Project highlights

  • Improve understanding of oceanic iodine emissions into the atmosphere.
  • Investigate suppression of iodine fluxes by dissolved organics in natural seawater.
  • New measurement methods for iodine species applicable at ambient conditions.


Halogen (Cl, Br, I) compounds markedly alter the chemistry of the troposphere (the lower part of the atmosphere). Gas-phase chemistry of halogens perturbs the radical chain reactions in the NOx and HOx chemical families, thereby affecting the concentration of OH radicals, the oxidising capacity of the troposphere, and the photochemical production of tropospheric ozone [1,2]. Iodine is particularly effective at controlling tropospheric ozone concentrations over the oceans, because ozone reacts with dissolved iodide ions at the ocean’s surface. Overall, iodine’s chemistry reduces the likelihood that high ozone events can occur (i.e. reduces the likelihood of poor air quality); also lower average ozone concentrations mean tropospheric ozone is less effective as a greenhouse gas [ref 2 and references therein].

Oceans are also the main source of iodine going 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 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.

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 (approx the same as 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


How to apply


Laboratory studies don’t always provide realistic representations of the real environment. Previous studies of the I(aq) + O3(g) reaction had to be perfomed at iodide and/or ozone concentrations vastly above those occuring 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 the 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) [5].  Experiments will investigate I2 fluxes from artifical seawater (buffered solutions of variable iodide, bromide, chloride concentrations) and natural seawater samples. Additionally, the BBCEAS instrument is scheduled to participate in oceanographic fieldwork in the Atlantic ocean in winter 2021/22, which will generate an observational dataset to compare with these laboratory results.

Training and skills

The student will join the Atmospheric Chemistry Group at Leicester University 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 use the broadband cavity enhanced absorption spectrometer (BBCEAS), its spectral fitting software, and other relevant supporting instrumentation available in the group. We offer various Master’s level 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. 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 interfacial model is used by atmospheric modellers to parameterise oceanic iodine emissions. The collaboration provides the student with a ready-made platform to discuss datasets 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, [email protected],

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

To apply to this project please visit:

Possible timeline

Year 1

Generic training from CENTA. Training specific to this project – the CENTA student will “learn on the job” to operate the broadband cavity enhanced spectrometer (BBCEAS) 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 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). We will also compare the lab results to oceanic I2 observations made in the Atlantic during the oceanographic ship cruise.

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 publication(s) to a high-impact, peer-reviewed journal(s) with the student as the first named author. Write and submit thesis.

Further reading

[1] Saiz-Lopez, A., et al. (2012), Atmospheric chemistry of iodine, Chemical Reviews, 112, 1773-1804.

[2] Carpenter L.J., et al. (2021), “Marine iodine emissions in a changing world”, Proceedings of the Royal Society A – Mathematical and Engineering Sciences, Volume 477, Issue 2247, Article Number 20200824.

[3] Carpenter, L.J., et al. (2013), Atmospheric iodine levels influenced by sea surface emissions of inorganic iodine, Nature Geoscience, 6, 108-111,

[4] MacDonald. S.M. et al. (2014), 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,

[5] Tinel, L., et al. (2020), Influence of the sea surface microlayer on oceanic iodine emissions, Environmental Science & Technology, 54, 13228-13237.


This is a laboratory-based project at the University of Leicester. The research labs are open and covid-safe. Experiments can run with artificial seawater samples at any time, synthesised using bought-in reagents. Some initial natural seawater samples are already available, and further seawater sampling (at UK locations) will be arranged according to when any prevailing COVID-19 restrictions allow. In the event of a full & prolonged lockdown, work can still continue online to (i) test and refine York’s model of iodine emissions against Leicester’s existing experimental dataset and (ii) to interpret/model oceanic I2 observations made during the winter 2021/22 oceanographic ship cruise.