Skip to main content

Hydrocarbon Oxidation Mechanisms Elucidated from Radical scavenging

Periodic Reporting for period 1 - HOMER (Hydrocarbon Oxidation Mechanisms Elucidated from Radical scavenging)

Reporting period: 2016-04-04 to 2018-04-03

It is estimated that air pollution causes 3.3 million premature deaths a year. It is therefore difficult to overestimate the impact of air pollution on society. Future projections of increasing urbanisation in many countries means that human populations will become increasingly affected in future years.
The objectives of this project were to develop experiments for elucidating hydrocarbon oxidation mechanisms, using various analytical methods, such as gas chromatography (GC) and laser techniques, and applying them to reaction mechanisms to explain how these reactions occur under atmospheric conditions. Furthermore, with the computational and atmospheric chemical modelling expertise in the group, it was possible to address several other questions, e.g. can these observations be explained with our current theoretical understanding of reaction mechanisms? and what are the global consequences of these types of chemical reactions?
1.Develop an experimental flow reactor for harvesting alkyl radicals formed in hydrogen abstraction reactions, associated analytical instrumentation and a detailed chemical model that can describe the effects of impurities upon reaction mixtures: A detailed chemical model was built in the initial stages of this project in order to identify the experimental limitations of the approach, e.g. the maximum concentration of impurities such as O2 that can be tolerated, and the concentrations and reaction times that would be optimum. Following this, an analytical sampling approach was developed. This consisted of refurbishing/ upgrading a system that can be used for concentrating a compound of interest from a dilute sample, extracting those compounds that will interfere with the analytical approach (mainly H2O), and delivering a sharp pulse of analyte into a GC with an electron-capture detector. Other tasks were producing the reactor and making the apparatus extremely vacuum tight, such that residual O2 could be effectively removed and that O2 leaking in from the atmosphere would have a negligible effect upon reactor chemistry.
2.Develop experiments for determining the fate of stabilised Criegee intermediates (CIs) with respect to several atmospheric gases, and understanding the global significance of these reactions: An existing experimental method was upgraded to afford measurements as a function of temperature, several systems were studied including different CIs with trifluoroacetic acid, methanol, ethanol, hydrogen sulphide and methyl mercaptan. This work provided the opportunity to mentor the 1st masters student associated with this project. These activities have produced two publications, with several others planned.
3.Develop methods for determining the fate of excited CIs and their effects in the atmosphere. Commissioning a chamber apparatus and developing a technique for quantifying the production of extremely long-lived compounds that are generated when commercial compounds (HFOs) react with ozone in the atmosphere. A series of kinetic experiments were conducted on several HFOs. Given the extremely slow reactions, it was necessary to employ new experimental techniques to reduce the effects of unwanted byproducts. These include stabilised CIs, and using kinetic data from phase 2 enabled the design of experiments where CIs were converted into a less reactive form. This provided the opportunity to train an EPSRC intern and the 2nd masters student of this project. One paper describing this work is being written up.
4.Communicating scientific ideas and results. Managing 3 undergraduate research projects, providing talks in student-/early career-led fora; providing seminars at UK universities (Cambridge, Leicester, Bristol) and NCAR, CO, USA. Two posters were presented, one at EGU, Vienna, Austria in 2017, and one at the Faraday Discussions, York, UK in 2017. I was able to reach out to development scientists at Honeywell, the corporation responsible for the development of HFOs and provided them with a lab tour and a summary of our findings. Furthermore, I was invited to become a participant of an international panel of experts on structure-activity relationships and reaction mechanisms, this is organised under the auspices of the CRC and serves the atmospheric science community by providing articles, datasets and other resources, enabling the prediction of many kinetic parameters that have yet to be measured.
Reactions measured in phase 2 are the first such data. Measurements are supported by various theoretical methods. Firstly, theory identified probable products of these reactions (AAAHs). A second theoretical calculation estimated the photochemical behaviour of AAAHs. Thirdly, chemical modelling determined the distribution of AAAHs in the atmosphere. Through this comprehensive approach we were able to highlight a large source of AAAHs in the world's forests, and that AAAHs can react further by photolysis, oxidation and aerosol uptake. This benefits society by improving knowledge of the natural world, and may increase understanding of which molecules contribute towards aerosol formation. This work resulted in a research article with gold open access, furthermore, the dataset is hosted on The UoB's data servers (publicly available) so that any future experiments can be compared in much more detail, an overlooked area in kinetic studies, where it is common to present only rates, without raw data, experimental conditions or fitting algorithms. This hampers efforts to explain discrepancies. One societal benefit of this proactive approach is that it may encourage other researchers to provide datasets, allowing future studies to be compared much more thoroughly.
Experiments in phase 3 were superior to current methods. Our control on the secondary chemical reactions was better than other studies because we had a better grasp of the reaction rates affecting these systems. The kinetic measurements of phase 2 afforded the design of experiments where CIs were removed from the system, whilst also scavenging OH radicals. This provided a reliable kinetic dataset. Experiments were then adapted to determine yields of long-lived fluorocarbons from these reactions, by interfacing a Medusa GC. This system has almost unrivalled precision and accuracy in determining fluorocarbon concentrations, and is far beyond what is available to other kinetic studies. This approach afforded the observation and quantification of a fluorocarbon (HFC-23) that originates from ozonolysis of certain HFOs. The results of this work were used to estimate the effect this chemistry has on the global warming potentials of these compounds. Furthermore, theoretical work has also been conducted, allowing us to understand the mechanism by which these long-lived compounds are formed. This work is currently being written up and is expected to result in a high-profile publication. Phase 3 can impact society by informing the design of fluorocarbons with the lowest environmental footprint, and therefore could result in a reduction in global warming. Economically, understanding the chemistry that makes these long-lived compounds will allow manufacturers to make better decisions on which fluorocarbons to produce, avoiding the potential of bringing a chemical to production, only to find that it results in a banned compound over its life-cycle. Such outcomes would be costly for any chemical manufacturer, and with our insights, this scenario can be avoided.
A summary image, showing some of the important components of this project