Total Radical Production and Degradation Products from Alkene Ozonolysis

Atmospheric composition is determined by a combination of emissions and chemical processing within the atmosphere. The removal of most hydrocarbons emitted to the atmosphere is initiated by reaction with the hydroxyl radical (OH). A series of degradation steps follows, leading eventually to CO2 and water. In the presence of nitrogen oxides, hydrocarbon oxidation leads to production of ozone / a pollutant harmful to human health, vegetation and materials. The extent to which ozone formation occurs depends upon details of the hydrocarbon oxidation steps. OH is formed naturally through the action of sunlight upon ozone. Following reaction with hydrocarbons, OH is converted into HO2 and RO2, hydro and organic peroxy radicals. RO2 and HO2 can then be converted back into OH if moderate levels of NO are present / an example of radical cycling. OH levels control the abundance of pollutants and global warming gases such as methane, and limit the rate of ozone production. To quantify these effects, we need to understand the processes which govern OH levels, while to predict the atmospheric impact of a given hydrocarbon, we must identify its degradation products / e.g., to assess if they have a long enough lifetime to be transported from their point of origin. Unsaturated compounds, alkenes, are those with one or more double bonds. They are emitted through industrial processes, and also from vegetation (forests are a large source of alkene molecules made up of 1-3 or more units of isoprene, C5H8). Alkenes react with ozone, with two effects: Radical species, including OH, are produced (without the need for sunlight), leading to faster oxidation of other compounds, and the alkene-ozone reaction produces further hydrocarbon species (degradation products), which can participate in atmospheric reaction cycles, potentially producing ozone. However, both of these effects have considerable uncertainties, which this project aims to address. The ozone-alkene reactions produce HO2 and RO2 radicals in addition to OH. RO2 and HO2 are readily converted into OH in the atmosphere; therefore, any production of RO2 or HO2 will lead to enhanced OH levels. Hitherto, the HO2 and RO2 yields have been inferred indirectly. Recently, we have been able to directly measure HO2 and RO2, and evidence has emerged that HO2 and RO2 yields (and hence, ultimately, OH production) from alkene ozonolysis is rather higher than previously thought, and varies with humidity. The first objective of this project is to measure total radical yields (OH, HO2 and RO2) from ozonolysis of a range of alkenes of both natural and man-made significance. Alkene-ozone reactions produce a range of degradation products. For biogenic alkenes (terpenes such as myrcene) up to 90 % of these gas-phase degradation products are unidentified. The second objective of this project is to identify these species, using a combination of conventional instruments together with two new approaches: A Chemical Ionisation Time-of-Flight Mass Spectrometer, uniquely suited to the identification of large oxygenated hydrocarbons, and measurement of the total reactivity of the unidentified species (with respect to reaction with OH), to further constrain their likely nature and atmospheric significance (lifetime). The experimental work will be carried out in the European Photoreactor facility (EUPHORE), in Valencia, Spain. EUPHORE consists of a 200 m3 simulation chamber, with a range of measurement instrumentation, which will be supplemented by the CIR-TOF-MS and a Peroxy Radical Chemical Amplifier, for measurement of RO2, from the University of Leicester. Following the experimental work, we will update the radical production and alkene degradation mechanism in an atmospheric model (the Master Chemical Mechanism), and use the revised model to reassess the impact of alkenes upon ozone production, in standard simulations of the transport of polluted air from Europe to the UK.