Atmospheric chemistry of Criegee intermediates: Kinetics and reaction mechanisms

Poor air quality represents the greatest environmental risk to public health in the UK (DEFRA, 2017), and causes over 4.2 million premature deaths worldwide, costing the UK economy ~£15 billion. The properties and impacts of our air are governed by atmospheric composition, the understanding of which is vital to the development of policies aimed at improving air quality and requires knowledge of the chemistry of reactive trace species.

Criegee intermediates are an important class of reactive intermediates produced in the atmosphere following the oxidation of unsaturated volatile organic compounds by ozone (Figure 1). Criegee intermediates react rapidly in the atmosphere, with potential impacts on the oxidising capacity of the atmosphere and formation of sulfate aerosol and secondary organic aerosol (Johnson and Marston, 2008; Khan et al., 2018). However, detailed assessments of the role and impacts of Criegee intermediates in the atmosphere are hindered by a lack of information regarding the production and subsequent chemistry of Criegee intermediates.

Figure 1: Production of Criegee intermediates in the atmosphere and their potential fates. Reaction kinetics and products of stabilised Criegee intermediate reactions are largely unknown and hinder our understanding of their impacts on atmospheric composition, air quality and climate.

A particular challenge since the proposal of the existence of Criegee intermediates in 1949 (Criegee & Wenner, 1949) has been their direct measurement owing to high reactivities and thus low concentrations. The advent of photolytic sources for Criegees, reported in 2012 (Welz et al., 2012), has facilitated laboratory studies of CIs and has enabled greater understanding of the chemistry and properties of Criegees, but there are still significant uncertainties regarding the kinetics and mechanisms of Criegee intermediate reactions under atmospherically relevant conditions.

Recent work in the Stone group has used photolytic methods to generate the Criegee intermediates CH2OO and CH3CHOO in the laboratory (Reactions R1-R4) coupled with a range of spectroscopic techniques to enable direct monitoring of either the Criegee intermediate or reaction products to determine the reaction kinetics and product yields (Stone et al., 2013; Stone et al., 2014; Stone et al., 2018; Mir et al., 2020; Onel et al., 2020).

(R1) CH2I2 + hν → CH2I + I

(R2) CH2I + O2 → CH2OO

(R3) CH3CHI2 + hν → CH3CHI + I

(R4) CH3CHI + O2 → CH3CHOO

These experiments have involved the development of time-resolved broadband UV absorption spectroscopy to monitor Criegee intermediates in real-time during the course of a reaction (Figure 2) and high-resolution time-resolved quantum cascade laser IR absorption spectroscopy to identify and monitor reaction products.

Figure 2: Time-resolved UV absorbance spectra used to monitor the Criegee intermediate CH2OO in real-time during its reaction with SO2. The Criegee intermediate is produced via photolysis of CH2I2/O2, with the maximum absorbance signal observed immediate following photolysis. The decay in absorbance is used to determine the reaction kinetics for CH2OO in the system (Mir et al., 2020).

We have also recently developed a novel technique to enable direct quantitative time-resolved measurements of Criegees produced in ozone-alkene reactions. This technique, based on cavity enhanced UV absorption spectroscopy, has been coupled to the atmospheric simulation chamber in the School of Chemistry which is also equipped with a wide range of instruments for the simultaneous measurement of reactants, free radical intermediates and stable products, and as such is ideally suited to investigate complex reaction mechanisms under atmospheric conditions. The development and subsequent application of the cavity enhanced system, coupled with existing capabilities in Leeds, will significantly enhance our understanding of the atmospheric production, chemistry, and impacts of Criegee intermediates, leading to improved understanding of atmospheric composition, air quality, and climate.

Objectives

You will develop capabilities for direct and simultaneous monitoring of reactants and products in the laboratory during the course of reactions taking place on microsecond timescales in real-time and apply this to studies of key atmospheric reactions of Criegee intermediates. You will apply this to investigations of the kinetics of Criegee reactions with water vapour, and of the atmospheric formation and impacts of Criegee-water complexes. You will also develop the experimental technique based on cavity enhanced UV absorption spectroscopy to explore Criegee intermediates directly, and quantitatively, in ozone-alkene reactions. You will apply this technique, and others including laser-induced fluorescence, in the atmospheric simulation chamber in Leeds in experiments to determine the yields and chemistry of reactive species including Criegee intermediates, hydroxyl radicals and peroxy radicals for a range of ozone-alkene reactions. This work will provide valuable information on the kinetics and mechanisms of Criegee intermediates for assessments of atmospheric composition, air quality and climate and links to the NERC objectives related to atmospheric pollution and human health, clean air, and climate.

You will have opportunities to develop skills in experimental design, kinetics, spectroscopy, atmospheric modelling and data analysis. You will develop experimental methodology that can be applied to a wide range of problems including those in atmospheric chemistry, combustion chemistry, astrochemistry, and materials chemistry.

Potential for high impact outcome

The role of chemistry in controlling atmospheric composition is of fundamental importance to our understanding of air quality and climate change. This work will provide valuable measurements of reaction kinetics and product yields to improve our understanding of atmospheric composition and chemistry, providing greater constraints on model calculations of global oxidising capacity and production of secondary organic aerosol. It is anticipated that this project will generate several papers, with potential for publication in high impact journals.

Training

The student will work under the supervision of Dr Daniel Stone, Professor Dwayne Heard and Professor Paul Seakins within the Atmospheric and Planetary Chemistry group in the School of Chemistry at the University of Leeds. You will be supported by a range of supervisions from monthly meetings and group presentations, through to daily informal chats with supervisors. 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 is part of the Atmospheric Measurement Facility, 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 numerical models and chemical mechanisms, including the Master Chemical Mechanism (MCM, Saunders et al., 2003; Jenkin et al., 2003). Activities in these three areas are intimately linked and interdependent, providing significant advantages. You will be supported to attend both national and international conferences, and will receive a wide range of training, for example in communication skills, project management, and with other technical aspects (for example LabView and computing). The PhD will provide a wide range of experience in the use of high power lasers, vacuum systems, optics, computer controlled data acquisition systems and methods in numerical modelling. You will also have access to training provided by the National Centre for Atmospheric Science. The successful PhD student will have access to a broad spectrum of training workshops that include managing your degree and preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

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

References

Criegee & Wenner, Liebigs Ann. Chem., 564, 9-15, 1949

DEFRA, Department for Food and Rural Affairs, Air Pollution in the UK 2016, September 2017

Johnson and Marston, Chem. Soc. Rev., 37, 699-716, 2008

Khan et al., Environ. Sci. Processes Impacts, 20, 437-453, 2018

Mir et al., Phys. Chem. Chem. Phys., 17, 9448-9459, 2020

Onel et al., J. Phys. Chem. A, 124, 31, 6287-6293, 2020

Stone et al., Phys. Chem. Chem. Phys., 15, 19119-19124, 2013

Stone et al., Phys. Chem. Chem. Phys., 16, 1139-1149, 2014

Stone et al., Phys. Chem. Chem. Phys., 20, 24940-24954, 2018

Welz et al., Science, 335, 204-207, 2012