Overview

The presence of halogens (Cl, Br I) in the troposphere markedly changes the chemistry of this part of the atmosphere [1]. In particular, halogens affect the concentration of OH radicals (the most important oxidant in the troposphere), alter the partitionning in chemical families (NOx, HOx etc), and act as sinks for tropospheric ozone. A robust understanding of halogen chemistry and the source rates of halogens into the atmosphere is therefore crucial for understanding trace gas composition and the oxidising capacity of the troposphere [2].

In the case of iodine, the major souce was initailly thought to be organic compounds (CH3I, CH2I2 etc) emitted by plankton. Subsequently very large inputs of molecular iodine (I2) were observed in coastal regions from seaweeds exposed to the air around low tides [3]. Recent attention has switched to non-biological iodine inputs occuring over the open ocean [4]. Here gas-phase ozone reacts iodide ions dissolved in the surface layer of the ocean to release molecular iodine and hypoiodous acid (HOI). The reaction is slow because ozone concentrations are typically only 30 ppbv (mixing ratio = 30 x 10-9) and the concentration of iodide in seawater is only around 0.1 micro-Molar. However the ocean is a big place, and so aggregated over a large area, even a slow reaction can introduce large fluxes of iodine to the atmosphere. Additionally, the reaction of ozone at the sea surface consitutes a significant sink for tropospheric ozone. This sink partically offsets the net global sources of tropospheric ozone from the chemistry of nitrogen oxides (NOx is mainly a manmade pollutant).

Large uncertainties remain in understanding ozone’s deposition fluxes to the ocean and the accompanying release of volatile iodine compounds. In part, this is because previous laboratory investigations of the ozone + iodide reaction were conducted at ozone and/or iodide concentrations significantlly above ambient concentrations in order for the reaction’s products to be detectable. In contrast, this project will use state-of-the-art analytical instruments to follow this reaction at ambient O3 and iodide concentrations.

A chemistry schematic of ozone loss and iodine release from the ocean surface

Methodology

We have developed instruments based on broadband cavity enhanced absorption spectroscopy (BBCEAS) that achieve detection limts of 5 pptv for molecular iodine (mixing ratio = 5 x 10-12). Previously we have used BBCEAS to measure iodine emitted by seaweed samples in coastal locations [3]. Recent experiments in our laboratory have shown that BBCEAS also has the sensitivity needed to quantify molecular iodine released when dilute ozone mixtures contact sea water samples. This project will additionally apply BBCEAS methods and other state-of-the-art analytical techniques to detect other iodine-containing compounds released from seawater.

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 use the broadband cavity enhanced absorption spectrometer (BBCEAS), and other relevant supporting instrumentation available in the group. We also offer various lecture courses that are directly relevant to the project: e.g. Earth System Science.

Timeline

Year 1: Generic training from CENTA. Training specific to this project – the student will be taught to operate the BBCEAS instrument and to work-up its data series. Laboratory experiments to quantify I2 released from ozone’s reaction with synthetic sea water as a function of ozone mixing ratios, concentration of aqueous-phase iodide, temperature and pH.

Year 2: Further experiments on I2 release, including from “real” sea water samples obtained from various coastal and open-ocean locations. Extension of BBCEAS and other methods to detect other iodine-containing compounds released from reactions with sea water. Basic training in atmospheric models.

Year 3 & 4: Consolidation of the dataset to produce a parameterisation of iodine production rates as a function of the relevant variables (concentrations etc). The parameterisation will be in a form suitable to be incorporated into advanced models of atmospheric chemistry. (Assuming things go well) 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)

We already have a joint project with two other UK universities about iodide in the ocean, and its impact on the atmospheric iodine flux and ozone loss. This collaboration provides the student with a ready-made platform to discuss results obtained during the CENTA PhD studentship, to receive advice/feedback on their work, 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, sb263@le.ac.uk,

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