Understanding the impact of trace level sulphur dioxide on air pollution and climate

This Project has been filled

Project summary

Particulate matter (PM) is a major air quality challenge, estimated to be responsible for >29,000 equivalent deaths a year in the UK alone, andalso represents a major uncertainty in current climate model predictions. Current UK targets, as outlined in the 2018 Clean Air Strategy, are to halve the number of people exposed to PM smaller than 2.5 μm (PM2.5) concentrations above 10 μg m-3by 2025. This is going to be a challenge as primary components of PM2.5 have been reduced through emissions controls, meaning secondary PM, produced through atmospheric chemical reactions, now dominate.Understanding secondary PM production is therefore vital if we are to develop effective policies to tackle both PM air pollution and climate.

The oxidation of gas phase sulphur dioxide (SO2) is central to the production of secondary inorganic PM.Significant reductions in SO2emissions in the developed world over recent years have resulted in background concentrations of SO2falling drastically. Although this is a major policy success, even at very low concentrations SO2is still thought to play an important role in PM production, but current models disagree on the impact further SO2emission reductions will have. The best way to improve our understanding of the chemistry occurring at low SO2concentrations is to directly measure it. Unfortunately current methods for measuring SO2are no longer sensitive enough to measure the low background levels, undermining work to better understand the production of secondary PM. Recently a new instrument has been developed at the University of York for the sensitive detection of trace levels of SO2. This PhD project will be the first to use this instrument to make the much needed observations capable of improving our understanding of  background SO2chemistry. This will be achieved through the following objectives:

  • Characterise new SO2instrument performance using lab and field experiments
  • Deploy instrument as part of 3 field projects, two in the UK and one aboard a research cruise to the Arctic
  • Use the data from these field projects to challenge and improve our understanding of atmospheric SO2chemistry through comparison with model predictions

This work will address an important knowledge gap in our understanding of atmospheric chemistry and thus directly improve our ability to design effective environmental policies.


Atmospheric aerosol, or particulate matter (PM), is the most significant air pollutant globally and impacts climate through the direct absorption and scattering of radiation and the formation of clouds. Policies aimed at tackling these environmental challenges rely on predictive models that can accurately describe PM production and loss under different scenarios. A significant fraction of global PM is secondary in nature, meaning it has not been directly emitted but instead has been formed through atmospheric gas phase chemical processes. It is well known that the oxidation of sulphur dioxide (SO2) to sulphuric acid plays a controlling role in the production of this secondary PM. In response to this UK emissions of SO2have been reduced by over 90% since the 1970s.  This pattern has been replicated across much of the developed world, where SO2concentrations are now well below the levels where they pose a direct health risk (Fig. 1), and this is celebrated as a major environmental regulatory success story.

Figure 1: UK emissions of SO2 from 1970 to the present, compared with emission target commitments. Defra report “Emissions of air pollutants in the UK, 1970 to 2017”.

Despite these reductions, the remaining low levels of SO2still play an important role in the formation of PM in the UK and globally, and this role needs to be understood if we are to achieve further improvements in air quality and understand the climate impacts. Background (suburban, rural, oceanic etc.) SO2concentrations are now below the limit of detection of current commercial instrumentation, meaningthe UK is currently unable to assess SO2ambient concentrations or test emissions inventories. These observations are also needed to compare with model predictions as a means to validate or identify and inform uncertainties in model assumptions. This lack of observations thus undermines our ability to accurately represent key processes in models which are used to inform policy on air pollution and climate.

There is a critical research need for sensitive measurements of SO2that are capable of testing our understanding of atmospheric chemistry at these low SO2concentrations. This need is even more urgent as recent international and national policy decisions are expected to reduce SO2concentrations even further, and the effect this will have on PM production needs to be understood if the true impact of these policies are to be assessed. This PhD project will use a newly developed and highly sensitive laser based instrument to provide vital data on trace level SO2as part of several NERC funded projects. These data will then be used to improve our understanding of atmospheric SO2chemistry and thus directly improve our ability to design effective environmental policies.

Project Aims

A new instrument developed at the University of York’s Wolfson Atmospheric Chemistry Labs, based on the laser induced fluorescence technique, is capable of measuring SO2levels in the parts per trillion range. This new instrument is orders of magnitude more sensitive than existing commercial SO2detectors, and will enable our understanding of atmospheric SO2chemistry to be properly challenged. This PhD project will be the first to use this instrument to make sensitive field measurements that can be compared with model predictions, as part of several recently funded NERC projects

The initial stages of the project will involve characterising the new instrument, in the lab and  field, and develop calibration methods and data analysis techniques. The instrument will then take part in three projects that will explore key uncertainties in our understanding of atmospheric SO2chemistry. The first of these projects will be the at the Plymouth Marine Laboratories Pen-Lee Point atmospheric observatory (PPAO in Fig. 2), where we will be assessing the impact on regional air quality of a legislated reduction in shipping fuel sulphur content that comes into force in early 2020. As shipping is an important source of global SO2emissions, this change in legislation is expected to have a significant effect on SO2and associated PM concentrations, which in turn will impact air quality and radiative forcing. This work will help contribute to the NERC funded ACRUISE project (Fig. 2), which aims to characterise the changes brought about by this important change in maritime legislation.

Figure 2: The ACRUISE project aims to assess the impact of a legislated reduction in shipping SO2emissions on PM production in the marine atmosphere. In particular the impacts that these changes will have on  cloud properties and coastal air pollution. This PhD will provide a key contribution to this work by making sensitive measurements of SO2 concentrations at the Pen-Lee Point atmospheric observatory (PPAO). Image taken from the ACRUISE case for support.

Following on from the ACRUISE project we will also make measurements to understand the importance of SO2in controlling UK urban PM air pollution. This will be as part of a large UK atmospheric chemistry project focussing on the current state of UK urban air pollution. The sensitive measurement of SO2concentrations will make a significant contribution to this project and help to understand the processes driving secondary PM formation in UK cities. It is also anticipated that the instrument will also be used as part of a NERC research cruise to the Arctic in 2021. The impact of a changing climate is being most strongly felt in the Arctic, yet large uncertainties still remain in our understanding of Arctic PM formation. The York SO2instrument is capable of measuring the low concentrations expected in the Arctic, and will thus provide a valuable dataset that can be compared with predictions from atmospheric chemistry models and used to improve our understanding of the important processes that control Arctic SO2and PM.

The data collected from the above mentioned field projects will be used in combination with measurements from project partners to challenge and improve our understanding of atmospheric sulphur chemistry. The new science from this PhD project will directly impact the reliability of future air pollution and climate model predictions, and thus inform environmental policy.


The instrumental and measurement focus of this PhD project means the successful candidate will have excellent practical / physical laboratory skills as well as a background in the physical sciences (e.g. Chemistry, Physics). The candidate will be enthusiastic about hands-on lab work, and show interest in environmental and/or public health issues. Computer programming experience, for both instrument control and data analysis, will be advantageous but significant training will be provided as part of the project (see Training section below).The Wolfson Atmospheric Chemistry Laboratories are a world class research centre, with significant technical support and resource for instrument development and data analysis. We appreciate that this PhD project encompasses several different science and technology areas, and we don’t expect applicants to have experience in many of these fields. The project is very well supported with experienced scientists and training in the new techniques and disciplines is all part of the PhD.


The student will work under the supervision of Dr Pete Edwards and Prof 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 laboratories comprise more than 1200 m3of new offices and labs, the most recent extension to the building being completed in April 2018. 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.The student will receive training on the new SO2instrument as well as techniques used to calibrate and characterise the instrument. Training will also be given on the computational skills needed to collect and analyse the data from the instrument. The University of York and the wider NERC PANORAMA DTP 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).