- Development of a full optimal estimation carbonyl sulphide (OCS) retrieval from atmospheric spectra taken by the IASI satellite instrument.
- Modelling of atmospheric OCS using the TOMCAT 3D chemical transport model.
- Interpretation of OCS observations/model outputs to provide insights into sources and sinks.
The carbon dioxide (CO2) taken up by leaves in the process of photosynthesis can be directly measured on the leaf scale. However on the global scale, i.e. on the scale of atmospheric measurements, it is impossible to distinguish it from the CO2 released by respiration, for example from non-photosynthetic parts of plants and soil microorganisms. This severely limits our understanding of carbon dioxide (CO2) sinks and sources on land.
The sulfur-containing analogue of CO2, carbonyl sulfide (OCS), has recently emerged as a proxy for the photosynthetic uptake of CO2. OCS and CO2 have the same diffusion pathway into leaves, followed by hydration reactions catalysed by the enzyme carbonic anhydrase. However, for OCS the hydration reaction,
OCS + H2O → H2S + CO2,
is irreversible, making this essentially a one-way flux into the land biosphere.
Despite the importance of OCS, its atmospheric sources and sinks are poorly quantified. The main sources of OCS are the oxidation of carbon disulfide (CS2) and dimethyl sulphide (CH3SCH3), direct emission from the upper ocean, and various anthropogenic sources. The primary sinks are vegetation, soil and photochemical loss in the stratosphere.
Although there are numerous ground-based measurements of OCS, these are rather sparse in nature and do not provide the global coverage required to quantify sources and sinks. Monitoring atmospheric OCS concentrations via satellite remote sensing is particularly powerful because it provides global coverage and a high density of data.
This project involves the development of a full optimal estimation OCS retrieval using observations by the Infrared Atmospheric Sounding Interferometer (IASI) instruments on board MetOp-A and MetOp-B.
The spectroscopic microwindow for OCS absorption (2030 – 2085 cm-1) is contaminated by a number of interfering species, such as CO2 and H2O, for which the spectroscopic line parameters are not sufficient to adequately calculate the absorption within experimental noise. The student will assess the quality of line parameters for this region and, where needed, derive more accurate line parameters using existing laboratory measurements. It is envisaged that data from the limb sounders MIPAS and ACE-FTS will be utilised for validation purposes.
The OCS IASI retrieval will make use of an existing retrieval algorithm known as ULIRS (University of Leicester IASI Retrieval Scheme), which the student will need to modify under supervised guidance. Retrievals will be run using the High Performance Computing (HPC) services at the University of Leicester.
The student will also run simulations of the TOMCAT 3D chemistry transport model (CTM) in collaboration with researchers at NCEO-Leeds. Modelled OCS global distributions will be compared with the new observations to help improve our understanding of OCS sources and sinks.
Training and Skills
This studentship provides an exciting multidisciplinary opportunity to work with cutting-edge satellite observations and modelling techniques in a challenging area of atmospheric science.
This project covers a wide range of topics: (1) atmospheric spectroscopy; (2) remote sensing techniques and the interpretation of retrieved quantities; (3) atmospheric chemistry & transport modelling; (4) data visualisation & analysis, including the comparison of model data with satellite observations.
The student will gain experience in many key areas of Earth observation (EO) and environmental science, as well as in using necessary tools such as FORTRAN, IDL and shell-scripting computer languages, and high performance computing on a Linux platform. The student will also learn how to present their work at conferences and for publication in peer-reviewed journals.
The student will work alongside researchers from the EOS group at the University of Leicester, the Atmospheric Chemistry Group at the University of Leeds, and the National Centre for Earth Observation (NCEO), who will provide additional training and guidance.
CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to CENTA research themes.
Year 1: Analyse and update spectroscopic line parameters using existing laboratory spectra. Begin development of retrieval algorithm (based on ULIRS).
Year 2: Finalise retrieval algorithm and process around ten years of IASI data. Derive OCS global distributions and analyse. Commence modelling work.
Year 3: Finalise model results. Test the model against observations. Interpret the observations with the aid of the model output.
Partners and collaboration (including CASE)
This studentship will be jointly supervised by NCEO researchers from the Universities of Leicester and Leeds. This studentship has been approved as a CASE award with the NCEO.
The TOMCAT CTM is hosted by the Atmospheric Chemistry Group based at the University of Leeds. The student will be expected to work closely with researchers in Leeds (including extended visits) to improve and develop computer code describing the atmospheric chemistry of OCS, and to use the outputs to help understand the satellite observations.
This studentship will benefit from the involvement of the supervisors in the NCEO, and regular NCEO meetings will give the student opportunities to interact with researchers from a number of leading UK institutions working in EO.
The NCEO (www.nceo.ac.uk) is a distributed NERC centre providing the UK with national capability in EO science.
Dr Jeremy Harrison is the NCEO’s spectroscopy leader and capability leader in atmospheric radiative transfer. Based in the Earth Observation Science (EOS) group, his expertise lies in atmospheric spectroscopy and the remote sensing of trace gases.
Prof Martyn Chipperfield is the NCEO’s capability leader for atmosphere-land surface data assimilation. His expertise is in chemistry-transport modelling and the interpretation of satellite observations of atmospheric chemistry.
Prof John Remedios is the Director of the NCEO, and an expert in the remote sensing of trace gases.
Interested applicants are invited to contact Dr Jeremy Harrison (firstname.lastname@example.org). Note that all potential applicants are strongly advised to make contact before applying.