Iodine oxides are a family of reactive compounds formed when iodine atoms or iodine-containing molecules combine with oxygen, and they punch well above their weight in Earth’s atmosphere. Despite existing at concentrations measured in parts per trillion, these molecules destroy ozone, seed new aerosol particles that can grow into cloud condensation nuclei, and connect the chemistry of the ocean surface to the composition of the air we breathe. The story of iodine oxides stretches from kelp beds at low tide to the Antarctic stratosphere, and even into nuclear reactor containment buildings and military research labs.
What Iodine Oxides Actually Are
The term “iodine oxide” covers several distinct chemical species. The simplest is iodine monoxide (IO), a radical with one iodine atom bonded to one oxygen. IO is the workhorse of atmospheric iodine chemistry: it forms quickly, reacts aggressively, and drives catalytic cycles that chew through ozone. A slightly larger relative, iodine dioxide (OIO), carries two oxygen atoms. When IO and OIO collide and recombine, they build up what atmospheric chemists call “higher-order iodine oxides,” written as IxOy, where x and y vary. Under dry conditions, these clusters eventually settle on a composition close to iodine pentoxide (I2O5).1PubMed Central. Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid Other forms, including I2O4 and I4O9, show up under different environmental conditions, each with its own stability and behavior.2Annals of Nuclear Energy. Study of the stability of iodine oxides (IxOy) aerosols in severe accident conditions
Alongside the oxides themselves, iodine oxoacids play a central role. Iodic acid (HIO3) and iodous acid (HIO2) form in the atmosphere when iodine oxides interact with water vapor. These acids turn out to be critical for particle formation, sometimes even more so than the oxides alone. In humid marine air, iodine-containing clusters and particles form through a mixture of IxOy and HIO3 aggregating together.3PubMed Central. Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid
Where Atmospheric Iodine Comes From
The ocean is the dominant source. When ozone in the air meets iodide dissolved in seawater, a surface reaction kicks off that releases volatile iodine compounds, primarily hypoiodous acid (HOI) and molecular iodine (I2). This reaction happens across the entire ocean surface and doubles as a major pathway for ozone to be deposited and removed over open water.4PubMed. Iodine emission from the reactive uptake of ozone to simulated seawater Dissolved organic matter in the surface microlayer of the sea can modify this process, altering both how fast ozone is consumed and how much molecular iodine escapes into the air.5PubMed. Modification of ozone deposition and I2 emissions at the air-aqueous interface by dissolved organic carbon of marine origin
Seaweed is another prolific emitter. Large brown kelps like Laminaria accumulate iodide inside their tissues as a chemical defense. When exposed to oxidative stress, they dump that iodide outward. On the surface of the kelp, iodide reacts with ozone, releasing bursts of molecular iodine. Those bursts have been directly linked to the formation of ultrafine iodine-containing particles in the coastal atmosphere.6Atmospheric Chemistry and Physics. Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine The kelp essentially uses iodide as an antioxidant shield, and the atmospheric side effects are substantial: the resulting iodine oxides form hygroscopic particles that can grow into precursors for cloud condensation nuclei.7PubMed Central. Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry
Volcanoes are a less obvious but real source. Satellite measurements following the major eruption of Kasatochi volcano in Alaska in 2008 detected strongly elevated levels of iodine monoxide in the volcanic plume, with high column amounts persisting along the plume trajectory for several days. That study provided the first observational evidence that volcanic eruptions inject iodine oxides directly into the atmosphere.8Atmospheric Chemistry and Physics. Space-based observation of volcanic iodine monoxide
How Iodine Oxides Destroy Ozone
Once molecular iodine or organic iodine compounds reach the air, sunlight breaks them apart, freeing iodine atoms. These atoms react with ozone to produce IO, which then participates in catalytic cycles: a single iodine atom can destroy many ozone molecules before being deactivated. The combined effect of these iodine-driven cycles is large. Modeling work has calculated that catalytic iodine reactions contribute two to five times more to tropospheric ozone loss than the combined bromine and chlorine cycles.9Atmospheric Chemistry and Physics. Iodine chemistry in the troposphere and its effect on ozone In the tropical marine boundary layer and the upper troposphere, iodine may account for roughly 9% to 27% of the annual ozone loss, depending on how efficiently higher-order iodine oxides are broken apart by sunlight.10Atmospheric Chemistry and Physics. Iodine chemistry in the troposphere and its effect on ozone
Whether those higher-order iodine oxides photolyze efficiently matters a lot. If sunlight rapidly breaks apart IxOy compounds, it recycles iodine back into forms like IO that keep the ozone-destroying cycle spinning. Laboratory measurements of these absorption cross sections at visible wavelengths support a “high photolysis” scenario in which IxOy species are efficiently removed during daytime, which simultaneously ramps up iodine-driven ozone depletion and reduces the formation of iodine particles.11Atmospheric Chemistry and Physics. Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm In other words, the atmosphere faces a fork: iodine either destroys ozone or forms particles, and photolysis rates tip the balance.
The Antarctic Ozone Hole Connection
Iodine’s reach extends into the stratosphere, where it contributes to the Antarctic ozone hole. Simulations covering 1980 to 2015 found that iodine accounts for about 10% of the lower stratospheric ozone loss during the Antarctic spring, equating to roughly 4% of the total stratospheric column loss. Iodine also shifts the timing of the ozone hole, advancing its onset and delaying its closure by three to five days. That timing shift increases the hole’s area by about 11% and its mass deficit by about 20%.12PubMed Central. The influence of iodine on the Antarctic stratospheric ozone hole Perhaps most striking, because iodine reactivates photochemically much faster than chlorine or bromine, it dominates halogen-mediated lower stratospheric ozone loss during summer and early fall, when chlorine and bromine are largely locked up in reservoir species. At that point, iodine is responsible for roughly 73% of the halogen-driven destruction, despite being present at far lower concentrations.13PubMed Central. The influence of iodine on the Antarctic stratospheric ozone hole
In the polar boundary layer closer to the surface, sea ice itself is a significant iodine source. Micro-algae living within and beneath sea ice produce iodide and hypoiodous acid, which diffuse upward through brine channels and accumulate in a thin brine layer on the ice surface. From there, iodine compounds escape into the atmosphere through snow cover, exposed ice surfaces, or fractures in the ice pack.14Atmospheric Chemistry and Physics. A mechanism for biologically induced iodine emissions from sea ice Field measurements in the Arctic have detected molecular iodine in the snowpack interstitial air at concentrations up to 35 parts per trillion when the surface was irradiated, and snow meltwater showed iodide enrichments of up to about 1,900 times above the normal seawater ratio, pointing to vigorous iodine recycling.15PubMed Central. Active molecular iodine photochemistry in the Arctic Modeling based on those observations shows that the I2 released can significantly accelerate local ozone depletion while producing IO at levels consistent with recent Arctic measurements.16PubMed Central. Active molecular iodine photochemistry in the Arctic
Seeding New Particles and Influencing Clouds
Iodine oxides and oxoacids do not just destroy ozone; they also build things. When IO and OIO recombine into IxOy clusters, those clusters can grow rapidly into ultrafine aerosol particles. In the real atmosphere, the dominant nucleation agent appears to be iodic acid (HIO3), which adds molecule by molecule to a growing cluster. Ambient measurements have confirmed this mechanism at the molecular scale: freshly formed clusters show a composition consistent with sequential HIO3 addition, followed by internal rearrangement toward I2O5 and release of water.17PubMed Central. Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3
Controlled experiments at CERN’s CLOUD chamber, which simulates atmospheric conditions with extreme precision, showed that HIO3 nucleation rates are rapid enough to compete with sulfuric acid and ammonia, which are the traditional workhorses of new particle formation. Below about 10°C, ion-induced nucleation involving iodate ions proceeds at the theoretical maximum rate. Neutral nucleation, meanwhile, relies on alternating additions of HIO2 and HIO3, with iodous acid playing a stabilizing role. Once formed, these particles grow quickly because HIO3 condenses onto them at or near the kinetic limit.18PubMed. Role of iodine oxoacids in atmospheric aerosol nucleation
The climate implications are real. In Antarctica, iodine-assisted particle formation can increase concentrations of cloud condensation nuclei by 20% to 100% under certain meteorological conditions.19PubMed Central. Role of Iodine-Assisted Aerosol Particle Formation in Antarctica More cloud condensation nuclei generally means more, smaller cloud droplets, which makes clouds more reflective and longer-lived. Over the remote Southern Ocean, where other particle sources are scarce, iodine could be one of the main drivers of cloud formation. This is not a small niche effect: pristine marine regions are the areas where Earth’s climate models are most sensitive to uncertainties in aerosol sources.
Mercury and Iodine Oxide Interactions
Iodine oxides do not only interact with ozone. Laboratory studies have shown that IO radicals react with gaseous elemental mercury, producing oxidized mercury compounds including mercuric iodide (HgI2), mercuric oxide (HgO), and compounds tentatively identified as HgIO or HgOI.20Canadian Journal of Chemistry. Reaction of gaseous mercury with molecular iodine, atomic iodine, and iodine oxide radicals — Kinetics, product studies, and atmospheric implications Oxidized mercury is far more soluble than elemental mercury, meaning it deposits out of the atmosphere much faster. In polar and coastal environments where both iodine and mercury are present, this chemistry could accelerate mercury deposition to sensitive ecosystems. The implications for biogeochemical cycling of mercury are still being worked out, but the basic kinetics suggest that iodine-mediated mercury oxidation could be a relevant pathway in regions with high iodine emissions.
Nuclear Safety and Iodine Oxide Aerosols
Iodine oxide chemistry takes on practical urgency inside nuclear reactor containment buildings. During a severe accident, radioactive iodine can escape as molecular iodine (I2) or methyl iodide (CH3I) into the containment atmosphere. There, the products of air radiolysis, particularly ozone and nitrogen oxides generated by intense radiation, react with these gaseous iodine species to produce iodine oxide particles.21Annals of Nuclear Energy. Experimental and modelling studies of iodine oxide formation and aerosol behaviour relevant to nuclear reactor accidents This gas-to-particle conversion is significant for safety analysis: iodine locked in aerosol particles behaves very differently from gaseous iodine. Particles can settle onto surfaces, be captured by filters, or be washed out by sprays, potentially reducing the radioactive iodine that could escape to the environment.
But the picture is not entirely reassuring. Experiments have confirmed that under radiation, iodine oxide aerosols decompose and release gaseous molecular iodine again.22Annals of Nuclear Energy. Study of the stability of iodine oxides (IxOy) aerosols in severe accident conditions This decomposition means that particle formation is not a permanent iodine sink: the iodine can cycle back into volatile form, complicating predictions about how much radioactive iodine stays airborne over time. Understanding the stability and decomposition rates of different iodine oxide forms (I2O4, I4O9, I2O5) under realistic accident conditions, including high humidity, elevated temperature, and radiation fields, remains an active area of nuclear safety research. Recent experiments using ozone oxidation of iodine vapor at room temperature have confirmed rapid formation of iodine oxide aerosols, achieving mass concentrations around 20 mg/m3.23Frontiers in Nuclear Engineering. Experimental insights into the formation and characterization of iodine oxide aerosols
Energetic Materials and Biocidal Applications
Iodine pentoxide has attracted attention from an entirely different community: researchers developing energetic materials. I2O5 is a strong oxidizer, and when paired with a metal fuel like aluminum in a nanothermite formulation, it reacts vigorously while releasing molecular iodine as a combustion product. That released iodine is a potent biocidal agent, meaning it kills bacteria and other pathogens. This combination of high energy release and biocidal delivery has made I2O5-based nanothermites a focus for defense and decontamination applications.24Combustion and Flame. Tuning the reactivity and energy release rate of I2O5 based ternary thermite systems
One practical challenge is that I2O5 absorbs water readily, which degrades its performance as an oxidizer over time. Researchers have addressed this by encapsulating I2O5 cores within thin iron oxide shells using aerosol spray pyrolysis, creating a passivated form that maintains the unhydrated oxidizer until the moment of use.25PubMed. Passivated iodine pentoxide oxidizer for potential biocidal nanoenergetic applications Other iodine oxides and iodine oxoacids, including HI3O8 and HIO3, have also been tested as thermite oxidizers, widening the palette of available formulations.26Combustion and Flame. Performance of iodine oxides/iodic acids as oxidizers in thermite systems
Detecting Iodine Oxides in the Field
Measuring iodine monoxide at parts-per-trillion concentrations in the open atmosphere is not trivial, and the development of detection methods has shaped how much we know about these compounds. The first measurements of IO in the Antarctic troposphere came in 1999 using differential optical absorption spectroscopy (DOAS), which identifies IO by its characteristic absorption bands in the 415 to 461 nanometer wavelength range.27Geophysical Research Letters. Spectroscopic measurements of tropospheric iodine oxide at Neumayer Station, Antarctica DOAS works by shining light through a long atmospheric path and looking for the spectral fingerprints of trace gases, making it well suited for remote environments where IO levels are very low.
More recently, cavity-enhanced absorption spectroscopy (CEAS) has allowed researchers to build compact, field-deployable instruments that detect multiple species simultaneously. One such instrument achieved a detection limit of 0.3 parts per trillion for IO alongside measurements of nitrogen dioxide, glyoxal, and ozone, all in a 22-minute measurement window.28Atmospheric Measurement Techniques. A compact incoherent broadband cavity-enhanced absorption spectrometer for trace detection of nitrogen oxides, iodine oxide and glyoxal at levels below parts per billion for field applications Earlier prototype instruments using LED light sources in the blue spectral range demonstrated the first cavity-enhanced DOAS detection of IO and achieved the first direct detection of methyl glyoxal in the same setup.29Atmospheric Measurement Techniques. Inherent calibration of a blue LED-CE-DOAS instrument to measure iodine oxide, glyoxal, methyl glyoxal, nitrogen dioxide, water vapour and aerosol extinction in open cavity mode On the chemical side, mass spectrometry using nitrate reagent ions can detect both iodine oxides and iodine oxoacids, though interpreting the signals requires care: roughly half of the iodate ion signal measured under humid conditions actually comes from IxOy rather than HIO3.30PubMed Central. Insights into the Chemistry of Iodine New Particle Formation: The Role of Iodine Oxides and the Source of Iodic Acid
How IO Forms from Organic Precursors
The atmospheric sources described above release molecular iodine and organic iodine compounds like methyl iodide and other iodoalkyl species. Once in the gas phase, sunlight breaks the carbon-iodine bond, freeing an iodine atom that quickly grabs an oxygen atom from ozone to form IO. But there is another pathway: laboratory experiments have shown that when iodoalkyl radicals (fragments left after UV cleavage of organic iodine) react directly with molecular oxygen at room temperature, IO can form without ozone being involved at all. This was demonstrated for a range of radicals using cavity ring-down spectroscopy.31Chemical Physics Letters. Formation of the iodine monoxide radical from gas-phase reactions of iodoalkyl radicals with molecular oxygen The practical significance is that IO production may begin as soon as organic iodine compounds start breaking apart, even before the freed iodine atoms encounter ozone. This expands the range of atmospheric conditions under which iodine oxide chemistry can get started.