Reactive organic compounds in the marine atmosphere have the potential to influence climate by changing the atmospheric oxidation capacity and by modifying the composition and size distribution of aerosol particles. These compounds are a sink for the hydroxyl radical (OH), which is the key atmospheric oxidant, responsible for regulating the lifetime of the powerful greenhouse gas methane (CH4). Formation of organic aerosol via oxidation of reactive organic species may contribute to cloud condensation nuclei (CCN) abundance in the marine atmosphere, and hence modify cloud brightness and lifetime. At the low CCN concentrations typical of the marine environment, cloud properties are highly sensitive to changes in CCN and respond non-linearly to aerosol concentrations. Due to the profound influence of marine stratocumulus clouds on global climate, and the extensive coverage of the oceans, there is high priority on improving understanding of the sources and sinks of reactive organic species in the remote marine atmosphere.
The oxygenated volatile organic compounds (oVOCs) glyoxal (CHOCHO) and acetaldehyde (CH3CHO) are ubiquitously present in the remote atmosphere. Globally, sources of these compounds are dominated by terrestrial emissions from anthropogenic and biogenic sources, and secondary atmospheric production from emitted precursor hydrocarbons. Both species have the potential to alter the atmospheric oxidative capacity, and glyoxal is a known precursor to organic aerosol formation. The atmospheric distributions of these compounds are highly uncertain, and current models are unable to explain limited observations of enhancements in both species in the atmosphere remote from the continents. Acetaldehyde has been shown to have a source from surface waters of the ocean, which increases its abundance in the remote surface atmosphere. There is some evidence that glyoxal may be produced from processes occurring in the organic microlayer on the ocean surface, however the prevalence and importance of such a source is currently unknown. Recent research in Leeds has suggested that acetaldehyde may be source of glyoxal in the remote atmosphere, although there are large uncertainties due to deficiencies in our knowledge of the mechanism for acetaldehyde photochemical oxidation.
Figure: Physical sea-air exchange and chemistry of volatile organic compounds (VOCs) and oxygenated volatile organic compounds (oVOCs) in the remote marine atmosphere. Figure from Yu and Li, (2021).
Aims, Objectives and Methods
This aim of this project is to improve our understanding of sources of acetaldehyde and glyoxal in the remote marine atmosphere, and their impacts on global-scale atmospheric composition and climate, through exploitation of novel atmospheric modelling and laboratory measurements. The student will work jointly between the School of Earth and Environment and School of Chemistry in Leeds, and have an opportunity to work with a global Earth system model and to be involved in laboratory experiments focussed on elucidating mechanisms for glyoxal production in the marine environment. Specific objectives of the project are to:
- Use a global Earth system model to investigate sensitivity of remote marine glyoxal abundance to acetaldehyde sources (ocean emissions, transport from continents, atmospheric production) and assumptions regarding acetaldehyde oxidation chemistry.
- Carry out laboratory experiments to quantify sources of oVOCs from sea surface microlayer proxies and samples, as a function of light intensity, ozone abundance, relative humidity, and to investigate the role of photosensitisers
- Develop parameterisations for inclusion of sea surface oVOC sources into the model based on laboratory measurements made during the project.
- Evaluate global distributions of glyoxal and acetaldehyde using the model, available in situ surface & aircraft measurements, and satellite measurements.
- Use the model to quantify oVOC impacts on oxidative capacity, methane lifetime, and marine CCN.
The project will use the Community Earth System Model (CESM) v2.2, which includes a detailed tropospheric chemistry scheme as well as an air-sea exchange scheme for consideration of ocean-atmosphere fluxes of oVOCs. In the laboratory an aerosol flowtube equipped with a sensitive laser-based detector will be used to quantify heterogeneous sources of oVOCs and reactive intermediates.
Research Training Environment
The student will benefit from training in expertise in numerical atmospheric chemistry-climate modelling, collaborating with partners from the US National Center for Atmospheric Research, analysis of satellite data, and in laboratory-based physical chemistry. The balance between modelling work and laboratory work in the project can be tailored to suit the interests of the student. The student will join the active Biosphere-Atmosphere (BAG) group in SEE, alongside ~20 PhD students and postdoctoral researchers working on atmospheric composition modelling, air quality, and biosphere interactions. The laboratory work will be taken within the Atmospheric and Planetary Chemistry research group within the School of Chemistry, which is active in fieldwork, laboratory studies and modelling, and which is supported by the National Centre for Atmospheric Science (NCAS, https://ncas.ac.uk/). For more information about our research and recent publications, see: https://environment.leeds.ac.uk/see/staff/1135/dr-steve-arnold, https://eps.leeds.ac.uk/chemistry/staff/4175/professor-dwayne-heard- . We encourage interested applicants to contact our research groups to discuss the project and our research further.