Understanding tropospheric ozone trends. Is it all just emissions?


Understanding changes in the concentration of ozone (O3) in the troposphere is of central importance to understanding the human impact on the Earth system. This project will explore our understanding of changes in the O3 concentration on the decadal-to-century scale, using a state-of-the-art numerical model. It will focus on the impact of atmospheric chemistry and the deposition of compounds to the Earth’s surface and clouds, to enhance or diminish the impact of human emissions on O3 in the atmosphere and on atmospheric oxidation more widely (OH, H2O2, etc). The student would work within Prof. Mat Evans’ group in the Chemistry Department of the University of York as part of the Wolfson Atmospheric Chemistry Laboratories. They would be funded as part of the NERC PANORAMA DTP and would benefit from the training and opportunities of the Chemistry Department at the University of York, the DTP, by NCAS and other providers. The student would ideally need a background in a numerical science discipline such as chemistry, physics, engineering, maths, or environmental science and be happy to develop and improve their computational skills, other backgrounds may also be suitable. For further details and an informal conversation about what the project would entail please contact Prof. Mat Evans (mat.evans@york.ac.uk)  https://www.york.ac.uk/chemistry/staff/academic/d-g/evansm/).


Atmospheric ozone (O3) plays a central role in the Earth’s environmental system. It is a significant climate gas, has adverse health impacts for animals and vegetation, and provides a control on the concentration of methane in the atmosphere. Understanding the changes in its concentrations over the last decades and centuries is important to understanding the impacts of humans on the Earth. There is strong evidence for O3 concentrations having increased over the last 4 decades or so, but the trend over the last centuries is highly uncertain due to a lack of observational constraints.

Atmospheric chemistry transport models (CTMs) represent our quantitative understanding of the processes that control the composition of the atmosphere. They simulate the composition of the atmosphere by numerically representing the different processes that impact its composition: emissions into the atmosphere, the chemistry within the atmosphere, deposition to the surface and to clouds, and transport by winds and clouds. They are also used for the development of policies for air pollution and climate change. Despite many decades of advances, these models do not appear to reproduce the observed trend in O3 particularly well. They appear to reproduce present-day ozone and oxidant concentrations but fail to represent the observed changes over the last decades and centuries. This failure is usually attributed to gaps in our understanding of the emissions of gases and particles into the atmosphere, however, other explanations might exist. It may be that errors in our model representation of the chemistry occurring in the atmosphere, or in the deposition of compounds to the surface or clouds may explain this failure.

This project will explore these alternative explanations using the GEOS-Chem model of atmospheric chemistry and transport. The model is used extensively globally and forms the basis of the NASA atmospheric composition forecasting system (GEOS-CF). You will learn how to use the model and develop simulations to represent the pre-industrial atmosphere, the recent past and the present day. Using observational data, you will assess the model’s performance and develop diagnostics to help understand the role of emissions, chemistry and deposition in determining the O3 concentration. You will then explore the model’s sensitivity to chemical and depositional processes and to the spatial resolution of the model. This exploration will then influence model choices that could be used to bring the model closer into agreement with the observed trends.

The results of this research will be useful for a number of internal programmes such as TOAR-II, HTAP , and CMIP which aim to inform policymakers about the state of the science of understanding of climate and air quality science.

You will be part of Prof Mat Evans’ research group, consisting of PhD students, post-doctoral researchers, staff from within the National Centre for Atmospheric Science, and a software engineer. You will also form part of the wider Wolfson Atmospheric Chemistry Laboratories. This is made up of around 70 atmospheric chemistry researchers, based at the Chemistry Department of the University of York. The research in the Laboratories consists of observational studies in locations around the world, laboratory experiments and simulation of the atmosphere.

Training for the project will occur within the Laboratories, the chemistry department of the University of Yor,  as part of the NERC-funded Leeds-York-Hull PANORAMA Doctoral Training Partnership and through other training provided by organizations such as NCAS. It will consist of a variety of hard and soft skills from learning how to run a complex numerical model on a high-performance computing platform, to how to give scientific presentations and write a scientific publication.

For further details and an informal conversation about what the project would entail please contact Prof. Mat Evans (mat.evans@york.ac.uk)  https://www.york.ac.uk/chemistry/staff/academic/d-g/evansm/).