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Mechanism development linked to the Master Chemical Mechansim

OH yields from O3 + alkene reactions

The OH radical is the major oxidant of volatile organic compounds (VOCs) in the atmosphere. Ozone alkene reactions are a non-photolytic source of HOx radicals and appear to be a significant source of new HOx radicals in urban and rural (especially forested) air. The dark reactions between O3 and trans-2-butene and isoprene have been studied in HIRAC. Only results from the O3+isoprene study will be discussed here but further information about the O3+trans-2-butene results have been published by Glowacki et al. (2007).


Pressure dependent (100 – 1000 mbar) OH yields from O3+isoprene have been measured both directly and indirectly in HIRAC.

Indirect measurements

Scavenger technique: Using cyclohexane as an OH scavenger, the yield of cyclohexanone was measured using GC-FID along with the concurrent loss rate of isoprene.

Kinetic technique: Kinetics studies using cyclohexane. A 10 fold excess O3 over isoprene was used to ensure that the decay of isoprene was pseudo-first order. The consumption of isoprene was monitored using both FTIR and GC-FID.

Direct measurements

OH and HO2 yields were measured using FAGE for chamber pressures between 300 and 1000 mbar. A simple kinetic model based on a subset of the Master Chemical Mechanism (MCM) was used to model the OH and HO2 concentrations and hence determine pressure dependent OH and HO2 yields.

Direct and indirect OH yields from O3+isoprene Measured and modelled OH from O3+isoprene Table of pressure dependent HO2 yields from O3+ispoprene
Pressure dependent OH yields determined by indirect and direct techniques. Direct FAGE OH measurements p = 1000 mbar (red) with modelled data (black) giving YOH = 0.26±0.02. Pressure dependent HO2 yields determined directly using FAGE.