OH Yields from Gas-Phase Ozonolysis of Isoprene

Environmental chambers are an important part of an integrated and multifaceted approach to understanding chemical processes in the atmosphere, by bridging the gap between laboratory measurements of individual elementary reactions and the complexity of the real atmosphere. The University of Leeds HIRAC (Highly Instrumented Reactor of Atmospheric Chemistry) chamber is a 2 m3 stainless steel vessel capable of operating at a variety of temperatures and pressures, enabling wide ranging kinetics experiments [D.R. Glowacki et al., Atmos. Chem. Phys., 7,5371,(2007)]. Ozone-alkene reactions are a non-photolytic source of OH, HO2 and RO2 radicals and are a significant source of these radicals in urban and rural air. We have used four different methods to measure OH yields in the HIRAC chamber. These are (1) laser- induced fluorescence at low pressure (FAGE) (detection limit of ~106 molecule cm-3); which provides a direct, calibrated measurement of the OH radical concentration. (2) An indirect method to measure OH yields by use of an OH radical scavenger (cyclohexane) producing stable products (cyclohexanol and cyclohexanone), the concentrations of which allow OH yields to be determined after suitable calibration. (3) A second indirect method utilises 1,3,5-trimethylbenzene (TMB) as an OH tracer, which is consumed by reaction with OH radicals. Analysis of the diminution in concentration of TMB allows the estimation of OH radical production yields. (4) Also a kinetic study was utilised, in which the decay of isoprene follows pseudo-first order kinetics using a 10 fold excess of ozone over alkene. The decay rate of isoprene was monitored using gas chromatography (GC) and FTIR, and was found to be exponential in all cases. From the decrease of the effective rate constant upon the addition of cyclohexane, the OH yield was determined. These 4 different techniques were used to measure the OH yield of O3 + isoprene. The three indirect techniques have been used to determine OH yields for ozone + isoprene of 0.267 ± 0.024, 0.265 ± 0.020 and 0.253 ± 0.026 for detection by OH scavenger, kinetic study and OH tracer, respectively. The reaction between O3 and isoprene has been studied by the direct detection the OH concentrations using FAGE, with excellent precision. A numerical model based on the Master Chemical Mechanism (MCM), with the OH yield as a variable parameter, was used to calculate the OH concentration, and a best fit to the OH FAGE concentration data, gave a yield of 0.255 ± 0.022 was determined. These techniques were then utilised to determine a pressure dependence of the OH yield for ozonolysis of isoprene.