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Peroxyl Radicals - Trapping and Laser Studies

Dr Terry Dillon (YDC), Dr Andrew Rickard (YDC), Dr Victor Chechik (YDC)

Contact email: terry.dillon@york.ac.uk

This is a project to study reactions of atmospherically important peroxy radicals (RO2) in lab-based experiments at the York Department of Chemistry (YDC). Free radicals initiate breakdown of the majority of volatile organic compounds (VOC) emitted to the atmosphere. Understanding radical chemistry is crucial therefore to calculate VOC lifetimes, assess impacts of VOC degradation chemistry on air quality, composition and climate, and confidently predict the impact of future emission scenarios. Recent results from laboratory, field (Fig. 1) and theoretical studies demonstrate that under relatively clean-air conditions, radical chemistry is poorly understood, for even the most important atmospheric oxidant, OH [1,2].

Radicals such as OH are consumed as they initiate the breakdown of VOC (1). Key intermediates in subsequent oxidation steps (reactions 2-3) are RO2. Their reactions (reaction 3, where X = NO, HO2, RO2) lead to a variety of products including closed-shell peroxides, carbonyl-containing VOC, O3, HO2 and the primary oxidant OH.

·OH + VOC

R· + H2O

(1)

R· + O2

RO2·

(2)

RO2· + X·

(products, including ·OH)

(3) 

Figure 1– radical measurements over rainforests suggest an unknown radical recycling role for RO2 derived (R1-R2) from isoprene (C5H8).

RO2 chemistry therefore controls the rates of radical termination and propagation in the atmosphere, and in other low-temperature oxidation environments (e.g. biofuel combustion). A detailed knowledge of RO2 reaction kinetics and products is crucial therefore to understand diverse subjects such as air pollution, climate change, fuel efficiency and impacts of vehicle emissions on air quality and health. Previous work identified significant OH recycling mechanisms for the breakdown of carbonyl-containing VOC, many of which are found in the atmosphere 1. In this project, we will target a series of important atmospheric RO2, including chlorinated RO2, for which virtually nothing is known. Laser-based experiments will be used for direct measurements of (reactions 2-3) kinetics and products, alongside the identification, speciation and quantification of different RO2 in a fast-flow reactor equipped with novel organic radical traps.

Science outcomes of this project will be key to answering big atmospheric questions such as “how is atmospheric oxidation power maintained in the tropics?”, “are intramolecular reactions important?”, “what is the role of chlorine in radical recycling chemistry?”, and “what level of detail do atmospheric chemical model mechanisms such as the Master Chemical Mechanism 3 actually need?” 

Objectives

You will work with leading atmospheric, laser kinetics and free-radical scientists in York. Expert supervision will ensure appropriate support and guidance. As the project progresses you will:

  1. use lasers for the generation of target RO2 and direct detection of OH, and hence determine rate coefficients and OH product yields for RO2 reactions (R2-R3)
  2. explore fundamental RO2 reaction mechanisms, e.g. via isotopic labelling experiments
  3. use fast flow radical-trap experiments, with analysis by modern mass-spectrometry methods to detect, identify and quantify new target RO2
  4. incorporate results into the Master Chemical Mechanism 3, and quantify atmospheric impacts using photochemical box models.

Potential for high impact outcome

Radicals control the removal and transformation processes for most pollutants in our atmosphere; radical chemistry impacts on important subjects such as air quality, composition and climate. Previous work on these issues resulted in multiple high-impact publications [1,5], presentations to international conferences and stimulated new collaborative research worldwide [2,6].

Training

Figure 2 – schematic of the YDC laser apparatus. RO2 are generated via 355 nm pulsed photolysis from laser B, with OH products detected via 282 nm laser induced fluorescence from laser A.

You will work under the supervision of Dr. Terry Dillon, Dr Andrew Rickard and Dr. Victor Chechik at the University of York Department of Chemistry (YDC). Dr Dillon and Dr Rickard are based in the Wolfson Atmospheric Chemistry Laboratories (WACL), a new facility bringing together experts in atmospheric measurements, Earth system models and lab-studies to form the largest integrated UK atmospheric science research team. This project provides a high level of specialist scientific training in: experimental kinetics and photochemistry, notably use of lasers and laser safety; data analysis; mass-spectrometry; computational methods (GAUSSIAN, MCM & AtChem online box modelling). Dr Dillon has a wealth of experience studying RO2 reactions, mechanisms and products, and together with the WACL team will provide comprehensive training in all kinetic techniques and instrumentation required. Dr Rickard has research interests that span mechanistic chemistry of complex gas- and condensed- phase systems, kinetic modelling of complex processes and the chemistry of reactive radical intermediates.  He currently curates the internationally renowned Master Chemical Mechanism. Dr Chechik is an expert on organic free-radical chemistry and has recently developed (along with Rickard) bespoke radical “traps” suitable for the stabilisation, identification and quantification of gas-phase free-radicals – a novel tool for the study of otherwise elusive atmospheric RO2.

This studentship is offered as part of the Leeds/York Chemistry SPHERES Doctoral Training Programme that will provide training in addition to that offered by YDC.  Courses specifically aid your development throughout the PhD, improving transferable skills, putting research into a wider scientific context and preparing for thesis presentations and viva. The University of York and the wider NERC SPHERES DTP provide comprehensive training programmes for students throughout their PhD studies, with a range of courses on both hard and soft skills (e.g. improving transferable skills, putting research into a wider scientific context and preparing for thesis presentations and viva).  Dr Rickard also works for and with the National Centre for Atmospheric Science (NCAS), and thus the student will have access to the wider resources that NCAS provides. You will also have access to training provided by NCAS such as the Arran instrumental Summer School, the Earth System Science Summer School (ES4), and future further developments in computations and data analysis.

Student profile

You will have a strong background in the physical sciences (good degree in chemistry, physics, engineering or similar), a keen interest in environmental issues, and an aptitude and enthusiasm for experimental work. We appreciate that this project encompasses several different science and technology areas, however the York team is well supported with experienced scientists. Training is part of the PhD, and no previous experience with specific techniques or instruments is necessary.

References

1. Lelieveld et al., “Atmospheric oxidation capacity sustained by a tropical forest”. Nature, 2008, 452, 737–740

2. Hofzumahaus, et al., “Amplified Trace Gas Removal in the Troposphere”, Science, 2009, 324, 1702

3. http://mcm.leeds.ac.uk/MCM/          

4. https://www.york.ac.uk/chemistry/research/wacl/

5. Taraborrelli et al. “Hydroxyl radical buffered by isoprene oxidation over tropical forests”. Nature Geoscience, 2012, 5, 190–193

6. Fuchs et al., “Experimental evidence for efficient hydroxyl radical regeneration in isoprene oxidation”, Nature Geos., 2013, 6, 1023

Related undergraduate subjects:

  • Atmospheric science
  • Chemistry
  • Natural sciences
  • Physics