Accurate, direct measurements of OH yields from gas-phase ozone-alkene reactions using an in situ LIF instrument

We present direct, pressure-dependent measurements of OH yields from gas-ph ase ozone-alkene reactions, using the Harvard HOx instrument to obtain sens itive, accurate, and precise OH concentrations. As in previous studies from our laboratory, steady-state [OH] is measured by laser-induced fluorescenc e (LIF); here, the accurate LIF calibration allows us to present absolute O H yields for the first time. To calibrate our original LIF system, we measu re yields from ozone plus tetramethylethylene (TME) as a function of pressu re. The pressure dependence agrees with our previous results, so our yield measurements are now of comparable accuracy to those from indirect studies. Prompt, low-pressure yields agree well with 1 atm yields measured over lon ger timescales, confirming that much of the OH arises from decomposition of stabilized carbonyl oxides. We also explore the pressure dependence of yie lds from ethene, isobutene, and isoprene. Yields from isobutene and isopren e are pressure-dependent, consistent with the formation of substituted carb onyl oxides. We observe no pressure dependence of the OH yield from ozone ethene, finding instead a constant yield of 14%, in line with most 1 atm s tudies but in contrast with the results of another pressure-dependent study .