Project Highlights:

  • Reaction of air pollutants on sea salt particles yields new chemical pathways.
  • Chlorine activation oxidises volatile organic compounds and reduces the lifetime of the climate-forcing gas methane.
  • Impacts for atmospheric composition and air quality, health and climate, especially in an island nation like the UK.



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

The removal of most trace gases from the atmosphere is initiated by molecules reacting with OH radicals, NO3 radicals, or with ozone [e.g. 2]. Recently however, chlorine atoms (Cl) have been recognised as another important oxidant [3,4,5]. Chlorine atoms are extremely reactive: this means that even small levels of atomic chlorine can enhance atmospheric oxidation rates, and thereby also promote formation of tropospheric ozone and other secondary air pollutants.

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

N2O5 + NaCl ® ClNO2 + NaNO3

Photolysis of ClNO2 the next morning produces Cl atoms:

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 from primarily manmade emissions 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 in central England [5,6]. Thus ClNO2 is widespread in the UK too. However its effects are not well understood. Since we inhabit a (somewhat) polluted island, ClNO2 chemistry could be especially important for the United Kingdom.

Figure 1: A schematic of ClNO2 chemistry. Inset = field observations of ClNO2 at Weybourne (Norfolk coast) showing night-time peak amounts (blue) and daytime photolytic loss rates (yellow).


Detection of ClNO2 at ambient 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 two clean(ish) UK coastal sites. These existing field data necessarily represent “snapshots” in time – in contrast, the aim of this CENTA project is to produce a long-term CIMS data series for Leicester in order to capture the full seasonal cycle in chlorine chemistry. We also want to deploy the instrument in a coastal UK city (e.g. Plymouth) to compare and contrast ClNO2 production with that in Leicester in the centre of the country. Atmospheric chemistry models will 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

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 other relevant instrumentation available in the group. In addition to the CENTA training, we offer lecture courses that are directly relevant to the project: e.g. Earth System Science.


Year 1: Generic CENTA training 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. Begin observations in Leicester.

Year 2: Conduct a year-long time 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 analytical technique – laboratory work will investigate the effectiveness of CIMS detection of other chlorine-containing (and non-chlorine) atmospheric gases, with the aim to generate time series of other atmospheric gases from the Leicester CIMS dataset.

Year 3: Deploy the CIMS instrument at Plymouth Marine Laboratory for a period of ca. 2 months. The deployment is likely to happen in winter or spring when the previous “snapshot” observations have indicated the greatest ClNO2 concentrations occur and when Cl-initiated oxidation is likely to have the greatest impact on atmospheric oxidising capacity. Write and submit a publication to a high-impact, peer-reviewed journal with the student as the first named author. Write and submit thesis.

Partners and collaboration (including CASE)

Dr Tom Bell and Dr Ming-Xi Yang (Plymouth Marine Laboratory) are willing to host the student and the CIMS instrument in their laboratory for a period of field work. PML is approximately 0.5 km from Plymouth city centre and a similar distance from shipping traffic in Plymouth Sound (i.e urban NOx and an additional marine source of NOx). PML also operates data buoys that provide measurements of the meteorology and sea conditions in the Western English Channel, upstream of Plymouth on the prevailing SW wind.

Further Details

Potential applicants are welcome to discuss the project informally and obtain further information from the project supervisors:

Dr Stephen Ball (sb263@le.ac.uk) and Prof Paul Monks (psm7@le.ac.uk)

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