Measurements of the kinetics of the hydroxyl radical-initiated oxidation of isoprene, methyl vinyl ketone, and methacrolein

The kinetics of some of the key reactions in the OH-initiated oxidation of isoprene, methyl vinyl ketone, and methacrolein that are responsible for tropospheric ozone production were investigated using the turbulent flow and low pressure discharge-flow techniques coupled with laser-induced fluorescence and resonance fluorescence detection of OH. Rate constants for the OH + isoprene, OH + methyl vinyl ketone, and OH + methacrolein reactions were found to be independent of pressure at 300 K, suggesting that these reactions have reached their high-pressure limits at 2 Torr. The measured rate constants at 300 K are in excellent agreement with present recommendations. This implies that the discrepancies between measured and predicted radical concentrations in the lower troposphere may not be due to uncertainties in the rate constants for OH reactions with isoprene, methyl vinyl ketone, and methacrolein, and may be due to uncertainties in the kinetics of the subsequent oxidation steps and kinetics of the oxidation of other important biogenic hydrocarbons. A pressure dependence was observed for the OH + isoprene and OH + methyl vinyl ketone reactions at temperatures above 321 K, suggesting that they are dominated by an OH-addition mechanism even at the lowest pressures and highest temperatures of these experiments. However, the oxidation mechanism for the OH + methacrolein reaction appears to be dominated by hydrogen abstraction at temperatures above 321 K since a pressure dependence was not observed, though the addition mechanism may still be significant at 300 K.; The overall rate constants for reactions of NO with isoprene-, methyl vinyl ketone-, and methacrolein-based peroxy radicals, including the branching ratio for organic nitrate formation, were indirectly measured to be consistent with current recommendations. This implies that the rate constants for RO 2 + NO and RC(O)O2 + NO reactions are independent of the structure of the R group. The large nitrate yield of ∼15% for isoprene agrees with some previous measurements, suggesting that isoprene can be a significant sink of anthropogenic NOx in biogenic hydrocarbon impacted areas. For methyl vinyl ketone and methacrolein, the nitrate yields are similar to other compounds of similar sizes.