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Rate constants for the reaction of ozone with methylvinyl ketone (H 2 C " CHC(O)CH 3 ), methacrolein (H 2 C" C(CH 3 )CHO), methacrylic acid (H 2 C "C(CH 3 )C(O)OH), and acrylic acid (H 2 C " CHC(O)OH) were measured at room temperature in the pres-(296 Ϯ 2 K) ence of a sufficient amount of cyclohexane to scavenge OH-radicals. Results from pseudofirst-order experiments in the presence of excess ozone were found not to be consistent with relative rate measurements. It appeared that the formation of the so-called Criegee-intermediates leads to an enhanced decrease in the concentration of the two organic acids investigated. It is shown that the presence of formic acid, which is known to react efficiently with Criegee-intermediates, diminishes the observed removal rate of the organic acids. The rate constant for the reaction of ozone with the unsaturated carbonyl compounds methylvinyl ketone and methacrolein was found not to be influenced by the addition of formic acid. Rate constants for the reaction of ozone determined in the presence of excess formic acid are (in ): methylvinyl ketone methacrolein 3 Ϫ1 Ϫ1 Ϫ18

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