Advancing our understanding of peroxy radical chemistry in the atmosphere

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Advancing our understanding of peroxy radical chemistry in the atmosphere

Professor Dwayne Heard (SoC, Leeds), Dr Lisa Whalley (SoC, Leeds), Professor Paul Seakins (SoC, Leeds)

 Contact email: d.e.heard@leeds.ac.uk

 

Photo-oxidation in the atmosphere is initiated by short lived radical species, in the daytime dominated by the hydroxyl radical, OH, and at night by either nitrate (NO3) radicals or ozone (Stone et al., 2012). Primary emissions, which may be biogenic in origin, for example hydrocarbons from vegetation and the oceans, or anthropogenic, from traffic, industry and domestic cooking/heating, are transformed into harmful secondary pollutants such as ozone, nitrogen dioxide and secondary organic aerosol (SOA). The latter is a major component of particulate matter (PM). There are now over 6 million deaths worldwide from air pollution, mainly owing to exposure to NO2, O3 and PM, with over 70,000 premature death in Europe alone from exposure to NO2. In the UK air pollution costs the economy £20 billion each year to health services and business, and is linked to 40,000 premature deaths corresponding to on average 6-7 months loss of life.

 

The focus of this project is to better understand the chemistry of peroxy radicals, RO2. As illustrated in Figure 1, RO2 radicals are critical intermediates along the pathway of chemical oxidation between primary emissions and secondary pollutants. There is growing evidence from both field and laboratory studies that our understanding of the detailed chemistry of RO2 is poor (Whalley et al., 2018), representing a significant limitation in our ability to predict secondary pollutant formation, and therefore to put in place effective policy measures to improve air quality. For example, CH3O2, the simplest organic peroxy radical, is formed from the oxidation of methane (an important greenhouse gas), yet there have been no measurements of the concentration of CH3O2 in the atmosphere. Peroxy radicals react quickly with nitric oxide, NO, a major emission from traffic and other industrial processes, to form NO2, which is rapidly photolysed by sunlight leading to ozone formation. In addition, oxidation of more functionalised volatile organic compounds (VOCs), for example those containing unsaturated C-C bonds, aromatic and carbonyl groups, leads to more complex RO2 which can undergo novel and poorly characterised chemistry. An example is repeated isomerisation involving a hydrogen-atom shift, followed by the addition of O2, which can result in rapid auto-oxidation leading to OH radical recycling and the formation of highly oxidised molecules (HOMs) (Jenkin et al., 2019). HOMs have low vapour pressures and can effectively partition to the aerosol phase as SOA (Berndt et al., 2016). However, the behaviour and chemical kinetics of these functionalised RO2 is poorly characterised, yet they form the critical link between emissions, radical recycling and secondary organic aerosol.

 

The overall goal of this project is to develop novel capability for the measurement of RO2 species in the field, to study the kinetics of RO2 in the laboratory using the HIRAC simulation chamber, and to incorporate new mechanistic findings into numerical models, for example those incorporating the Master Chemical Mechanism, which describes in detail chemical oxidation in the atmosphere. The project will lead to an improved representation of RO2 within chemical oxidation processes for use in models to calculate the abundance of health and climate related trace gases and particles. In turn this will lead to an improved predictive capability of atmospheric composition aiding the formulation of effective policy measures.

Figure 1. Chemical mechanism showing the OH initiated oxidation of VOCs in the presence of NOx, leading to the formation of ozone and secondary organic aerosol.

Figure 2. The Leeds FAGE instrumentation for measurement of radicals deployed during fieldwork in Cape Verde.

Figure 3. The Leeds Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC).

Figure 4. Measurements of the CH3O2 radical in the HIRAC chamber generated using the Cl atom initiated oxidation of methane. At t = 400 s the photolysis lamps in the chamber were switched off and the radical concentration decays via its self-reaction. See Onel et al., 2017.

 

Objectives

The specific objectives of the project are:

(1) The development of a new field instrument, based on the FAGE technique (Figure 2), to make the first ever measurements in the atmosphere of the concentration of the simplest peroxy radical, CH3O2. We have already developed a laboratory prototype with the required sensitivity and selectivity which is based on the chemical conversion to CH3O radicals, followed by detection of CH3O using laser-induced fluorescence spectroscopy at 298 nm (Onel et al., 2017). Following commissioning, the instrument will be deployed in collaborative field campaigns in both clean and polluted environments to make measurements alongside OH and HO2 radicals, the sum of peroxy radicals, and OH reactivity (the rate at which OH is removed from the atmosphere) (Whalley et al., 2018). Future opportunities include deployment as part of the The Clean Air: Analysis & Solutions programme, and at the University of Leeds Farm, where a new atmospheric observatory is being established.

(2) To study the kinetics and mechanisms of reactions of a range of peroxy radicals. Making use of our laboratory facilities, for example the Highly Instrumented Reactor for Atmospheric Chemistry (HIRAC, Figure 3), peroxy radicals will be generated from photo-oxidation of individual parent VOCs (of both biogenic and anthropogenic origin) under controlled conditions, and detected using either the FAGE method (Onel et al., 2017) or the ROxLIF technique (Whalley et al., 2013). RO2 with varying degrees of functionality can be generated in the HIRAC chamber from a range of oxidants (OH, Cl atoms, NO3, O3), and the kinetics of RO2 reactions will be studied over a wide range of NOx representative of different ambient environments. An example of a kinetic decay of CH3O2 measured in HIRAC is shown in Figure 4. Reaction products will be quantified using a range of analytical techniques, including absorption spectroscopy and proton-transfer reaction mass spectrometry.

(3) To perform data analysis and interpretation of field and laboratory/chamber data and undertake numerical modelling to calculate concentrations of RO2 and other species. The model will incorporate the detailed Master Chemical Mechanism, which contains ~17,000 reactions and ~7,000 species, in order to calculate radical concentrations which will be compared with field or laboratory measurements. In this way it is possible to quantitatively evaluate how accurately the chemistry of RO2 is represented in the mechanism, for example, the validity of assumptions about the reactivity of more complex, functionalised RO2 species compared with simple RO2. The impact of the new kinetic and mechanistic data derived from objective (2) above can therefore be evaluated.

 

Potential for high impact outcome

The research will lead to an improved representation of chemical oxidation mechanisms which control the abundance of non-CO2 greenhouse gases and lead to the formation of secondary pollutants that are harmful to humans and ecosystems. This in turn will lead to an improved predictive capability of air quality and climate models for atmospheric composition to support policy decisions. The work will be of particular interest to the Department of Business, Energy and Industrial Strategy, and the Department for Environment, Food and Rural Affairs, and will also provide an ideal vehicle for Science in Society activities, for example presentations in Schools. The results from the project will be disseminated widely to the scientific community through high quality publications in leading international journals and at international conferences.

 

Training

The student will work under the supervision of Professor Dwayne Heard, Dr Lisa Whalley and Professor Paul Seakins from the School of Chemistry at Leeds, who are all members of the Atmospheric and Planetary Chemistry Group. The supervisors lead active and vibrant research groups exploring the role of gas-phase and aerosol chemical processes in the atmosphere, using experimental and modelling approaches. We have experience with the ultra-sensitive detection of radicals using laser-induced fluorescence spectroscopy (Stone et al., 2012; Heard and Pilling, 2003), the development of novel methods to detect RO2 in the HIRAC chamber (Onel et al., 2017), as well as chemical modelling using both local “box” (zero-dimensional) models (Whalley et al., 2018) and larger-scale models. The project will provide opportunities to work with other atmospheric scientists in the UK as part of collaborative fieldwork.

You will work in well-equipped laboratories and be part of an active, thriving and well-funded atmospheric chemistry community. The Leeds group receives funding from the National Centre for Atmospheric Science (NCAS) and the FAGE instrumentation is part of the Atmospheric Measurement and Observation Facility (AMOF), and has an internationally leading reputation in atmospheric chemistry for field measurements of atmospheric composition, laboratory studies of chemical kinetics and photochemistry, and the development of advanced numerical models and chemical mechanisms. Activities in these three areas are intimately linked and interdependent, providing a significant advantage. The PhD will provide a broad spectrum of experience and training in the use of high power lasers, vacuum systems, optics, electronics, computer controlled data acquisition systems and methods in numerical calculations. By working with expert investigators the student will receive advanced technical training and enhance their skills base considerably. We strongly support students to write publications during their PhD and you will be supported to attend both national and international conferences. You will have access to a broad spectrum of training workshops in scientific writing, numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).You will also have opportunities for training provided by the National Centre for Atmospheric Science such as the Atmospheric Measurement Summer School and other courses.

 

Student Profile

You should have an interest in atmospheric chemistry, air quality and global environmental problems, with a strong background in physical chemistry or similar (e.g. physics, engineering, environmental science). Standard NERC eligibility rules apply.

 

References

Berndt, T., S. Richters, T. Jokinen, N. Hyttinen, T. Kurten, R. V. Otkjaer, H. G. Kjaergaard, F. Stratmann, H. Herrmann, M. Sipila,̈ M. Kulmala, M. Ehn, Hydroxyl radical-induced formation of highly oxidized organic compounds, Nat. Commun., 7, 13677, 2016.

Heard D.E.; Pilling M.J. Measurement of OH and HO2 in the troposphere, Chemical Reviews, 103, 5163-5198, 2003.

Jenkin, M. E., Valorso, R., Aumont, B., and Rickard, A. R.: Estimation of rate coefficients and branching ratios for reactions of organic peroxy radicals for use in automated mechanism construction, Atmos. Chem. Phys., 19, 7691–7717, 2019.

Onel, L., Brennan, A., Seakins, P. W., Whalley, L., and Heard, D. E.: A new method for atmospheric detection of the CH3O2 radical, Atmos. Meas. Tech., accepted, 2017.

Stone, D.; Whalley, L.K.; Heard, D.E. Tropospheric OH and HO2 radicals: Field measurements and model comparisons, Chemical Society Reviews, 2012, 41, 6348-6404.

Whalley, L. K., Blitz, M. A., Desservettaz, M., Seakins, P. W., and Heard, D. E.: Reporting the sensitivity of laser-induced fluorescence instruments used for HO2 detection to an interference from RO2 radicals and introducing a novel approach that enables HO2 and certain RO2 types to be selectively measured, Atmos. Meas. Tech., 6, 3425–3440, 2013.

Whalley, L. K., Stone, D., Dunmore, R., Hamilton, J., Hopkins, J. R., Lee, J. D., Lewis, A. C., Williams, P., Kleffmann, J., Laufs, S., Woodward-Massey, R., and Heard, D. E.: Understanding in situ ozone production in the summertime through radical observations and modelling studies during the Clean air for London project (ClearfLo), Atmos. Chem. Phys., 18, 2547–2571, 2018.