Project highlights

  • Improve understanding of inorganic iodine emissions at the ocean’s surface.
  • Investigate suppression of iodine fluxes by dissolved organics in natural seawater.
  • New measurement methods for iodine species applicable at ambient conditions.


The presence of halogens (Cl, Br, I) in the troposphere markedly changes the chemistry of this part of the atmosphere [1]. Gas-phase chemistry of halogens perturbs the radical chain reactions in the NOx and HOx chemical families, thereby affecting the concentration of OH radicals and the photochemical production rates of tropospheric ozone. Iodine is particularly effective at controlling tropospheric ozone concentrations over the oceans. The oceans are a sink of tropospheric ozone because ozone reacts with dissolved iodide anions at the ocean’s surface. Overall, iodine’s chemistry reduces the likelihood that high ozone events can occur (i.e. iodine plays a role in ameliorating poor air quality) and lower average ozone concentrations mean tropospheric ozone is less effective as a greenhouse gas [2].

Oceans are also the main source of iodine into the atmopshere. Some iodine is biological in origin: it comes from volatile organic iodine compounds (CH3I, CH2I2 etc) emitted by plankton and from molecular iodine (I2) emitted by seaweeds growing in coastal locations. But there is now strong evidence from laboratory and modelling studies to show that the largest flux of iodine into the atmosphere comes from I2 and HOI produced from the inorganic I(aq) + O3(g) reaction at the ocean surface [3,4]. Iodide reacts rapidly with ozone (six orders of magnitude faster than the equivalent Br(aq) + O3(g) reaction), and thus this reaction is effective even at the very low iodide concentrations found naturally in seawater ([I] » 100 nanoMolar).

Recently our research group used a highly sensitive spectroscopic technique to quantify I2 fluxes generated when ambient concentrations of ozone (20-150 ppbv) react with solutions containing iodide concentrations down to 150 nanoMolar (i.e. very close to ambient oceanic iodide amounts) [5]. We found that organic compounds present in natural seawater reduced the I2 fluxes by an order of magnitude compared to I2 produced from the reaction of ozone with synthetic artifical seawater (buffered potassium iodide solutions), see Figure 1. This suppression of I2 fluxes has profound implications for translating laboratory measurements into modelling the “real-world” iodine release from the oceans. More experimental data are urgently required to confirm these results.



University of Leicester


  • Climate and Environmental Sustainability


Project investigator

  • Dr Stephen Ball, Department of Chemistry, University of Leicester



  • Prof Paul Monks, Department of Chemistry, University of Leicester

How to apply


Laboratory studies don’t always provide realistic representations of the real environment. Previous literature studies of the I(aq) + O3(g) reaction [3,4] had to be perfomed at iodide and/or ozone concentrations vastly above those occurring in the natural environment, raising questions about how reliably laboratory results can be extrapolated to ambient conditions.

In contrast, this project uses state-of-the-art analytical instruments to follow this reaction at ambient iodide and O3 concentrations. The CENTA student will use our existing broadband cavity enhanced absorption spectroscopy (BBCEAS) instrument, which has a proven detection limit for molecular iodine of 4 pptv (= 4´1012 mixing ratio). The BBCEAS light beam is integrated into a bespoke, thermostated reaction vessel and records I2 concentrations 3.5 cm above the liquid’s surface [5].  Experiments will investigate I2 fluxes from artifical seawater (buffered solutions of iodide, bromide, chloride) and “natural” samples of subsurface seawater and the surface microlayer.

Training and skills

The student will join the Atmospheric Chemistry Group at the University of Leicester and thereby benefit from the group’s extensive expertise in trace gas detection methods, data analysis techniques, atmospheric modelling, fieldwork skills and logistics planning. Targeted training will be given to use the broadband cavity enhanced absorption spectrometer (BBCEAS), its spectral fitting software, and other relevant supporting instrumentation available in the group. We offer various lecture courses that are directly relevant to the project: e.g. Earth System Science.

Partners and collaboration

We have an existing collaboration with colleagues at York University from our consortium NERC award “Iodide in the ocean: distribution and impact on iodine flux and ozone loss”. Experimental data gathered by the CENTA student will feed into, and be used to refine, York’s model of the chemical mechanism for the I(aq) + O3(g) reaction. This collaboration provides the student with a ready-made platform to discuss results obtained during the CENTA PhD studentship and to integrate their results into collaborative atmospheric modelling activities.

Further details

Potential applicants are welcome to discuss the project with the lead supervisor:

Dr Stephen Ball,,

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

Possible timeline

Year 1

Generic training from CENTA. Training specific to this project – the CENTA student will work alongside another PhD student already in post. They will “learn on the job” to operate the BBCEAS instrument and to work-up its data series. Laboratory experiments to quantify I2 and HOI released from ozone’s reaction with synthetic sea water and samples of natural seawater as a function of ozone mixing ratios, concentration of aqueous-phase iodide, temperature and pH.

Year 2

In our published study [5], experiments were performed on a very limited set of natural seawater samples. This phase of the project will investigate iodine emissions from the reaction of ozone with natural seawater samples obtained from different geographical locations and different times of the year (seasonal variability).

Year 3

Consolidation of the dataset to produce a parameterisation of iodine production rates as a function of the relevant variables. The parameterisation will be in a form suitable to be incorporated into advanced models of atmospheric chemistry. Submit a publication to a high-impact, peer-reviewed journal with the student as the first named author. Write and submit thesis.

Further reading

[1] Saiz-Lopez, A., et al.: Atmospheric chemistry of iodine, Chemical Reviews, 112, 1773, (2012)

[2] Sherwen, T., et al.: Effects of halogens on European air-quality, Faraday Discussions, 200, 75-100 (2017),

[3] Carpenter, L. J., et al.: Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine, Nature Geoscience, 6, 108, (2013),

[4] MacDonald. S.M., et al.: A laboratory characterisation of inorganic iodine emissions from the sea surface: dependence on oceanic variables and parameterisation for global modelling, Atmos. Chem. Phys., 14, 5841–5852, (2014),

[5] Tinel, L., et al.: Influence of the sea surface microlayer on oceanic iodine emissions, Environmental Science & Technology, accepted (2020),


This is a laboratory-based project at the University of Leicester (no fieldwork), and the research labs are open, and covid-safe. Some initial samples are already available, and further seawater sampling will be arranged for times when COVID-19 restrictions allow. In the event of a full & prolonged lockdown, work can still continue online to test (and refine) York’s model of iodine emissions against the existing experimental dataset.