DEK photolysis follows conventional Stern Volmer behaviour. MEK and DEK are quenched less efficiently than acetone; quenching efficiency increases with decreasing temperature (213-295 K). At 320 nm Stern Volmer plots of the RCO quantum yields show evidence for the involvement of multiple states in the dissociation. The wavelength dependence of this phenomenon is compared with that for acetone and the atmospheric implications for MEK and DEK lifetimes have been investigated by converting the measured quantum yields to photolysis rates. The calculated rates under typical atmospheric conditions are a factor 2-3 lower than if the quantum yields in the literature are used, influencing both the overall atmospheric lifetime of these ketones and their relative rates of removal by reaction with OH and by photolysis.