Understanding atmospheric oxidation in the marine tropical atlantic.

Project summary 

The methane (CH4) and ozone (O3) are two key greenhouse gases in the Earth’s atmosphere. Methane is directly emitted into the atmosphere by natural and human activity and it is removed by the action of the OH radical. This removal can lead to the production of O3. Thus CH4, O3 and OH are linked chemically. We need to predict their concentrations into the future if we are to understand our changing climate. But we need to be confident that we understand their chemistry in the present day if we are to have confidence in predictions. The TropOx campaign will fly the UK’s large research aircraft into the air over the Atlantic’s tropical marine environment to explore this chemistry. Teams from the Universities of York, Leeds and Manchester will measure the concentrations of a large number of atmospheric components in both the gas and aerosol phase in order to explore our understanding of the chemistry in the tropical marine atmosphere. The student will form part of the team on the aircraft in the field but will predominantly be exploring the measurement using a chemistry transport model to evaluate our understanding of this chemistry and improve future predictions. 

Background

The chemistry of the troposphere (the lowest 12 km of the atmosphere) is central to understanding of our changing climate. Its oxidation chemistry determines the concentration of key climate gases such as methane, ozone, and the production of solid and liquid particles suspended in the air (aerosol) which can impact the scattering of light and the distribution of clouds. This chemistry is mainly driven by sunlight, so the tropical regions of the planet (subject to the most intense solar radiation) play a critical role. The marine environment dominates the tropics (~75%) and so if we are to understand tropospheric oxidation we need to understand the photochemically driven chemistry in the tropical marine regions. 

Although, in some ways, the marine atmosphere is less complicated than the terrestrial, it is still a challenging environment to understand. Emissions of compounds from the oceans (sea-salt, DMS, VOCs, iodine) and the deposition to the oceans (O3, SO2, organics) modulate the composition. The transport of material from the land (human emissions, forest fire emissions, dust, emissions from vegetation etc) provide inputs. Direct emissions from ships can also have a significant influence on the composition. Recent work has shown that our understanding of the chemistry in the region is incomplete with novel chemistry (halogens, nitrate photolysis, DMS oxidation chemistry) being regularly discovered. This incomplete understanding has impacts on the balance between methane emission and loss, the importance of O3 as a climate gas, etc. and therefore increases the uncertainty about the future state of the atmosphere. 

 

The project. 

In 2027 the £2.5M NERC funded TropOx campaign will fly the UK’s large research aircraft (www.faam.ac.uk) into the tropical Atlantic specifically to explore this oxidation chemistry. Based out of Cape Verde and Ghana, the aircraft will measure a large number of different atmospheric constituents, under a range of conditions. Measurements will be made by teams from the University of York, Leeds and Manchester using a range of instrumentation. This will allow us to explore the processes controlling this oxidation chemistry, challenging our understanding of this chemistry and reducing the uncertainty in our predictions of the future state of the atmosphere.  

This studentship will take part in the field experiment in Africa. This will involve supporting the forecasting tools necessary to plan flights. However, the main activity will be to use the data collected by other groups to challenge our understanding of tropospheric chemistry. 

The tool for this will be the GEOS-Chem atmospheric chemistry transport model. This tool represents our understanding of the chemistry, emissions, transport and deposition of chemicals in the atmosphere as computer code and allows us to be able to predict the concentration of compounds such as OH, O3, CH4 etc. By virtually “flying” the aircraft through the model atmosphere, it is possible to evaluate the model’s performance compared to those observations. This allows a comparison of when the model does well and when it does badly. As well as the dataset collected by TropOx, data collected year round at the Cabo Verde Atmospheric Observatory and the NASA AToM missions will be used.  

Rather than taking an approach of running the model once and comparing the observations to that single model simulation, the focus will be on developing an uncertainty based approach. All of the parameters going into the model (chemical rate constants, photolysis rates, Henry’s law constants, emission rate etc) have a degree of uncertainty associated with them and this approach will compare the model prediction and the calculated uncertainties with the observations within their associated uncertainty. Our previous work highlighted the uncertainty from the chemical rate constants (https://doi.org/10.5194/acp-17-14333-2017), but this will be extended here to consider a wider range of factors. 

Ultimately the project will identify whether the observed oxidation chemistry is consistent within the uncertainty of our current understanding. If it is the processes contributing the largest uncertainties will be identified so that future laboratory or field students could be developed to reduce those uncertainties. If it is not consistent with understanding, the student will explore the conditions under which the model failure is most pronounced and identify the potential processes which could rectify that disagreement. 

 

The student 

The project will involve some work with the research aircraft but will mainly involve developing our understanding of tropospheric chemistry as represented by the GEOS-Chem model. This will involve working in both the FORTRAN and Python programming languages and so any student will need to be able to develop skills in these areas. The project would suit students with a background in chemistry but also any natural science with a significant mathematical / computational element (e.g. environmental science, physics, biology, engineering, mathematics etc.) 

 

Training

The student will work under the supervision of Profs Mat Evans and James Lee and will be based at the Department of Chemistry’s Wolfson Atmospheric Chemistry Laboratory at the University of York. The Wolfson Atmospheric Chemistry Laboratories are home to more than 65 researchers with interests in all aspects of atmospheric chemistry, from stratospheric ozone, through to urban pollution, personal exposure and health. The labs support an exceptional environment for research, have access to state-of-the-art facilities and include a range of different disciplines and researchers. 

Project specific training will be provided through online training material (reathedocs, YouTube), and support from current PhDs, Post-Docs and Software Engineers in the Evans group. 

The University of York and the wider NERC YES-DTN provide comprehensive training programmes for PhD students with a range of courses on both hard (e.g. data carpentry) and soft (e.g. presentation) skills. 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. The student 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).