Total Radical Production and Degradation Products from Alkene Ozonolysis

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

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.

Publications

10 25 50
 
Description Unsaturated compounds, alkenes, 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). Once emitted to the atmosphere, alkenes react with ozone, with two effects: Radical species, including OH, are produced, and the alkene-ozone reaction produces further hydrocarbon species (degradation products), which can participate in atmospheric reaction cycles, potentially producing ozone. Both of these effects have considerable uncertainties, which this project addressed.



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, 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 may vary with humidity. 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 key aims of the TRAPOZ project were to directly measure the radical yields from ozonolysis of a range of alkenes, and to identify their degradation products using a range of instruments including a novel Chemical Ionisation Time-of-Flight Mass Spectrometer.



The experimental work was 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 was supplemented by the CIR-TOF-MS and a Peroxy Radical Chemical Amplifier, for measurement of RO2, from the University of Leicester. In addition, offline analysis of aerosol samples collected on filters was performed by the University of York. Ozonolysis experiments were perfomed on ethene, propene, 1-butene, cis- and trans-2-butene, isoprene, a-pinene, b-pinene, limonene, myrcene, a-cedrene and methyl chavicol; OH and HO2 yields were measured for all of these species, while RO2 data were obtained for ethene, propene and butene.
The data have shown that OH yields are broadly consistent with those currently assumed, while HO2 yields are somewhat higher than those presently incorporated within atmospheric chemical mechanisms (for example, the Master Chemical Mechanism, version 3.1) - for example, for ethene, a yield of 0.27 was obtained, compared with the previous value of 0.13. Taken together, atmospheric model simulations have shown that nighttime OH and HO2 levels increase by 29 and 40 % (clean environments) and 8 and 12 % (polluted environments) respectively, if the TRAPOZ results are used, implying increased tropospheric oxidising capacity. The findings will be used to improve the accuracy of current chemical mechanisms (e.g. the MCM), and will be archived in the open access EUROCHAMP database for future use by other scientists working on related problems.
Exploitation Route Project has led to subsequent funded research on alkene interference on NOx measurements, and the atmospheric chemistry of Oil Palm emissions - listed under subsequent funding.

Results will be incorporated in updates to the MCM and related atmospheric chemical mechanisms, which underpin air quality models.
Sectors Environment

 
Description Results from the project have been published in peer-reviewed journals and presented at international conferences. They will be used in updates to Atmospheric Chemical Mechanisms (for example, the Master Chemical Mechanism, MCM) which in turn support scientific advice in support of air quality policy. Improved accuracy in mechanisms and models translates infto more effective and efficient policy measures to improve air quality.
First Year Of Impact 2010
Sector Environment
Impact Types Policy & public services

 
Description FP7: AtMeCh: The Atmospheric Chemistry of Methyl Chavicol
Amount € 42,000 (EUR)
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 01/2011 
End 12/2012
 
Description FP7: NOxINT: Nitrogen Dioxide Measurement Interference
Amount € 25,600 (EUR)
Organisation European Commission 
Department Seventh Framework Programme (FP7)
Sector Public
Country European Union (EU)
Start 01/2010 
End 12/2010
 
Description ASAP-Delhi: IITD 
Organisation Indian Institute of Technology Delhi
Country India 
Sector Academic/University 
PI Contribution Expertise in air pollutant source identification / source apportionment
Collaborator Contribution Access to measurement sites/expertise in Indian air pollution climatology, causes, variations.
Impact Project currently ongoing
Start Year 2016
 
Description TRAPOZ Collaboration with EUPHORE 
Organisation Centre for Environmental Studies of the Mediterranean (Centro de Estudios Ambientales del Mediterráneo)
Country Spain 
Sector Public 
PI Contribution Application of the EUPHORE simulation chamber to improve atmospheric chemical mechanisms for the prediction of air quality issues
Collaborator Contribution Scientific leadership to identify science questions and experiment plans across EU FP7 facility access awards
Impact Multiple journal papers and subsequent funding awards, detailed under the indicvidual NERC awards, where appropriate
Start Year 2007