Iodine Emissions Research Articles

Depletion of iodide in ageing aerosols and the role of humidity: A case study of mixed sodium iodide-malonic acid aerosol

Global sea to air iodine emissions, along with organic emissions and their oxidation products, have increased tremendously. This work presents a comprehensive analysis of the humidity mediated changes in ageing aerosols comprising iodide and water soluble dicarboxylic acid using aerosol micro-Raman spectroscopy. The studies in the model system, sodium iodide-malonic acid mixed aerosols, unveiled the depletion in iodide. Mechanistic insights gleaned through comparative studies conducted under inert (nitrogen) and oxidative (air) atmospheres reveal the iodide depletion occurs possibly via oxidation to molecular iodine. The reaction involves gaseous components, diffusion of which across the particles will be impacted by the physical state of the particles, such as viscosity, which in turn is intricately linked to ambient humidity levels. To this end, studies on the temporal evolution of the reaction at three distinct RHs covering 30–80% revealed the enhanced progression of the reaction with increasing humidity. Given that geographical locations serving as major sources for atmospheric iodine typically experience high humidity, these reactions could emerge as an additional process controlling iodine speciation in ageing aerosols.

An improved estimate of inorganic iodine emissions from the ocean using a coupled surface microlayer box model

Abstract. Iodine at the ocean's surface impacts climate and health by removing ozone (O3) from the troposphere both directly via ozone deposition to seawater and indirectly via the formation of iodine gases that are released into the atmosphere. Here we present a new box model of the ocean surface microlayer that couples oceanic O3 dry deposition to inorganic chemistry to predict inorganic iodine emissions. This model builds on the previous work of Carpenter et al. (2013), improving both chemical and physical processes. This new box model predicts iodide depletion in the top few micrometres of the ocean surface due to rapid chemical loss to ozone competing with replenishment from underlying water. From this box model, we produce parameterized equations for HOI and I2 emissions, which are implemented into the global chemical transport model GEOS-Chem along with an updated sea surface iodide climatology. Compared to the previous model, inorganic iodine emissions from some tropical waters decrease by as much as half, while higher-latitude emissions increase by a factor of ≫10. With these large local changes, global total inorganic iodine emissions increased by ∼49 % (2.99 to 4.48 Tg) compared to the previous parameterization. This results in a negligible change in average tropospheric OH (<0.2 %) and tropospheric methane lifetime (<0.2 %). The annual mean tropospheric O3 burden decreases (−1.5 % to 325 Tg); however, higher-latitude surface O3 concentrations decrease by as much as 20 %.

β-Ketoenamine-linked covalent organic framework for efficient iodine capture.

Exploring the materials that effectively capture radioactive iodine is crucial in managing nuclear waste produced from nuclear power plants. In this study, a β-ketoenamine-linked covalent organic framework (bCOF) is reported as an effective adsorbent to capture iodine from both vapor and solution. The bCOF's high porosity and heteroatom-rich skeleton offer notable iodine vapor uptake capacity of up to 2.51 g g-1 at 75 °C under ambient pressure. Furthermore, after five consecutive adsorption-desorption cycles, the bCOF demonstrates high reusability performance with significant iodine vapor capacity retention. The adsorption mechanism was also investigated using various ex situ structural characterization techniques, and these mechanistic studies revealed the existence of a strong chemical interaction between the bCOF and iodine. The bCOF also showed good iodine uptake performance of up to 512 mg g-1 in cyclohexane with high removal efficiencies. The bCOF's performance in adsorbing iodine from both vapor and solution makes it a promising material to be used as an effective adsorbent in capturing radioactive iodine emissions from nuclear power plants.

Anthropogenic iodine-129 tracks iodine cycling in the Arctic

Global warming has accelerated the iodine cycle in the Arctic Ocean, amplifying its significance in the global iodine cycle. However, some intricacies of the iodine cycle in the Arctic marine environment remain unclear. In this study, we present concentrations of stable- and radio-iodine (127I and 129I) in total iodine, iodide (I–), and iodate (IO3–) in seawater from the Beaufort, Chukchi, and Bering Seas. Results show a wide range of 129I/127I ratios of total iodine (3.2 × 10–12–9.3 × 10–9), indicating that the total 129I/127I ratio is a valuable fingerprint for identifying water masses and comprehending mixing processes. Iodine loss in surface seawater was quantitatively assessed using 129I, revealing that Arctic rivers substantially contribute to the emission of iodine into the atmosphere from the Arctic and subarctic seas. Our redox simulation in the Arctic intermediate layer shows the concentrations and 129I/127I ratios of I– and IO3– during its migration and provides the first evidence of an IO3– reduction process masked by the I– oxidation in the intermediate Arctic Ocean. Oxidation and reduction processes in the intermediate Arctic Ocean were characterized by extremely slow rates, at 0.95–2.41 and 0.26 ± 0.13 nM/year, respectively. Our findings demonstrate the effectiveness of anthropogenic 129I in tracking iodine cycling in the Arctic marine environment.

Iodine oxoacids and their roles in sub-3 nm particle growth in polluted urban environments

Abstract. New particle formation contributes significantly to the number concentration of ultrafine particles (UFPs, d ≤ 100 nm) and has a great impact on human health and global climate. Iodine oxoacids (HIOx, including iodic acid, HIO3, and iodous acid, HIO2) have been observed in pristine regions and proved to dominate new particle formation (NPF) at some sites. However, the knowledge of HIOx in polluted urban areas is rather limited. Here, we conducted a long-term measurements of gaseous iodine oxoacids and sulfuric acid in Beijing from January 2019 to October 2021 and also in Nanjing from March 2019 to February 2020 and investigated the contribution of HIOx to UFP number concentration in both urban environments. HIO3 is highest in summer, up to 2.85 × 106 and 2.78 × 106 cm−3 in Beijing and Nanjing, respectively, and is lowest in winter by 96 % and 75 %, respectively. HIO3 exhibits more prominent variation than H2SO4 in both urban sites. HIO3 concentration shows a clear diurnal pattern at both sites with a daily maximum at around noontime, similar to the atmospheric temperature, solar radiation, and ozone (O3) levels. HIO2 concentration has the same diurnal and seasonal trend as HIO3 but is overall about an order of magnitude lower than HIO3 concentration. Back trajectory analysis suggests that the sources for inland iodine species could be a mix of marine and terrestrial origins, with both having peak iodine emission in warm seasons. While the contribution of HIO2 to particle growth is marginal in Beijing and Nanjing, our results demonstrate that HIO3 enhances the particle survival probability of sub-3 nm particles by about 40 % (median) and occasionally by more than 100 % in NPF events, suggesting HIOx are significant contributor to UFPs in polluted urban areas. As the growth contribution from HIO3 and H2SO4 is similar on a per-molecule basis, we propose that the sum of HIO3 and H2SO4 could be used to estimate sub-3 nm particle growth of inorganic acid origin in polluted atmospheres with a significant amount of HIOx.

The paradigm for exceptional iodine capture by nonporous amorphous electron-deficient cyclophanes

Nuclear power emerges as a beacon of hope in tackling the energy crisis. However, the emission of radioactive iodine originating from nuclear waste and accidents poses a serious danger to nature and human well-being. Therefore, it becomes imperative to urgently develop suitable adsorbents capable of iodine capture and long-term storage. It’s generally recognized that achieving high iodine capture efficiency necessitates the presence of electron-rich pores/cavities that facilitate charge-transfer (CT) interactions, as well as effective sorption sites capable of engaging in lone pair interactions with iodine. In this study, an unprecedented iodine capture paradigm by nonporous amorphous electron-deficient tetracationic cycloalkanes in vapor and aqueous solutions is revealed, overturning preconceived notions of iodine trapping materials. A newly reported tetracationic cyclophane, BPy-Box4+, exhibited an exceptional iodine vapor sorption capacity of 3.99 g g−1, remarkable iodine removal efficiency in aqueous media, and outstanding reusability. The iodine capture mechanism is unambiguously elucidated by theoretical calculations and the single-crystal structures of cyclophanes with a gradual increase in iodine content, underlining the vital role of host-guest (1:1 or 1:2) interactions for the enhanced iodine capture. The current study demonstrates a new paradigm for enhanced iodine capture by nonporous amorphous electron-deficient cyclophanes through host-guest complexation.

Differences in iodine chemistry over the Antarctic continent

High concentrations of iodine oxide (IO) have been reported over west Antarctica, with areas around the Weddell Sea showing a peak in spring. However, stations in east Antarctica show much lower values during summer, although observations over spring are still missing. Here, we present the first year-long observations of IO outside the Weddell Sea region using a multi-axis differential optical absorption spectrometer (MAX-DOAS) over the Bharati station (69.41°S, 76.19°E). Observations show that iodine chemistry is less active than over the Weddell Sea, even during springtime, with IO mixing ratios below 2 pptv throughout the sunlit period. A slight increase in IO is observed in spring, although it is a factor of 10 lower than the Weddell Sea region. We identify the variations in drivers in the different regions using sea ice concentrations, sea ice thickness and chlorophyll concentrations. We use a global model which uses a parameterization for iodine emissions based on a combination of these factors. The model reproduces the high concentrations over the Weddell Sea and the low concentrations over Bharati throughout the year, shedding light on the environmental factors, sources and chemistry of iodine in Antarctica. Even at small concentrations, iodine can enhance ozone loss caused by bromine chemistry over east Antarctica, although this impact is lower than in the west Antarctic.

Vibrational and Structural Properties of Two-Dimensional Tin Mixed-Halide Perovskites

The emergence of two-dimensional (2D) hybrid metal-halide perovskites has garnered significant attentions for optoelectronic devices and light-emitting applications. Since the toxicity of lead-based perovskites could potentially be harmful to the environment, several works have attempted to change the active metal to tin (Sn). Here, we investigate the characterization of (PEA)2SnBrxI4-x mixed halide perovskites using X-ray fluorescence (XRF), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Qualitative XRF analysis suggests the presence of tin, bromine and iodine emissions under the mid-Z and high-Z ranges. In mid-Z range, Br-Kα peak appeared on 11.96 keV and Br-Kβ was detected on 13.3 keV. Meanwhile Sn-Kα, I-Kα, I-Kβ1, and I-Kβ2 peaks were detected in high-Z range on 25.24 keV, 28.6 keV, 32.35 keV and 33.11 keV, respectively. Thus, the elemental composition of mixed halide components exhibits an indicative control that bromine-rich or iodine-rich can be synthesized via rational chemical design. XRD pattern display a systematic progression at the peak 5.18° (corresponds to (002) plane), which unambiguously demonstrated the feasibility to tune halide composition in tin-based hybrid perovskite. It also confirms that (2D) hybrid metal-halide with tunable halide have identical structure for both bromine-rich and iodine-rich composition. Furthermore, the 2θ peaks slightly shifted to lower angle with increasing bromine composition. The presence of C−I bonding on ~500 cm-1 and C-Br bond on ~600 cm-1 in FTIR spectra highlights the functional group of organic cations. These experimental results promote a foundation to implement compositional engineering on 2D-tin mixed-halide perovskites for optoelectronics and scintillators.

Comparing the Importance of Iodine and Isoprene on Tropospheric Photochemistry

AbstractIsoprene, arguably the most studied biogenically emitted gas, is thought to have a large impact on tropospheric composition. Other naturally emitted species have been considered to play a less important role. Here the GEOS‐Chem model is used to compare the impacts of isoprene and iodine emissions on present‐day tropospheric composition. Removing isoprene emissions leads to a 3.4% decrease in tropospheric O3 burden, a smaller absolute change than the 5.9% increase from removing iodine emissions. Iodine has a negligible impact on global mean OH concentrations and methane lifetime (+0.6% and +0.05%) whereas isoprene has a substantial impact on both (−4.3% and −4.2%). Isoprene emissions and chemistry are seen as essential for tropospheric chemistry models, but iodine is often not. We suggest that iodine should receive greater attention in model development and experimental research to allow improved predictions of past, present, and future tropospheric O3.

Sensitive and High-Throughput Analysis of Volatile Organic Species of S, Se, Br, and I at Trace Levels in Water and Atmospheric Samples by Thermal Desorption Coupled to Gas Chromatography and Inductively Coupled Plasma Mass Spectrometry.

Emissions of volatile organic sulfur (S), selenium (Se), bromine (Br), and iodine (I) species from aquatic ecosystems represent an important source of these elements into the atmosphere. Available methods to measure these species are either not sensitive enough or not automated, which hinder a full understanding of species distribution and production mechanisms. Here, we present a sensitive and high-throughput method for the simultaneous and comprehensive quantification of S, Se, Br, and I volatile organic species in atmospheric and aqueous samples using a preconcentration step onto sorbent tubes and subsequent analysis by thermal desorption coupled to gas chromatography and inductively coupled plasma mass spectrometry (TD-GC-ICP-MS). Selected commercially available sorbent tubes, consisting of mixed porous polymer and graphitized black carbon, offered the highest trapping capacity and lowest loss of species when stored at -20 °C for 28 days after sampling. After optimization of the TD-GC-ICP-MS method, absolute detection limits were better than 3.8 pg, 9.1 fg, 313 fg, and 50 fg, respectively, for S, Se, Br, and I species. As a proof of concept, the concentrations of target species were determined in aqueous and continuously collected atmospheric samples during a cruise in the Baltic and North Seas. Moreover, unknown S, Br, and I volatile species were detected in both aqueous and atmospheric samples demonstrating the full potential of the method.

Iodine emission from the reactive uptake of ozone to simulated seawater.

The heterogeneous reaction of ozone and iodide is both an important source of atmospheric iodine and dry deposition pathway of ozone in marine environments. While the iodine generated from this reaction is primarily in the form of HOI and I2, there is also evidence of volatile organoiodide compound emissions in the presence of organics without biological activity occuring [M. Martino, G. P. Mills, J. Woeltjen and P. S. Liss, A new source of volatile organoiodine compounds in surface seawater, Geophys. Res. Lett., 2009, 36, L01609, L. Tinel, T. J. Adams, L. D. J. Hollis, A. J. M. Bridger, R. J. Chance, M. W. Ward, S. M. Ball and L. J. Carpenter, Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions, Environ. Sci. Technol., 2020, 54, 13228-13237]. In this study, we evaluate our fundamental understanding of the ozonolysis of iodide which leads to gas-phase iodine emissions. To do this, we compare experimental measurements of ozone-driven gas-phase I2 formation in a flow tube to predictions made with the kinetic multilayer model for surface and bulk chemistry (KM-SUB). The KM-SUB model uses literature rate coefficients used in current atmospheric chemistry models to predict I2(g) formation in pH-buffered solutions of marine composition containing chloride, bromide, and iodide compared to solutions containing only iodide. Experimentally, I2(g) formation was found to be suppressed in solutions containing seawater levels of chloride compared to solutions containing only iodide, but the model does not predict this effect using literature rate constants. However, the model is able to predict this trend upon adjustment of two specific reaction rate constants. To more closely represent true oceanic conditions, we add an organic component to the proxy seawater solutions using material generated from Thalassiosira pseudonana phytoplankton cultures. Whereas the rate of ozone deposition is unaffected, the formation rate of I2(g) is strongly suppressed in the presence of biological organic material, indicative of a sink or reduction of reactive iodine formed during the oxidation process.

The gas-phase formation mechanism of iodic acid as an atmospheric aerosol source

Iodine is a reactive trace element in atmospheric chemistry that destroys ozone and nucleates particles. Iodine emissions have tripled since 1950 and are projected to keep increasing with rising O3 surface concentrations. Although iodic acid (HIO3) is widespread and forms particles more efficiently than sulfuric acid, its gas-phase formation mechanism remains unresolved. Here, in CLOUD atmospheric simulation chamber experiments that generate iodine radicals at atmospherically relevant rates, we show that iodooxy hypoiodite, IOIO, is efficiently converted into HIO3 via reactions (R1) IOIO + O3 → IOIO4 and (R2) IOIO4 + H2O → HIO3 + HOI + (1)O2. The laboratory-derived reaction rate coefficients are corroborated by theory and shown to explain field observations of daytime HIO3 in the remote lower free troposphere. The mechanism provides a missing link between iodine sources and particle formation. Because particulate iodate is readily reduced, recycling iodine back into the gas phase, our results suggest a catalytic role of iodine in aerosol formation.

Oxygen-rich microporous carbons with exceptionally high adsorption of iodine

The effective adsorption of radioactive iodine from nuclear waste using porous carbons remains significantly challenging, which currently appears to have an upper limit for their uptake capacity. Here, to overcome this upper limit, we explore the potential of oxygen-rich microporous carbons derived from cellulose diacetate via a general hydrothermal carbonization and KOH activation route. These activated carbons show exceptionally high adsorption of radioactive iodine due to the combined effect of their high surface area, large pore volume and intrinsic oxygen-rich nature. Our results indicate that these activated carbons have large surface areas of 1829–2997.7 m2 g−1, large pore volumes of 0.88–1.40 cm3 g−1 and high oxygen contents of 19.32–26.14%. They possess high iodine adsorption capacities of up to 6.44 g g−1 in dry conditions and 2.44 g g−1 even at high humidity (74.0 ± 0.9%). Further DFT calculations reveal that the oxygen-rich nature of activated carbon enhances its interaction with iodine molecules, resulting in a high pore accessibility. Also, the porous carbon shows good iodine retention and good reusability for practical applications. These results have fundamental implications for understanding the iodine uptake of oxygen-rich porous carbon and designing materials to handle radioactive iodine emission.

Multiple ionization of iodine for 2.5\u20135.0 MeV I22+ ions impacting on Fe target

The L-shell X-ray emissions of iodine are investigated as a function of the incident energy for I22+ ions impacting on Fe target in the energy region near the Bohr velocity. Six distinct L-subshell X-rays, Lι, Lα1, 2, Lβ1, 3, 4, Lβ2, 15, Lγ1 and Lγ2, 3, 4, 4', are observed. Compared to the atomic data, the energy of the experimental X ray shifts to the higher energy side. The relative intensity ratios of Lι, Lβ1, 3, 4, Lβ2, 15, to Lα1, 2, Lι to Lβ2, 15 and Lγ2, 3, 4, 4/ to Lγ1 are enhanced, but has no obvious change with the increase of projectile energy in the present energy region. That is interpreted by the multiple ionization effect of the M-, N- and O-shell electrons.

Mixing state and distribution of iodine-containing particles in Arctic Ocean during summertime

Iodine chemistry plays a key role in ozone destruction and new aerosol formation in the marine boundary layer (MBL), especially in polar regions. We investigated iodine-containing particles (0.2-2 μm) in the Arctic Ocean using a ship-based single particle aerosol mass spectrometer from July to August 2017. Seven main particle types were identified: dust, biomass combustion particles, sea salt, organic S, aromatics, hydrocarbon-like compounds, and amines. The number fraction of iodine-containing particles was higher inside the Arctic Circle (>65°N) than outside (55–65°N). According to the air mass back trajectories, the latitudinal distribution of iodine-containing particles can be mainly attributed to iodine emissions from the sea ice edge region. Diurnal trends were found, especially during the second half of cruise, with peak iodine-containing particle number fractions during low-light conditions and relatively low number fractions at midday. These results imply that solar radiation plays a significant role in modulating particulate iodine in the Arctic atmosphere.

Climate changes modulated the history of Arctic iodine during the Last Glacial Cycle

Iodine has a significant impact on promoting the formation of new ultrafine aerosol particles and accelerating tropospheric ozone loss, thereby affecting radiative forcing and climate. Therefore, understanding the long-term natural evolution of iodine, and its coupling with climate variability, is key to adequately assess its effect on climate on centennial to millennial timescales. Here, using two Greenland ice cores (NEEM and RECAP), we report the Arctic iodine variability during the last 127,000 years. We find the highest and lowest iodine levels recorded during interglacial and glacial periods, respectively, modulated by ocean bioproductivity and sea ice dynamics. Our sub-decadal resolution measurements reveal that high frequency iodine emission variability occurred in pace with Dansgaard/Oeschger events, highlighting the rapid Arctic ocean-ice-atmosphere iodine exchange response to abrupt climate changes. Finally, we discuss if iodine levels during past warmer-than-present climate phases can serve as analogues of future scenarios under an expected ice-free Arctic Ocean. We argue that the combination of natural biogenic ocean iodine release (boosted by ongoing Arctic warming and sea ice retreat) and anthropogenic ozone-induced iodine emissions may lead to a near future scenario with the highest iodine levels of the last 127,000 years.

Iodine chemistry in the chemistry–climate model SOCOL-AERv2-I

Abstract. In this paper, we present a new version of the chemistry–climate model SOCOL-AERv2 supplemented by an iodine chemistry module. We perform three 20-year ensemble experiments to assess the validity of the modeled iodine and to quantify the effects of iodine on ozone. The iodine distributions obtained with SOCOL-AERv2-I agree well with AMAX-DOAS observations and with CAM-chem model simulations. For the present-day atmosphere, the model suggests that the iodine-induced chemistry leads to a 3 %–4 % reduction in the ozone column, which is greatest at high latitudes. The model indicates the strongest influence of iodine in the lower stratosphere with 30 ppbv less ozone at low latitudes and up to 100 ppbv less at high latitudes. In the troposphere, the account of the iodine chemistry reduces the tropospheric ozone concentration by 5 %–10 % depending on geographical location. In the lower troposphere, 75 % of the modeled ozone reduction originates from inorganic sources of iodine, 25 % from organic sources of iodine. At 50 hPa, the results show that the impacts of iodine from both sources are comparable. Finally, we determine the sensitivity of ozone to iodine by applying a 2-fold increase in iodine emissions, as it might be representative for iodine by the end of this century. This reduces the ozone column globally by an additional 1.5 %–2.5 %. Our results demonstrate the sensitivity of atmospheric ozone to iodine chemistry for present and future conditions, but uncertainties remain high due to the paucity of observational data of iodine species.

Targeted knockout of the gene OsHOL1 removes methyl iodide emissions from rice plants

Iodine deficiency represents a public health problem worldwide. To increase the amount of iodine in the diet, biofortification strategies of plants have been tried. They rely on the exogenous administration of iodine to increase its absorption and accumulation. However, iodine is not stable in plants and can be volatilized as methyl iodide through the action of specific methyltransferases encoded by the HARMLESS TO OZONE LAYER (HOL) genes. The release of methyl iodide in the atmosphere represents a threat for the environment due to its ozone depletion potential. Rice paddies are among the strongest producers of methyl iodide. Thus, the agronomic approach of iodine biofortification is not appropriate for this crop, leading to further increases of iodine emissions. In this work, we used the genome editing CRISPR/Cas9 technology to knockout the rice HOL genes and investigate their function. OsHOL1 resulted a major player in methyl iodide production, since its knockout abolished the process. Moreover, its overexpression reinforced it. Conversely, knockout of OsHOL2 did not produce effects. Our experiments helped elucidating the function of the rice HOL genes, providing tools to develop new rice varieties with reduced iodine emissions and thus more suitable for biofortification programs without further impacting on the environment.

Reversible iodine vapor capture using bipyridine-based covalent triazine framework: Experimental and computational investigations

To address the environmental issues arising from the emission of radiotoxic iodine from nuclear waste streams, developing high-capacity and recyclable adsorbents is urgently demanded. In this study, a nitrogen-rich covalent-triazine framework (CTF-bpy) was synthesized through the ionothermal synthetic method and was used as a reusable adsorbent to capture iodine vapor for sequential cycles. The obtained CTF-bpy adsorbent showed ultrahigh iodine vapor capture capacity of 4.52 g.g−1 at 90 °C and atmospheric pressure, which ranks among the highest values reported to date. CTF-bpy could be simply recycled by washing and heating while preserving above 89.6% of its initial iodine capture capacity after five consecutive cycles, demonstrating its excellent structural stability. Assessment of the adsorption kinetics of the iodine vapor through the fractal-like pseudo-first-order (FL-PFO) kinetic model revealed that the diffusion through micropores was the rate-controlling mechanism. Moreover, the density functional theory (DFT) calculations further demonstrated the significance of the surface's basicity and aromaticity of the structure in efficiently capturing the iodine species. This study may shed light on designing and developing novel adsorbents suitable for solving one of the main environmental issues.

Oxidation of iodide to iodate by cultures of marine ammonia-oxidising bacteria

Reaction with iodide (I−) at the sea surface is an important sink for atmospheric ozone, and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate this process into chemical transport models, improved understanding of the factors controlling marine iodine speciation, and especially sea-surface iodide concentrations, is needed. The oxidation of I− to iodate (IO3−) is the main sink for oceanic I−, but the mechanism for this remains unknown. We demonstrate for the first time that marine nitrifying bacteria mediate I− oxidation to IO3−. A significant increase in IO3− concentrations compared to media-only controls was observed in cultures of the ammonia-oxidising bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus oceani (Nc10) supplied with 9–10 mM I−, indicating I− oxidation to IO3−. Cell-normalised production rates were 15.69 (±4.71) fmol IO3− cell−1 d−1 for Nitrosomonas sp., and 11.96 (±6.96) fmol IO3− cell−1 d−1 for Nitrosococcus oceani, and molar ratios of iodate-to-nitrite production were 9.2 ± 4.1 and 1.88 ± 0.91 respectively. Preliminary experiments on nitrite-oxidising bacteria showed no evidence of I− to IO3− oxidation. If the link between ammonia and I− oxidation observed here is representative, our ocean iodine cycling model predicts that future changes in marine nitrification could alter global sea surface I− fields with potential implications for atmospheric chemistry and air quality.