Understanding Volatile Organic Compounds in the tropical Atlantic Ocean

Volatile organic compounds (VOCs) play a key role in important atmospheric chemical processes. They are precursors of ozone and secondary organic aerosols, both of which are responsible for negative effects on human health and the environment, while also having climatic consequences. VOCs undergo oxidation mainly by reaction with the hydroxyl radical (OH). OH is responsible for the removal of numerous pollutants in the atmosphere, initiating oxidation processes that usually transform airborne species into more oxygenated and, therefore, more water-soluble compounds. The reaction between VOCs and OH can effectively deplete OH and diminish the oxidative capacity of the atmosphere, thereby increasing the lifetime of pollutants and greenhouse gases such as methane. It is therefore crucial to understand the sources, transport and chemistry of VOCs, especially in tropical regions where OH levels are highest. It is currently known that there is a “missing” OH sink, primarily attributable to unknown organic compounds, in particular over tropical forests, in areas affected by biomass burning and in the remote tropical marine boundary layer.

In this project, measurements of a wide range of VOCs (including oxygenated and chlorinated compounds) will be made in the remote tropical North Atlantic Ocean and the Gulf of Guinea from the UKs large atmospheric research aircraft (www.faam.ac.uk). A new state of the art gas chromatography – mass spectrometer (GC-MS) will be flown on the aircraft during 2-week periods in mid-2027 (based in Cabo Verde) and early 2028 (based in Accra, Ghana). These are part of TropOx, a large multi-institution project that aims to understand multiple aspects of tropical tropospheric oxidation capacity. Flights will target: 1) regions affected by sea-salt, dust and a combination of both to measure sources and effects of Cl compounds and the impact of dust; 2) the vertical extent of oceanic oxygenated VOCs; 3) emissions from biomass burning. The GC-MS can measure 100s of discrete compounds and thus these measurements will provide a unique dataset to greatly improve our understanding of the sources, atmospheric processing and wider effect of VOCs in the region. Data will also serve as inputs and tests to detailed photochemical and regional transport models that will be run as part of the TropOx project.

In addition to the ambient measurements, a detailed set of calibration and characterization experiments will be carried out in the laboratory. Due to the extremely low concentrations of many of these species in the remote tropical atmosphere, a detailed calibrations are essential to deliver accurate quantitative data. Work will be carried out in collaboration with the University of Manchester, who will operate a Chemical Ionization Mass Spectrometer (CIMS) on the aircraft during the project, providing complimentary data that will also be used in the data analysis.

As well as the UK partners on the project, there will be an opportunity to work with colleagues at the National Centre for Atmospheric Research (NCAR) in Boulder, CO, USA.   At NCAR there is a coordinated effort on research in Africa, aligning our capabilities and expertise across the organisation. They carry out atmospheric chemistry modelling, with a focus on understanding VOC chemistry and OH, as well as understanding methane. The visit will take place after the data has been taken, providing a great opportunity for the student to discuss the results and put them in context of previous measurement and modelling studies in the region.

The student will work under the supervision of Prof. James Lee, Dr Jim Hopkins and Dr Steve Andrews and will be based at the Wolfson Atmospheric Chemistry Laboratories, part of the Department of Chemistry, University of York. Here, the student will develop transferrable skills in making atmospheric measurements using GC-MS, numerical and data skills associated with source apportionment, chemical reactivity and model/measurement comparison using a variety of statistical techniques. The student will also have access to training provided by the UK National Centre for Atmospheric Science such as the Introduction to Atmospheric Science course and Atmospheric Measurement Summer School on the Isle of Arran, and the Scientific Computing Course. They will have the opportunity to present their work to the scientific community at national and international meetings and conferences, and will also be encouraged to take part in outreach events in order to disseminate the research beyond the immediate scientific community (e.g. to policymakers and the general public).