Abstract. We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constams for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298 +_ 2 K and are found to be independent of pressure with an averaged value of (10.1 + 0.8) x 10 '• cm 3 molecule 4 s 4. The error limit given is within 1 standard deviation; a systematic error is estimated to be +_15%. We also describe direct observation of the OH-isoprene adduct based on ion-molecule reactions by using the CIMS method. The formation of the OH-isoprene adduct was used to extract the rate constam between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH-isoprene adduct in the presence of oxygen molecules, an overall rate constam between OH-isoprene adduct and 02 has been first determined, with an averaged value of (2.8 +_ 0.7) x 10 '15 cm 3 molecule 'l s 'l at 76 torr and an estimated systematic error of +_50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed.

The kinetics of the isoprene−OH reaction have been studied both experimentally and computationally. Experimental rate constants at pressures in the range 0.5−20 Torr have been determined at 295 K using pulsed photolysis/laser-induced fluorescence detection of the OH radical. A rate constant of (0.99 ± 0.05) × 10-10 molecules-1 cm3 s-1 at 20 Torr in argon was determined, which is consistent with previous results for the high-pressure limiting rate constant. We present the first experimental observation of the falloff region for this reaction and have modeled the pressure dependence of the rates using the Troe formalism. Canonical variational transition state theory calculations were performed on the basis of recent ab initio calculations to determine the relative branching among the four possible isoprene−OH adducts in the high-pressure limit. We find OH addition to the outer carbon positions dominates OH addition to the inner carbon positions. We have employed RRKM/master equation calculations to evaluate the pressure dependence of the overall rate and the rates for the individual isomers in the pressure range 0.25−1000 Torr. The excellent agreement between the calculated and experimental falloff behavior provides an independent test of the ab initio energetics and RRKM/ME treatment. The results shed light on the mechanisms for oxidation of isoprene in the troposphere.