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Project Highlights: 3 points as a bulleted list

  • Atmospheric chemistry’s impacts on air quality, health and climate
  • Investigate critical questions about a poorly-understood source of atmospheric oxidants
  • State-of-the-art analytical instrumentation

Human activity is changing the composition of our atmosphere. Anthropogenic emissions change composition directly by increasing the atmospheric burden of the emitted species, and indirectly by altering (usually speeding up) the chemistry that oxidises trace gases in the atmosphere. The emitted gases and their oxidation products affect air quality, environmental and public health, and influence climate [1]. For example, emissions of NOx from road vehicles have recently been making news headlines – irrespective of which manufacturers’ vehicles are more (or less) polluting, it is certainly the case that, collectively, vehicle emissions are a large contributor to NOx and aerosol particles in urban environments. NOx is also a precursor of tropospheric ozone, itself a harmful air pollutant and a greenhouse gas.

The removal of most trace gases from the atmosphere is initiated by chemical reaction with OH radicals. OH is a product of ozone’s photochemistry and of the photolysis of certain other molecules such as nitrous acid, HONO – the focus of this project.

HONO + light ® OH + NO               [R1]

At first the only source of HONO in the atmosphere was thought to be the reaction OH + NO, i.e. the reverse of reaction R1, and that consequently HONO production and its subsequent photolysis was a null cycle yielding no nett production or loss of OH radicals [2,3]. But recent measurements of HONO [2,3] and OH [4] clearly show other HONO sources must be present. Current knowledge about these “extra” sources is poor, even though they are potentially very important for atmospheric chemistry because extra HONO leads to increased OH concentrations (via R1) which in turn enhances the atmosphere’s capacity to oxidise a wide range of trace gases.

Emission of HONO by road vehicles is one of the possible missing sources of HONO into the atmosphere. We have made measurements in a road tunnel (Fig 1) that show HONO concentrations closely tracked NO2 concentrations. But whilst vehicles are known to emit NO2, it is less clear whether the HONO in Fig 1 was also emitted directly or whether it was formed “post-tailpipe” by heterogenous reactions on exhaust particles and/or on the tunnel walls, e.g.

2 NO2 + H2O (+ light?) ® HONO + HNO3   [R2]


HONO and NO2 in an inner-city road tunnel. The concentrations of both species rise around 06:00 (morning rush hour) and drop away in the evening. The spike around 17:30 coincides with a traffic jam.


The project will conduct field observations and laboratory process studies to better constrain the uncertainties on the vehicular HONO source. Roadside deployments will investigate HONO emitted directly from traffic; laboratory work will examine HONO produced heterogeneously on particulate matter collected from vehicle exhaust. In both cases, HONO will be detected by state-of-the-art, highly sensitive, broadband cavity enhanced absorption spectrometers (BBCEAS). It is a fortuitous coincidence that the wavelength range used to detect HONO also facilitates the co-measurement of NO2 and aerosol particles.

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to CENTA research themes.

The successful candidate will join Leicester’s Atmospheric Chemistry Group (approx 20 people) and thereby benefit from the group’s extensive expertise in atmospheric trace gas detection methods, data analysis techniques & tools, atmospheric modelling, field work skills and logistics planning. Targeted training will be given on the BBCEAS and other supporting instrumentation. In addition to the dedicated CENTA training, the student should attend one master’s level lecture course (from Chemistry, Physics or Geography) in an area relevant to the project, e.g. Earth System Science.


Year 1: Training in research methodology and atmospheric chemistry; targeted specific training in the operation of the BBCEAS instrument and its data work-up. The student will also contribute to ongoing efforts to further optimise the performance of the BBCEAS instrument through testing new optical components. Student deploys instruments for roadside measurements of HONO.

Year 2: Obtain a long time series of HONO, NO2 and aerosol extinction measurements, e.g. by deploying the BBCEAS instrument from the AURN air monitoring station on the Leicester University campus. Recommision laboratory apparatus for generating controlled HONO amounts using the “Febo synthesis”. Begin laboratory investigation into heterogeneous HONO production/losses on particulate matter [Reaction R2].

Year 3: Conclude laboratory work. Further roadside measurements and/or other opportunistic field work deployment. Data synthesis & drawing conclusions. Submit a publication to a high-impact, peer-reviewed journal with (assuming things go well) the student as the first named author. Write and submit thesis.

Partners and collaboration (including CASE)

The project’s main supervisor has a collaboration with Birmingham University (another CENTA partner institution) on the chemistry of HONO in the atmospheric boundary layer. This collaboration provides the student with a platform to: contribute results from their own research; receive advice/feedback on their work from the team; integrate their results into collaborative atmospheric modelling activities; and enhance the knowledge generated by the collaboration.

It is also likely that, within the project’s lifetime, the student will deploy the BBCEAS instrument in a multi-institution field work campaign in the UK or abroad.


Further Details

Dr Stephen Ball,

Department of Chemistry

University of Leicester