Project highlights

  • Contribute towards a better understanding of nitrogen compounds’ atmospheric chemistry and their roles in determining air quality
  • Gain expertise in using highly sensitive instrumentation to record air pollutants
  • Conduct collaborative field work at one of the UK’s three air quality supersites.


Air pollution is responsible for around 350,000 premature deaths per year in Europe [1] and 7 million globally [2]. In the UK and in Europe, the three most toxic air pollutants are NO2 (nitrogen dioxide), tropospheric O3 (ozone) and PM (particulate matter). NO2 and PM are emitted directly from various human-made sources and a few natural sources. But the majority of NO2 comes from road vehicles: substantial decreases in NO2 concentrations were observed in many UK and European cities during COVID lockdown restrictions when people’s activity was much reduced [1]. In contrast, tropospheric ozone is not emitted directly but rather it is produced within the atmosphere itself from the photochemical oxidation of volatile organic compounds in the presence of NOx (NO+NO2). Some types of PM are also formed and/or chemically transformed in the atmosphere. Controlling these secondary air pollutants is challenging because it requires control of their precursors (such as NO2 for tropospheric ozone), and such efforts must be informed by a thorough understanding of the chemical pathways.

Observations of air pollutants are vital. The Automated Urban and Rural Network (AURN) has 150 air monitoring sites located around the UK [3]. Additionally, three “air quality supersites” have been established that contain more extensive instrumentation for monitoring atmospheric composition than is available at regular AURN sites. One supersite is on Birmingham University’s campus [4], where this project will deploy additional research instruments to quantify oxidised nitrogen compounds (collectively termed “NOy”).

To further complicate the picture, air pollutant concentrations are highly dynamic, especially close to pollution sources.

Graph of time series of emissions from road vehicles

Figure 1: A time series of emissions from road vehicles. NO2 mixing ratios are in red, overlaid by wavelength-resolved aerosol extinctions (coloured as stated in the legend). The BBCEAS instrument sampled at 1 second time resolution from the second-storey window of a building at Leicester University that overlooks a public road approximately 10 metres away.


University of Leicester


  • Climate and Environmental Sustainability


Project investigator

Dr Stephen Ball, University of Leicester ([email protected])


How to apply


The CENTA student will use a broadband cavity enhanced absorption spectrometer (BBCEAS) built at Leicester University. This instrument acquired the data shown in Figure 1. Multiple reflections of light between the instrument’s mirrors generates the “cavity enhancement” – photons typically travel 5 km through the gas sample, thereby producing the instrument’s high sensitivity. The instrument is configurable at different wavelength regions: (i) near-UV to measure HONO and NO2 [5] (HONO is a source of the daytime oxidant OH radicals), (ii) blue wavelengths for fast NO2 measurements like in Figure 1, or (iii) red wavelengths to quantify the night-time reservoir compound N2O5 [6]. It is anticipated the student will measure different NOy species at different stages of the project to complement the ongoing “core” observations at the supersite, plus a more intense measurement period targeting the formation of aerosol nitrate from the deposition of N2O5 and HNO3.

Training and skills

Students will be awarded CENTA2 Training Credits (CTCs) for participation in CENTA2-provided and ‘free choice’ external training. One CTC equates to 1⁄2 day session and students must accrue 100 CTCs across the three years of their PhD.

The student will receive training in trace gas detection methods, data analysis techniques, field work skills and logistics planning. There will be extensive opportunities to collaborate with and learn from colleagues at Birmingham who run the BAQS supersite and operate its suite of state-of-the-art commercial instruments. Leicester’s School of Chemistry offers lecture courses that are directly relevant to the project: e.g. Earth System Science. There is also an AURN monitoring station on the Leicester University campus to provide reference data against which to test instruments prior to their deployment in the field.

Partners and collaboration

University of Birmingham: The Birmingham Air Quality Supersite (BAQS) is a state-of-the-science urban background air quality monitoring station, on the Edgbaston campus of the University of Birmingham, 2 miles south-west of the city centre.  Instrumentation at BAQS covers gas-phase species (NOx, O3, NH3, VOCs, CO2), aerosol physical characteristics (ultrafine particles, PM mass concentration, PM size distribution), aerosol chemical composition (inorganics and metals), and meteorological & photochemical parameters. There is space to host visiting instruments.  Recently, BAQS has supported baseline air quality measurements for the Commonwealth Games, and an intensive campaign within the NERC-funded OSCA project to explore NOy speciation in wintertime.

Further details

Potential applicants are welcome to discuss the project informally with the project supervisors:

Dr Stephen Ball ([email protected]), School of Chemistry, University of Leicester.

Prof Bill Bloss ([email protected]), School of Geography Earth and Environmental Sciences, University of Birmingham.

If you wish to apply to the project, applications should include:

  • A CV with the names of at least two referees (preferably three and who can comment on your academic abilities)

Applications to be received by Wednesday 31st May 2023. 

Possible timeline

Year 1

CENTA’s research skills training and training specific to this project. The student will be taught how to operate the BBCEAS instrument and how to use the software needed to fit BBCEAS spectra to retrieve the concentrations of target gases. The instrument’s most straightforward configuration is to measure NO2; thus the student will perform a “dry run” campaign measuring ambient NO2 on the Leicester University campus and compare their results with NO2 data from the Leicester AURN site.

Year 2

Measurements at BAQS. An instrument inter-comparison of HONO measurements by BBCEAS at UV-wavelengths (also co-measures NO2) and HONO measured by BAQS’s quantum cascade laser (QCL) system. BBCEAS measurements of N2O5 at BAQS (and potentially upwind of BAQS), together with the site’s core instrumentation, to investigate aerosol composition and concentration change as air is advected through the city, with a focus on the production of aerosol nitrate.

Year 3

(Year 3 into 4) The BBCEAS instrument is sufficiently compact to be operated from battery power for several hours. This enables measurements to be performed from a moving vehicle. Possible further investigation of real on-road NO2 and HONO concentrations [5], or transects of NOy species across the city. Student prepares & submits a publication to a high-impact, peer-reviewed journal with the student as the first named author. Write and submit thesis.

Further reading

[1] “Air quality in Europe – 2021 report”, European Environment Agency (2021),

[2] The Lancet Commission on pollution and health, P.J. Landrigan et al., (2018) vol 391, issue 10119,  pages 462-512,

[3] Automatic Urban and Rural Network (AURN),

[4] Air quality supersites, and

[5] Nitrous acid (HONO) emissions under real-world driving conditions from vehicles in a UK road tunnel, L.J. Kramer, L.R. Crilley, T.J. Adams, S.M. Ball, F.D. Pope, and W.J. Bloss, Atmos. Chem. Phys., 20, 5231–5248, (2020),

[6] Atmospheric chemistry at night, S.M. Ball, (2014),


This project contains periods of instrument testing and optimisation in the laboratory at Leicester University and fieldwork at the Birmingham supersite. Since much of the deployment of the BBCEAS instrument to BAQS is to add extra capability to the continuous monitoring made by the BAQS’s core instrumentation, visits can be scheduled to avoid any travel restrictions. Once the student has gained experience of operating the instrument, the BBCEAS can be “ready to go” at a couple of weeks’ notice, so the timetable is adaptable. If restrictions become extensive, work can continue wholly in Leicester (and online) to work-up/interpret/model earlier datasets.