Ozone: right answer, wrong reason?

Ozone (O3) plays a central role in the Earth system: it protects the surface from harsh UV radiation, it is a pollutant, it reduces crop yields and is a climate gas. Computer models of atmospheric chemistry and transport are used to evaluate our understanding of its distribution around the world. In general, the O3 concentrations calculated by these models, agree both with themselves and with observations .

Despite the concentrations agreeing, the models disagree on how they reach this concentration. Some models have more chemical ozone production, some models have more transport from the stratosphere, etc. The models may be right, but for the wrong reasons. The ‘budgets’ (adding up the source and destruction terms) for O3 between the models can be quite different. The reasons for these differences are not clear, but they have existed for over 20 years and little progress has been made in advancing the diagnosis of why.

If models don’t agree on the terms making up the ozone budget, our confidence in their predictions, for either the past or the future is in doubt. These differences could reflect differences in the chemical rate constants, the organic compounds considered in the models, wet deposition etc. However, without an understanding of why there are differences it is difficult to progress. The standard diagnostics of O3 production and loss have not been helpful in this understanding as they focus on the fast internal chemistry of the models rather than the influence of slower external factors such as emissions and deposition.

Over the last few years, some progress has been made. Edwards and Evans, (2017) developed a new diagnostic for the production of ozone based on the conservation of quantum mechanical spin. This allowed for a new way of quantifying the production of ozone, which traced O3 back to the emission of organic compounds. Figure 1 shows the new description of O3 production which traces back to the emission of organic compounds into the atmosphere on the left handside, chemical processes in the middle and the production of O3 on the right hand side. Of the potential 778 Terra moles of global O3 production, only 112 Terra moles is actually produced. The model efficiency in this case is only 14%.

Figure 1 shows the new perspective on tropospheric ozone production taken by Edwards and Evans, 2017. O3 production is a by-product of the oxidation of VOCs in the presence of oxides of nitrogen. This diagnostic traces the spin paired electrons in each VOC bond that could theoretically go on to produce O3 through to the production of O3.The project will reproduce this diagnostic for the different models to understand their different ozone budgets.Figure taken from Edward and Evans, 2017.

Similarly, Bates and Jacob (2020) developed an extended Oy family within a chemistry transport model and showed it provided significant insight into new considerations for the production of ozone.

Figure 2  shows the new perspective on the tropospheric ozone budget taken by Bates and Jacob (2020). This considers a much wider Oy family and so gives a wider perspective on the sources and sinks of O3 in the model. The project will reproduce this diagnostic for the different models to understand their different ozone budgets. Figure taken from Bates and Jacob, 2020.

This project will continue with these methods to develop and diagnose O3 and Oy production and loss, and it to a number of different atmospheric chemistry transport models. Initial work will be done with the GEOS-Chem model, which is also the platform for the Edwards and Evans, and Bates and Jacob developments. The group in York has extensive experience of using this model to develop novel insights into the chemistry of the atmosphere. The group also has access to significant computational resources through the University of York’s, Viking High-Performance Computing capability.

Initial experiments will involve diagnosing GEOS-Chem using both the Edwards and Evans, and the Bates and Jacob, approaches. The emphasis will then turn to developing a new diagnostic for ozone loss, based on the ozone production approach of Edwards and Evans. This would then provide a complete set of diagnostic tools for understanding the budget of O3 within the GEOS-Chem model.

Working with partners at the University of Rochester (Prof. Lee Murray ) and the UK Met Office ( Dr Fiona O’Connor ) the two approaches, developed for GEOS-Chem, will be included into two climate models (GISS ModelE and UK-ESM1), and then the new budgets for O3 evaluated and compared. It will be possible to diagnose why the budgets for these models are different based on these diagnostics. This would provide more confidence in the predictions of the model both for the past and the future and help support activities in the wider CMIP activity.

The studentship would be based in the University of York’s, Wolfson Atmospheric Chemistry Laboratories (WACL), supervised by Prof Mat Evans and Dr Pete Edwards. The student would form part of the wider atmospheric modelling community at WACL which researches a range of problems using numerical models, data analysis and machine learning. The student would have the opportunity to travel to the University of Rochester and the Met-Office to learn how to run the necessary simulations for the project. Other training would be provided in-house within WACL, by the Department of Chemistry, or by the NERC PANORAMA DTP.

The project would be suitable for a student with an undergraduate background in a numerate natural-science subject. Experience of computational methods and tools (FORTRAN, Python, etc) would be an advantage, as would previous experience in atmospheric chemistry.

The Wolfson Atmospheric Chemistry Laboratory (WACL) consists of 11 academic staff, 23 postdoctoral staff, 32 PhD students, and 8 technical / support staff. Based in a purpose-built building, it constitutes one of the, if not the, largest groupings of atmospheric chemists in the United Kingdom. The research covers all aspects of atmospheric chemistry, outdoors and indoors, from the stratosphere to the marine surface. By bringing together researcher making observations, in the field, laboratory studies and theoretical calculations within the same building, often side by side, it allows for a rapid exchange of interdisciplinary knowledge. Connections with policymaking are strong, with staff working with Defra and BEIS on generating new policies for air quality and climate change. Colaboration with industrial partners is also underpins much of the activity. .