Chemosensors for Atmospheric Peroxyl Radical Investigations (CAPRI)

This is a project to study of atmospheric free-radical reactions in laboratory experiments at the University of York Department of Chemistry (YDC).

Atmospheric chemistry lies at the heart of a wide range of environmental issues, which have substantial societal and economic impacts. Whether it is a changing climate, a reduction in air quality affecting human health or the degradation of ecosystems due to air pollution, the details of this chemistry determines the severity of the impacts. Many thousands of volatile organic compounds (VOCs) are emitted to the atmosphere from both natural and human activities. Free-radicals such as hydroxyl (·OH) initiate VOC breakdown (Figure 1) leading to the formation of peroxyl radicals (HO2· and organic peroxy radicals, RO2·) [1,2]. RO2• are the key intermediates in subsequent oxidation steps, directly involved in the formation of ground-level ozone, photochemical smog and the production of secondary organic aerosols – significantly impacting upon air quality and climate [1,2].

Figure 1: Simplified schematic highlighting the atmospheric free-radical propagated cycle for OH initiated VOC oxidation, which illustrates the fast photochemical processes that form ozone and oxygenated products. Peroxy radical intermediates are highlighted in the red boxes [2].
Radical chemistry thus controls the oxidation capacity of the atmosphere, the yields of the most harmful secondary pollutants and hence the severity of the environmental impacts. Computer models are used to simulate this complex chemistry and allow us to predict and mitigate air quality problems. It is of critical importance therefore that the detailed atmospheric chemical mechanisms used in such models accurately and adequately represent the most important processes. However, recent results from laboratory, field and theoretical studies demonstrate that RO2· chemistry is poorly understood, for even the most important atmospheric species [1-6]. Improved lab-based experiments are urgently needed, but to date, radical measurements have posed considerable challenges to analytical chemistry. Owing to their low concentrations, high reactivity, and short lifetimes, radicals cannot easily be sampled. Although a number of highly sensitive techniques have been developed [5], selectivity, full structure determination, portability and cost remain challenging obstacles to atmospheric radical analysis.

Figure 2a: A generic radical (R.) reacts with a “chemosensor” species to eliminate the stabilised radical leaving group (TEMPO: (CH2)3(C(CH3)2)2NO). The original radical structure is maintained in the ‘trapped’ molecule (in red box) to be detected off line using mass spectrometry
Figure 2b: Example use of chemosensors to identify the two main RO2• formed following OH + n-nonane (in the presence of O2). Trapped products are again highlighted in red boxes

This project builds upon previous work to develop a series of novel radical trapping compounds (or “chemosensors”) that efficiently and selectively react with gas-phase RO2·, producing non-radical reaction products that conserve the structure of the original radical, allowing off-line analysis using a range of mass spectrometric techniques (Figure 2a). This approach allows accurate determination of the radical structures (Figure 2b). In this project, we will incorporate these chemosensor traps into afast-flow reactor (Figure 3) for studies of RO2· chemistry in a range of important atmospheric reactions (Figure 1). We will target a series of important atmospheric RO2·, including chlorinated RO2·, for which virtually nothing is known. Science outcomes of this project will be key to answering big atmospheric questions such ashow 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 actually need?

Figure 3: Schematic of the fast flow apparatus set up. Radical precursor species (e.g. H2O/O2, Cl2) pass over the photolysis lamp to generate primary radicals (e.g. •OH, Cl•) that can react with the chosen VOC of interested, rapidly forming RO2•/HO2• species. Flow tube reaction times prior to capture by the chemosensor may be varied from tens of milliseconds to many seconds. The flow tube system can also be connected to online detection systems such as a Chemical Ionisation Mass Spectrometer (CIMS).

Objectives

You will work closely with an interdisciplinary team of leading atmospheric, gas-kinetic and free-radical scientists in York. Expert supervision will ensure appropriate support and guidance. As the project progresses you will:

  • Couple chemosensor “trapping” to the fast-flow apparatus (Figure 3) for characterisation experiments on simple chemical systems e.g. CH3O2 + CH3O2·.
  • Use mass-spectrometry methods to detect, identify and quantify new target RO2 species.
  • Use the fast-flow apparatus (Figure 3) to determine rate coefficients and product yields for target RO2 reactions, for example:
    • Systematic experiments of structurally diverse alkene ozonolysis systems, which are known to be important atmospheric sources of ·OH, HO2 and RO2· [6]
    • OH abstraction reactions of long chain alkanes in order to look at the formation of primary RO2 species and the branching ratio of secondary/tertiary RO2 species formed via hydrogen shift reactions (“autooxidation” [4]) (see Figure 1b).
  • further explore reaction mechanism pathways, e.g. via isotopic labelling experiments.
  • use chemical box models incorporating the Master Chemical Mechanism (mcm.york.ac.uk) to design and optimise experiments for the target atmospheric RO2· species.

Potential for high impact outcome

Radical chemistry impacts on important subjects such as air quality, composition and climate. Previous work resulted in multiple high-impact publications, presentations to international conferences and stimulated new collaborative research worldwide [1-7]. This project aims to investigate a range of important atmospheric chemistry processes through the accurate detection and speciation of low concentration, highly reactive gas-phase radicals. The chemosensors have been used elsewhere (e.g. heterogeneous catalysis [7]) but represent a new approach to the detection of atmospheric radicals.  We therefore envisage a range of high impact publications stimulating interest from across the atmospheric science community.

You will work under the supervision of Dr. Terry Dillon, Dr Andrew Rickard and Dr. Victor Chechik at University of York Department of Chemistry (YDC). You will be based in the Wolfson Atmospheric Chemistry Laboratories (WACL), a unique facility bringing together experts in atmospheric measurements, Earth system models and laboratory chemistry to form the largest integrated UK atmospheric research team. Dr Dillon has a wealth of experience of studying RO2 reactions in the laboratory. Dr Rickard has 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 (mcm.york.ac.uk). Drs Dillon, Rickard and the wider WACL team will provide comprehensive training in all kinetic techniques and instrumentation required. Dr Chechik is an expert on organic free-radical chemistry and recently developed bespoke radical chemosensor suitable for the stabilisation, identification and quantification of gas-phase free-radicals – a novel tool for the study of otherwise elusive atmospheric RO2. You will develop transferrable skills in design and preforming fast flow gas-phase kinetic experiments, spectrometric characterisation techniques (e.g., Chemical Ionisation Mass-Spec (CIMS), HPLC with mass spectrometry), chemical mechanism development and evaluation, numerical / data skills and kinetic model analysis. Training will be provided in all areas, and we expect to establish international collaborations with a number of colleagues in the area of chemical kinetics and mechanism development.

The University of York and the wider PANORAMA 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 you will have access to the wider resources 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 scientific background (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 is highly interdisciplinary and encompasses several different science and technology areas. However the York team is well supported with experienced scientists and technical support; all training will be provided, and no previous experience with specific techniques or instruments is necessary.

References

  1. Fuchs et al.,Experimental evidence for efficient hydroxyl radical regeneration in isoprene oxidation”, Nature Geos., 2013, 6, 1023
  2. Orlando and Tyndall (2012): Laboratory studies of organic peroxy radical chemistry: an overview with emphasis on recent issues of atmospheric significance. Chem. Soc. Rev., 41, 6294–6317. DOI: 10.1039/c2cs35166h.
  3. Winiberg et al., (2016) Direct Measurements of OH and Other Product Yields from the HO2 + CH3C(O)O2 Atmospheric Chemistry and Physics, 16, 4023-4042, doi:10.5194/acp-16-4023-2016.
  4. Crounse et al., (2013): Autoxidation of Organic Compounds in the Atmosphere. Phys. Chem. Lett., 4, 3513-3520. dx.doi.org/10.1021/jz4019207.
  5. Whalley et al., (2013): 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, Meas. Tech., 6, 3425-3440, doi:10.5194/amt-6-3425-2013.
  6. Alam et al., (2013): Radical Product Yields from the Ozonolysis of Short Chain Alkenes under Atmospheric Boundary Layer Conditions. Phys. Chem. A., 117 (47), 2468–12483. DOI: 10.1021/jp408745h.
  7. Conte and Chechik (2010), Spin trapping of radical intermediates in gas phase catalysis: cyclohexane oxidation over metal oxides, Commun., 46, 3991-3993. 10.1039/c0cc00157k.