Understanding the atmospheric composition over the Southern Ocean.
Summary
The Southern Ocean surrounds Antarctica and covers a vast area. The air over the ocean is unique. The high windspeeds means that there is significant ocean-atmosphere exchange moving atmospheric species into the ocean and emitting oceanic species into the atmosphere. Far from major human activities, the atmosphere over the ocean is as close to being “pristine” as anywhere on Earth – offering a laboratory for the atmospheric chemistry occurring before the industrial revolution and potentially into the future. The region edges onto Antarctica in the south, with its unusual atmosphere-cryosphere interactions and onto South America, Southern Africa, and Oceania in the north with its anthropogenic, biomass burning and natural sources. Thus, the Southern Ocean represents one of the cleanest marine areas in the world, a region with enormous air-sea exchange and the meeting point of several usual sources into the atmosphere. It offers a unique challenge to our understanding of atmospheric chemistry and is likely to throw up new challenges and opportunities for scientific exploration.
Only relatively sparse observations are available of the composition over the area. It is a long way from highly studied Northern Hemisphere population centres, mainly covered in ocean and the climate is often inhospitable. This offers challenges and opportunities. However, there is evidence that the atmosphere of this region has been changing over the last decades (Kumar et al., 2021).
This project will use the available observations from surface sites, aircraft and satellites coupled to a chemistry transport model to explore the atmospheric composition of the region and understand the processes controlling the concentration of key compounds such as Ozone, Methane, OH and aerosols. The project will collate the long term surface observations collated from around the Southern Ocean (ebas.nilu.no), short term observations made at locations such as Halley and Cape Grim, with aircraft observations from programmes such as AToM and with satellite data from various instruments and species. The GEOS-Chem atmospheric chemistry transport model will be used to predict the concentration of compounds and compared to those observed. The model will be updated to include emissions of more compounds from the ocean (carbon monoxide (Conte et al., 2021) and alkyl nitrates (Burger et al., 2022)) and potentially to improve the deposition of others to the ocean. Model diagnosis and sensitivity studies will allow the processes controlling key compounds (O3, OH, aerosols) in the model to be explored and understood.
Training in appropriate scientific and transferable skills will be provided by the PANORAMA DTP, the University of York, the department of Chemistry and by the Wolfson Atmospheric Chemistry Laboratory. There will be opportunities to work with the observational groups outside of York to understand their data and to provide context and to work with the wider international GEOS-Chem community on developing new understanding of the model.
The project will involve significant data process and programming. It would be suitable for students with a background in a numeric physical science (chemistry, physics, environmental science etc.). Please contact Mat Evans (mat.evans@york.ac.uk) for more details and an informal conversation about the studentship.
Further details
The Southern Ocean surrounds Antarctica, covers a vast area. The high wind speeds leads to significant movement of chemical species from the atmosphere into the ocean and from the ocean into the atmosphere. Ocean-atmosphere exchange is likely more important here than anywhere else on Earth for determining the composition of the atmosphere. The ocean is far from large population centres, meaning it is as close to being “pristine” as anywhere on Earth. These low concentrations of pollutants found in this region make it similar to the state of the marine atmosphere before industrialisation, and may offer a proxy for the atmosphere if extreme emission controls are implemented into the future. In the south the region interfaces with the ice and snow of Antarctica, where as in the north it is bounded by South America, Southern Africa and Oceania where anthropogenic, forest fires and natural sources can be found. The air over the southern ocean represents a unique challenge to our understanding of atmospheric chemistry, atmosphere-ocean exchange and long range transport of pollutants.
Despite this unique environment it lies far from most of our observational capability for atmospheric chemistry. Long term observations are limited to a few sites at the bottom of South America, Southern Africa, and Oceania or along the coast of Antarctica (see Figure 1). Short term intensive observation campaigns have occurred at some of the surface sites such as Halley (e.g. Lewis et al., 2001), and a small number of aircraft campaigns and research cruises have reached the region e.g. HiPPO and AToM. Satellite data is available for some of the region and this may offer a unique dataset for analysing the region.
Figure 1. Locations of surface monitoring sites in the Southern Ocean (from ebas.nilu.no). Some
By considering meteorology, emissions, clouds, chemistry, etc Atmospheric Chemistry Transport Models (CTMs) represent out best understanding of the processes that control the concentration of compounds in the atmosphere. Few studies have explicitly looked at model performance in the region, but some more general studies have identified problems. For example, models appear to systematically underestimate the O3 concentrations in the region (see Figure 5 in Griffiths et al., 2021) which may be attributable to natural sources which are missing in these models. There appears to be an oceanic source of CO missing from models (Conte et al., 2021) and missing sources of oceanic NOy (Burger et al., 2022).
Another unique characteristics of the region is the importance of impact of halogen chemistry in the region. The chemistry of halogens (Cl, Br, I) emitted into the air from the ocean (mainly as sea-salt) may play an important role here. Given the rapid air-sea exchange the southern ocean is particularly susceptible to infuennces from the halogens. Figure 2 shows the impact of halogen chemistry on O3 concentrations. Despite this sensitivity we have little constraint on halogens in the southern ocean.
Figure 2. Taken from Sherwen et al. (2016). Shows the impact on annual mean ozone concentrations (tropospheric column , surface and zonal) of switching off halogen chemistry. Upper panels represent absolute changes, lower panels represent fractional changes. The largest fractional impacts of halogen chemistry is located around the southern oceans.
Deposition of ozone to the ocean surface is also more significant in this region than in others due to the importance of air-sea exchange. Figure 3 shows the impact of updating ozone dry deposition to the ocean on ozone concentrations (Pound et al., 2020). The largest impacts on O3 concentration are found around the southern ocean.
Figure 2. Taken Pound et al., (2020) this shows change in modelled surface and zonal ozone concentrations from updating the parameterisation of ozone dry deposition too the ocean. The largest impacts are felt over the Southern Ocean.
Thus, the atmosphere in the southern ocean region shows some unique sensitivities, especially to ocean-atmosphere exchange making it different from other parts of the world.
What will the project do:
- Collate the available surface, aircraft, ship and satellite observations of tropospheric composition data from sites in the Southern Ocean. There are a number of long term observational sites (https://ebas.nilu.no ), surface field campaign (e.g. https://data.ceda.ac.uk/badc/chablis), aircraft dataset (e.g. ATOM https://daac.ornl.gov/ATOM/), cruise data (e.g. https://acp.copernicus.org/articles/19/7233/2019/) and satellite datasets (https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1163/ )
- Compare these observations to simulations made by the GEOS-Chem atmospheric chemistry transport (geos-chem.org) to try to understand model successes and failures.
- Improve the model representation of natural emissions in the region (including oceanic CO, NOx etc, crysopheric NOx) to assess whether these could improve the model performance.
- Assess the model’s sensitivity to factors such as halogen chemistry, ocean deposition, ice deposition, stratospheric ozone, shipping emissions etc to look for potential explanations for model failure.
- Assess the model budget for ozone and other species in the region. This diagnoses the emissions, deposition and chemical loss and chemical production of species in the region and balances this against the transport of the regions into and out of Antarctica and the North.
- Assess how the composition of the region has changed from the preindustrial to the present day and may change in the future with changing global emissions.
- Identify locations where future observations could provide critical constraints on composition and processes.
Research environment
The student will form part of Prof Evans’ research group, the wider Wolfson Atmospheric Chemistry Laboratories and University of York’s Chemistry Department, and the NERC PANORAMA Doctoral Training Programme. The Wolfson Labs are a large interdisciplinary research group consisting of around 70 researcher exploring atmospheric chemistry using field observations, laboratory experiments and computational methods.
Training in hard (programming, high performance computing, atmospheric chemistry etc.) and soft skills (presentations, writing, ethics, careers, etc.) to undertake the research will be provided within the group and through training courses provided by the Department, University and PANORMA. They will have access to the new University of York Viking 2 High Performance Computing platform for their simulation.
The student will develop collaborations with the wider international GEOS-Chem modelling community and those group making observations in the southern ocean region.
Suitable background.
Suitable students could have a broad background in a numerate physical science (chemistry, physics, environmental science, engineering etc) but an interest in developing their skills in an interdisciplinary research area. Specific training will be provided in areas where the student may be weak (atmospheric chemistry, ocean chemistry, computational methods, programming etc.).
Further information
For further information and discussion of the project please contact Prof Mat Evans (mat.evans@york.ac.uk)
References:
Griffiths et al., Tropospheric ozone in CMIP6 simulations, ACP, 21, 4187–4218, 2021