Natural and manmade processes emit huge quantities of gases into the atmosphere. Were it not for oxidation chemistry, such emissions would build up to harmful concentrations. The chemical processing of gas emissions, their oxidation products, and their atmospheric lifetimes affect atmospheric composition, air quality, environmental and public health, and influence climate. [1]

The removal of most trace gases from the atmosphere begins with molecules reacting with OH radicals during the day, with NO3 radicals at night, or by direct reaction with ozone [e.g. 2]. Recently however, chlorine (Cl) has been recognised as another important oxidant [3,4,5]. Chlorine atoms are extremely reactive. This means that even small levels of chlorine can enhance oxidation rates, and hence also promote formation of tropospheric ozone and other secondary air pollutants.

Chlorine is activated in a two step process starting with the night-time reaction of N2O5 with chloride in particles of sea salt. 

N2O5 + NaCl ClNO2 + NaNO3

Photolysis of the ClNO2 product yields Cl atoms the next morning:

ClNO2 + light Cl + NO2

Note how the first step involves the reaction of a natural component of the atmosphere (sea salt aerosol) with N2O5, a species derived primarily from the manmade pollutants of nitrogen oxides (NOx).

Early studies conducted in the USA focussed on coastal regions, the assumption being that ClNO2 production would be limited by the availability of sea salt. However later studies observed ClNO2 at inland locations 1000 km or more from the sea [3,4]. Our group has observed ClNO2 at two UK coastal sites and on almost every night when we made measurements in Leicester [5]. Thus ClNO2 is widespread. Its effects however are still not well understood. Since we inhabit a (somewhat) polluted island, ClNO2 chemistry could be especially important for the United Kingdom.

ClNO2 chemistry. Inset = field measurements of ClNO2 at Weybourne (Norfolk coast) showing night-time peak amounts and daytime photolytic loss.


Detection of ClNO2 at ambient atmospheric concentrations requires a highly sensitive analytical technique: this project uses Chemical Ionisation Mass Spectrometry (CIMS). We have successfully deployed a CIMS instrument to measure ClNO2 and molecular chlorine (Cl2) from the Leicester University campus and at measurement stations at two UK coastal sites. However these measurements represent “snapshots” in time – the aim of the CENTA project is to produce a long-term CIMS data series in order to capture the full seasonal cycle in chlorine chemistry. Atmospheric chemistry models will then be used to identify the ambient conditions under which ClNO2 is most likely to form, and to model the effects of chlorine chemisty on downstream atmospheric processes.

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD, including a placement of at least two weeks with an industrial/government partner. 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 the student's projects and themes. 

The student will join the Atmospheric Chemistry Group at Leicester University (approx 20 people) and thereby benefit from the group’s extensive expertise in trace gas detection methods, data analysis techniques, atmospheric modelling, field work skills and logistics planning. Targeted training will be given to operate the CIMS instrument, a broadband cavity enhanced absorption spectrometer (BBCEAS), and any other relevant supporting instrumentation available in the group. We also offer several lecture courses that are directly relevant to the project: e.g. Earth System Science.


Year 1: Generic training (CENTA and locally) and training specific to this project. The student will be taught to operate the CIMS instrument and other research and commercial instruments capable of making the supporting measurements of chemically-related species, notably a broadband cavity spectrometer (BBCEAS) for detecting N2O5, the main ClNO2 precursor. Start training in atmospheric models.

Year 2: Conduct a long-term series of ambient observations in Leicester. Begin chemical modelling of the ClNO2 sources, sinks and downstream Cl oxidation chemistry, focussing initially on “interesting” case studies from the observational period. Additionally, chemical ionisation mass spectrometry is a versatile technique. We have observed additional signals in the mass spectra of ambient samples, but it is currently unclear which atmospheric species produce these peaks. Laboratory work will investigate the assignment of these signals, with the aim to leverage measurements of other atmospheric gases by CIMS.

Year 3: More advanced modelling work will investigate the pervasiveness of ClNO2 chemistry in the UK context, its affects on oxidising capacity, and on formation rates for tropospheric ozone. The Atmospheric Chemistry Group regularly deploys instruments for atmospheric field work and for experiments at atmospheric simulation chambers: Year 3 provides the opportunity to piggy-back the student and his/her CIMS instrument into a relevant field-work deployment in the UK or Europe, if such an opportunity exists.


Partners and collaboration (including CASE)

Atmospheric trace gas measurements are an inherently collaborative endeavour. The supervisory team has numerous collaborations with other UK and international investigators. It is probable that, within the lifetime of the PhD project, the student will deploy the CIMS instrument in collaborative field work in the UK or abroad.

Further Details

Potential applicants are welcome to discuss the project with the supervisory team:

Dr Stephen Ball, sb263@le.ac.uk,

Prof Paul Monks, psm7@le.ac.uk,

Department of Chemistry, University of Leicester, Leicester LE1 7RH