Assessment of ClNO2 as a missing oxidant in the UK atmosphere

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.

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

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

Entry requirements
Applicants are required to hold/or expect to obtain a UK Bachelor Degree 2:1 or better in a relevant subject. The University of Leicester English language requirements apply where applicable.

How to apply
Please refer to the CENTA Studentship application information on our website for details of how to apply.

As part of the application process you will need to:
• Complete a CENTA Funding form – to be uploaded to your PhD application
• Complete and submit your PhD application online. Indicate project CENTA2-CHEM5-BALL in the funding section.
• Complete an online project selection form Apply for CENTA2-CHEM5-BALL