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Advancing our understanding of dimethyl sulfide oxidation products by field observations of formaldehyde and photolysis frequencies


Key facts

Type of research degree
Application deadline
Ongoing deadline
Country eligibility
International (open to all nationalities, including the UK)
Professor Dwayne Heard and Dr Lisa Whalley
<h2 class="heading hide-accessible">Summary</h2>

Air Quality, Atmosphere and Climate, Climate.

<h2 class="heading hide-accessible">Full description</h2>

<p>Dimethyl sulfide (DMS) from marine biogenic emissions is the largest natural source of sulphur in the atmosphere and a major aerosol precursor (Carslaw et al., 2010). With anthropogenic sources of SO2&nbsp;decreasing, knowledge of the fate of DMS is critical for understanding the formation of new sulphate aerosol particles, growth of existing aerosols to cloud condensation nuclei (CCN), and the burden/composition of aerosols in the remote marine environment.</p> <p>DMS is oxidised predominantly by hydroxyl radical (OH) either via an addition pathway producing methane sulfonic acid (MSA) (via DMSO), or via abstraction which forms a peroxy radical CH3SCH2OO (MSP) which, until recently, was thought to primarily undergo bimolecular reactions with NO or another peroxy radical. Following subsequent reactions of these DMS intermediates, SO2&nbsp;forms which, can either be oxidised by OH or undergo aqueous phase reactions in clouds to form H2SO4. The importance of DMS as a new particle source has previously been questioned, because the SO2+OH reaction is slow, and sulfuric acid (H2SO4) formed by this reaction tends to lead to growth of existing aerosols, rather than new particle formation. The recent discovery of a new molecule,&nbsp;hydroperoxy methylthioformate&nbsp;(HOOCH2SCHO; HPMTF) which is formed following the autoxidation of the peroxy radical, MSP, has highlighted, that there are uncertainties in our knowledge of the main pathways and products of DMS oxidation (Veres et al., 2020). HPMTF is now understood to be the major intermediate formed from the oxidation of DMS and the oxidation of HPMTF could lead to much faster rates of aerosol formation than the SO2+OH pathway (Veres et al., 2020).</p> <p><img alt="" decoding="async" sizes="(max-width: 748px) 100vw, 748px" src="" srcset=" 300w, 1024w, 768w, 1380w" /></p> <p>Figure 1. Chemical mechanism outlining our current understanding of the gas-phase oxidation of DMS by OH. Reactions in black represent the previously known DMS oxidation pathways in which peroxy radicals undergo a bimolecular reaction. Reactions in red highlight the new autoxidation route that forms HPMTF (figure taken from Ye et al., 2020).&nbsp;</p> <p>The step controlling the production of HPMTF is currently very uncertain yet is crucial to determining the yield of HPMTF. There is also little consensus about the fate and oxidation products of HPMTF. A fraction of HPMTF is likely oxidized by OH but photolysis is also another potential loss pathway (Khan et al., 2021).</p> <p>To address these key uncertainties in DMS oxidation, a large field and modelling project (Constraining the role of the marine sulfur cycle in the Earth System (CARES)) has been developed. As part of CARES, a comprehensive suite of measurements will be made on board a research ship and from the&nbsp;<a href="">FAAM aircraft</a>&nbsp;(fig. 2) flying off the west coast of Ireland. Of particular relevance to this studentship, actinic flux observations, coupled with absorption cross sections for the functional groups (aldehydic and hydroperoxide) present in HPMTF will be measured and will enable photolysis rates for HPMTF to be determined and the photolytic products to be derived. HCHO observations, coupled with HCHO photolysis frequencies and observations of a suite of VOCs, CH4, CO, NOx (from which &alpha;, the effective HCHO yield weighted over all OH reactions, can be derived) will provide a proxy for the OH concentration (Wolfe et al., 2019) and enable the loss of HPMTF via OH oxidation to be determined as well as the DMS oxidation rate via OH. Through these targeted field observations and subsequent modelling studies, an improved understanding of the role DMS plays in the marine sulfur cycle and the impact on the climate system will be achieved.</p> <p><img alt="" decoding="async" loading="lazy" sizes="(max-width: 534px) 100vw, 534px" src="" srcset=" 300w, 344w" /><img alt="" decoding="async" loading="lazy" sizes="(max-width: 426px) 100vw, 426px" src="" srcset=" 300w, 768w, 1024w" /></p> <p>&nbsp;</p> <p>&nbsp;</p> <p>&nbsp;</p> <p>&nbsp;</p> <p>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Figure 2. FAAM research aircraft&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Figure 3. Ground FAGE instrument</p> <p>&nbsp;</p> <h4>Objectives</h4> <p>Specifically, in this project you will:</p> <p>(1) Participate in the aircraft field campaign, operating the Leeds HCHO LIF (laser-induced fluorescence) instrument for the in situ measurement of HCHO from which OH concentrations can be derived, as well as a spectral radiometer for the determination of photolysis frequencies.</p> <p>(2) Perform analysis and interpretation of the field data collected from the aircraft to determine OH concentrations and photolysis rates of HPMTF and other relevant gas-phase photolysis rates. Using a box model based on the&nbsp;<a href="">Master Chemical Mechanism</a>&nbsp;the dominant loss processes for HPMTF (and subsequent product formation) as well as other short and intermediate lifetime gas-phase species will be investigated.</p> <p>(3) Depending on your research interests, get involved in the laboratory developments of a new HCHO laser system, new aircraft LIF instrumentation for the measurement of OH, peroxy radicals and OH reactivity or help develop new fieldwork instrumentation for the measurement of HO2&nbsp;reactivity.</p> <p>(4) Where opportunities arise, take part in ground-based field observations of radicals, OH reactivity, HCHO and photolysis frequencies from the FAGE mobile laboratory (fig. 3) to assess oxidation processes in different environments and the impact on air quality.</p> <p>&nbsp;</p> <h4>Training</h4> <p>The student will work under the supervision of&nbsp;<a href="">Dr Lisa Whalley</a>&nbsp;and&nbsp;<a href="">Professor Dwayne Heard</a>&nbsp;from the&nbsp;<a href="">School of Chemistry</a>&nbsp;at Leeds, who are all members of the&nbsp;<a href="">Atmospheric and Planetary Chemistry Group</a>. 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) as well as chemical modelling using both local &ldquo;box&rdquo; (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.</p> <p>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&nbsp;<a href="">National Centre for Atmospheric Science</a>&nbsp;(NCAS) and the FAGE instrumentation is part of the&nbsp;<a href="">Atmospheric Measurement and Observation Facility</a>&nbsp;(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. You will also have opportunities for training provided by the&nbsp;<a href="">National Centre for Atmospheric Science</a>&nbsp;such as the&nbsp;<a href="">Atmospheric Measurement Summer School</a>&nbsp;and other&nbsp;<a href="">courses</a>.</p> <p>&nbsp;</p> <h4>Student Profile</h4> <p>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.</p> <p>&nbsp;</p> <h4>References</h4> <p>Carslaw, et al., A review of natural aerosol interactions and feedbacks within the Earth system, Atmos. Chem. Phys. 10(4), 2010</p> <p>Heard D.E.; Pilling M.J. Measurement of OH and HO2 in the troposphere, Chemical Reviews, 103, 5163-5198, 2003</p> <p>Khan, et al., Impacts of Hydroperoxymethyl Thioformate on the Global Marine Sulfur Budget, ACS Earth and Space Chemistry, 2021</p> <p>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</p> <p>Veres, et al., Global airborne sampling reveals a previously unobserved dimethyl sulfide oxidation mechanism in the marine atmosphere, PNAS, 117(9), 2020</p> <p>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&ndash;2571, 2018</p> <p>Wolfe, et al., Mapping hydroxyl variability throughout the global remote troposphere via synthesis of airborne and satellite formaldehyde observations, PNAS, 116(23), 2019</p> <p>Ye et al., Organic Sulfur Products and Peroxy Radical Isomerization in the OH Oxidation of Dimethyl Sulfide, ACS Earth and Space Chemistry 2021 5 (8), 2013-2020</p>

<h2 class="heading">How to apply</h2>

<p>Formal applications for research degree study should be made online through the&nbsp;<a href="">University&#39;s website</a>. Please state clearly in the research information section&nbsp;that the research degree you wish to be considered for is Advancing our understanding of dimethyl sulfide oxidation products by field observations of formaldehyde and photolysis frequencies as well as&nbsp;<a href="">Dr Lisa Whalley</a>&nbsp;as your proposed supervisor.</p> <p>If English is not your first language, you must provide evidence that you meet the University&#39;s minimum English language requirements (below).</p> <p><em>As an international research-intensive university, we welcome students from all walks of life and from across the world. We foster an inclusive environment where all can flourish and prosper, and we are proud of our strong commitment to student education. Across all Faculties we are dedicated to diversifying our community and we welcome the unique contributions that individuals can bring, and particularly encourage applications from, but not limited to Black, Asian, people who belong to a minority ethnic community, people who identify as LGBT+ and people with disabilities. Applicants will always be selected based on merit and ability.</em></p>

<h2 class="heading heading--sm">Entry requirements</h2>

Applicants to research degree programmes should normally have at least a first class or an upper second class British Bachelors Honours degree (or equivalent) in an appropriate discipline. The criteria for entry for some research degrees may be higher, for example, several faculties, also require a Masters degree. Applicants are advised to check with the relevant School prior to making an application. Applicants who are uncertain about the requirements for a particular research degree are advised to contact the School or Graduate School prior to making an application.

<h2 class="heading heading--sm">English language requirements</h2>

The minimum English language entry requirement for research postgraduate research study is an IELTS of 6.0 overall with at least 5.5 in each component (reading, writing, listening and speaking) or equivalent. The test must be dated within two years of the start date of the course in order to be valid. Some schools and faculties have a higher requirement.

<h2 class="heading">Contact details</h2>

<p>For further information please contact Dr Lisa Whalley:&nbsp;<a href=""></a></p>