Understanding atmospheric oxidation in the marine tropical Atlantic.

Project summary 

Methane (CH₄) and ozone (O₃) are two key greenhouse gases in the Earth’s atmosphere. Methane is emitted by natural and human activity and removed by the hydroxyl (OH) radical — a process that can also produce O₃. These three species are therefore tightly linked through the atmosphere’s oxidation chemistry. Predicting their future concentrations, and thus the pace of climate change, requires a detailed understanding of this chemistry in the present day. The most chemically active region of the global atmosphere is the tropical marine boundary layer, yet our understanding here remains poor owing to a lack of detailed observations. Key open questions include:

• What is the role of chlorine (Cl) radical chemistry in oxidising methane?
• How important is nitrous acid (HONO) production from aerosol nitrate photolysis in driving the local oxidising environment?
• Are there undiscovered sinks for OH that would extend the atmospheric lifetime of methane?
• How do halogen oxides (IO, BrO) modulate ozone and OH concentrations?

The NERC-funded TropOx campaign will deploy a world-leading suite of atmospheric instruments at the Cape Verde Atmospheric Observatory (CVAO) on São Vicente, Cabo Verde, to address these questions directly. Teams from the Universities of York, Leeds and Manchester will build on the CVAO’s decade-long measurement record to provide a significantly more comprehensive dataset spanning gas-phase radicals, volatile organic compounds, aerosol composition, and halogen species. The student will join the field team at CVAO and then lead the analysis of these measurements within a global atmospheric chemistry model to evaluate and improve our understanding of tropical oxidation chemistry.

Background

The chemistry of the troposphere (the lowest 12 km of the atmosphere) is central to understanding our changing climate. Oxidation chemistry controls the concentration of key climate-active gases such as methane and ozone, as well as the formation of aerosol particles that influence how sunlight reaches the Earth’s surface and how clouds form. This chemistry is driven by sunlight and water vapour, so the tropical regions — subject to the most intense solar radiation and the highest atmospheric moisture — play a disproportionately important role. Since the marine environment covers around 75% of the tropics, understanding the photochemistry of the tropical marine boundary layer is essential to understanding the global atmosphere.

In some respects, the marine atmosphere is simpler than the terrestrial, but it is still highly complex. Ocean emissions — including sea salt, dimethyl sulphide (DMS), volatile organic compounds (VOCs), and iodine — as well as deposition of ozone (O₃), sulphur dioxide (SO₂), and organic compounds to the sea surface all modulate atmospheric composition. Long-range transport brings dust from the Sahara, biomass burning plumes from West Africa, and anthropogenic pollution from both hemispheres. Recent research has repeatedly uncovered chemistry that models had not previously captured — including novel halogen reactions, aerosol nitrate photolysis as a source of OH, and new DMS oxidation pathways — demonstrating that our understanding of this environment remains incomplete. Each gap in understanding contributes to uncertainty in predictions of methane lifetime, ozone burden, and future climate.

The project

The £2M NERC-funded TropOx campaign will deploy a state-of-the-art instrument suite at the Cape Verde Atmospheric Observatory (CVAO) on São Vicente, Cabo Verde, across two seasonal campaigns in 2027: autumn and winter. These contrasting seasons capture different atmospheric regimes — the summer campaign targets clean marine air and dust-influenced conditions to the west of the islands, while the winter campaign captures periods of intense Saharan dust outflow and long-range transport of biomass burning plumes from the Guinea Coast. Instruments deployed include the MR-CIMS (measuring halogen and nitrogen species, organic acids), THOr GC-MS (VOCs and halocarbons), LOKI-LIF (NO, NO₂ and oxidised nitrogen), AirFAGE (OH, HO₂, RO₂ radicals and OH reactivity), AMS and SP2 (aerosol mass and composition), and PALMS-NG (single-particle composition). Together these provide an unprecedented characterisation of the factors controlling oxidation chemistry in the tropical marine atmosphere.

The student will join the field team at CVAO for both campaigns, gaining hands-on experience in atmospheric measurement and in the logistical planning of a major field experiment. The primary research activity, however, will be the analysis of TropOx observations alongside long-term CVAO data and existing datasets (such as the NASA ATom global aircraft surveys) using the GEOS-Chem global atmospheric chemistry transport model.

GEOS-Chem (www.geos-chem.org) is a state-of-the-art global model used by hundreds of research groups worldwide, making it one of the most widely recognised and transferable tools in atmospheric chemistry. It encodes our understanding of atmospheric chemistry, transport, emissions, and deposition as computer code, enabling simulation of species including OH, O₃, and CH₄ across the globe. By ‘flying’ the model through the conditions observed at CVAO, the student will be able to evaluate directly where model predictions agree with observations and where they diverge.

A key methodological innovation of this project is an uncertainty-based approach to model evaluation. Every parameter in GEOS-Chem — chemical rate constants, photolysis rates, Henry’s law constants, emission factors, and more — carries an associated uncertainty. Rather than a single model run, the student will systematically explore how these uncertainties propagate through to the model’s predictions. For example, the rate constant for the reaction OH + CH₄ has a stated uncertainty of ~20%; the student will assess how varying this (and dozens of other parameters) within their known uncertainty ranges changes the model’s predicted OH concentrations relative to what was observed. This extends and builds directly on our previous work on rate constant uncertainties (doi.org/10.5194/acp-17-14333-2017).

The project will ultimately answer whether the tropical marine oxidation chemistry observed during TropOx is consistent with our current modelling framework within realistic uncertainties. Where the model and observations agree, the student will identify which parameters drive the largest remaining uncertainties, providing a roadmap for future experimental work. Where model and observations disagree, the student will diagnose the most likely missing processes and test candidate solutions. Finally, the improved model representation will be used to assess the sensitivity of future CH₄ and O₃ projections to these processes — directly informing the evidence base for climate policy.

The student

This project will suit students with a background in chemistry, physics, environmental science, engineering, mathematics, or any other natural science with a strong quantitative component. A 2:1 or above in a relevant undergraduate degree is typically required. The most important qualities are curiosity about the Earth’s atmosphere, the willingness to develop computational skills, and the ability to work both independently and as part of a large, multi-institution team.

The modelling work will primarily use Python for data analysis, visualisation, and scripting, alongside some Fortran for development within GEOS-Chem itself. Prior experience with either language is an advantage but is not essential — training will be provided and students from non-computing backgrounds regularly develop strong skills during the PhD. Some experience with any programming language (MATLAB, R, Julia, etc.) would be helpful. The student does not need a background in atmospheric chemistry: the necessary science will be learned during the PhD.

Training and research environment

The student will be supervised by Prof Mat Evans and Prof James Lee, and will be based in the Wolfson Atmospheric Chemistry Laboratories (WACL) in the Department of Chemistry at the University of York. WACL is one of the UK’s leading atmospheric chemistry research centres, with more than 65 researchers spanning stratospheric ozone, urban air quality, climate chemistry, personal exposure, and health. The student will join a vibrant community that includes regular group meetings, an active seminar programme with visiting national and international scientists, and strong links to NCAS, and the broader GEOS-Chem international community.

Within the Evans group specifically, the student will receive hands-on mentoring from their supervisors, alongside day-to-day support from current PhD students, postdoctoral researchers, and a dedicated research software engineer. Model-specific training is available through the GEOS-Chem documentation (geos-chem.readthedocs.io) and tutorial videos, supplemented by an active global user community forum.

The University of York and the YES•DTN doctoral training network together offer a comprehensive programme of professional development covering hard skills (data carpentry, scientific computing, statistics) and soft skills (scientific writing, presentation, science communication, and project management). The student will be supported and funded to present their work at national and international conferences throughout the PhD, and will be encouraged to engage in public outreach and policy communication activities.