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The formation and growth of iodine oxide particles (IOPs), originating from molecular iodine precursor, has been studied at room temperature as a function of water vapour, and sulphuric and oxalic acid vapours. A linear variation in total IOP mass was observed over a wide range of iodine atom production rates under both dry and humid formation conditions. Particle formation was also observed in the absence of ozone, and was found to be temperature sensitive, with elevated temperatures resulting in reduced particle number and mass. Electronic structure calculations are used to show that particle formation is initiated by polymerization of I2O4 with I2O3, or with itself. Formation of IOPs in humid conditions results in lower numbers and smaller particles than formed in the absence of water vapour, because H2O forms relatively stable complexes with molecules such as I2O3 and I2O4, inhibiting their polymerization. Addition of H2O to particles formed under dry conditions shows the collapse of fractal-like, aggregate particle structures. The uptake of sulphuric acid vapour onto humidified particles was studied over a wide range of relative humidity (RH) at room temperature, with the calculated accommodation coefficient (α) for this process increasing with RH to a value of 0.75±0.05 at RH = 90%. In contrast, growth of particles exposed to oxalic acid vapour was not observed on the experimental timescales employed, indicating an upper limit for α of 10−3.

Sea ice is an integral part of the earth's climate system because it affects planetary albedo, sea-surface salinity, and the atmosphere–ocean exchange of reactive gases and aerosols. Bromine and iodine chemistry is active at polar sea ice margins with the occurrence of bromine explosions and the biological production of organoiodine from sea ice algae. Satellite measurements demonstrate that concentrations of bromine oxide (BrO) and iodine oxide (IO) decrease over sea ice toward the Antarctic interior. Here we present speciation measurements of bromine and iodine in the TALDICE (TALos Dome Ice CorE) ice core (159°11' E, 72°49' S; 2315 m a.s.l.) spanning the last 215 ky. The Talos Dome ice core is located 250 km inland and is sensitive to marine air masses intruding onto the Antarctic Plateau. Talos Dome bromide (Br⁻) is positively correlated with temperature and negatively correlated with sodium (Na). Based on the Br⁻/Na seawater ratio, bromide is depleted in the ice during glacial periods and enriched during interglacial periods. Total iodine, consisting of iodide (I⁻) and iodate (IO₃⁻), peaks during glacials with lower values during interglacial periods. Although IO₃⁻ is considered the most stable iodine species in the atmosphere it was only observed in the TALDICE record during glacial maxima. Sea ice dynamics are arguably the primary driver of halogen fluxes over glacial–interglacial timescales, by altering the distance between the sea ice edge and the Antarctic plateau and by altering the surface area of sea ice available to algal colonization. Based on our results we propose the use of both halogens for examining Antarctic variability of past sea ice extent

Iodine oxide particles are known to nucleate in the marine boundary layer where gas phase molecular iodine and organoiodine species are produced by macroalgae. These ultra-fine particles may then grow through the condensation of other materials to sizes where they may serve as cloud condensation nuclei. There has been some debate over the chemical identity of the initially nucleated particles. In laboratory simulations, hygroscopic measurements have been used to infer that they are composed of insoluble I₂O₄, while elemental analysis of laboratory generated particles suggests soluble I₂O₅ or its hydrated form iodic acid, HIO₃ (I₂O₅·H₂O). In this paper we explore the response of super-micron sized aqueous iodic acid solution droplets to varying humidity using both Raman microscopy and single particle electrodynamic traps. These measurements reveal that the propensity of an iodic acid solution droplet to crystallise is negligible on drying to ~0% relative humidity (RH). On applying mechanical pressure to these droplets they shatter in a manner consistent with an ultra-viscous liquid or a brittle glass. Water retention in amorphous material at low RH is important for understanding the hygroscopic growth of aerosol particles and uptake of other condensable material. Subsequent water uptake between 10 and 20% RH causes their viscosity to reduce sufficiently that the cracked droplets flow and merge. The persistence of iodic acid solution in an amorphous state, rather than a crystalline state, suggests they will more readily accommodate other condensable material and are therefore more likely to grow to sizes where they may serve as cloud condensation nuclei. On increasing the humidity to ~90% the mass of the droplets only increases by ~20% with a corresponding increase in radius of only 6%, which is remarkably small for a highly soluble material. We suggest that the small growth factor of aqueous iodic acid solution droplets is consistent with the small aerosol growth factors observed in previous experiments

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Marine aerosol is highly enriched in iodine, mostly in the form of iodate (IO(3)(-)) ions, compared to its relative abundance in seawater. This paper describes a laboratory study of the photochemical reduction of IO(3)(-) in the presence of humic acid. Spectroscopic analysis showed that ~20% of IO(3)(-) was converted to "free" iodide (I(-)) ions and this fraction remained constant as a function of time. Direct detection of an organically fixed fraction (i.e., ∼ 80%) was not possible, but a number of test reactions with surrogate organic compounds containing functional groups identified in humic acid structures indicate that efficient substitution of iodine occurs at aromatic 1,2 diol sites. These iodinated humic acids are stable with respect to photolysis at near-UV/visible wavelengths and are likely to account for a significant proportion of the soluble iodine-containing organic material occurring within aerosols. In the lower atmosphere, oxidation of I(-) to I(2) in marine aerosol occurs mostly through the uptake of O(3), with H(2)O(2) playing a very minor role. A model of iodine chemistry in the open ocean tropical boundary layer, which incorporates these experimental results, is able to account for the observed enrichment of iodine in marine aerosol.

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Abstract. Secondary aerosol produced from marine biogenic sources in algal-rich coastal locations will initially be composed of iodine oxide species, most likely I 2 O 5 , or its hydrated form HIO 3 , formed as a result of iodine gas-phase chemistry. At present, there is no quantitative hygroscopic data for these compounds and very little data available for iodate solutions (HIO 3 and I 2 O 5 share a common aqueous phase). With increased interest in the role of such aerosol in the marine atmosphere, we have conducted studies of (i) the deliquescence behaviour of crystalline HIO 3 and I 2 O 5 at 273-303 K, (ii) the efflorescence behaviour of aqueous iodate solution droplets, and (iii) properties (water activity, density, and viscosity) of subsaturated and saturated iodate solutions.The deliquescence of I 2 O 5 crystals at 293 K was observed to occur at a relative humidity (DRH) of 80.8±1.0%, whereas for HIO 3 , a DRH of 85.0±1.0% was measured. These values are consistent with measured water activity values for saturated I 2 O 5 and HIO 3 solutions at 293 K of 0.80±0.01 and 0.84±0.01 respectively. At all temperatures, DRH values for HIO 3 crystals were observed to be higher than for those of I 2 O 5 . The temperature-dependent DRH data, along with solubility and water activity data were used to evaluate the enthalpy of solution ( H sol ) for HIO 3 and I 2 O 5 . A H sol value of 8.3±0.7 kJ mol −1 was determined for HIO 3 which is consistent with a literature value of 8.8 kJ mol −1 . For I 2 O 5 , we report for the first time its solubility at various temperatures and H sol = 12.4±0.6 kJ mol −1 . The measured water activity values confirm that aqueous iodate solutions are strongly non-ideal, consistent with previous reports of complex ion formation and molecular aggregation.