I2 Emission Research Articles

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 %.

Spontaneous Iodide Activation at the Air-Water Interface of Aqueous Droplets.

We present experimental evidence that atomic and molecular iodine, I and I2, are produced spontaneously in the dark at the air-water interface of iodide-containing droplets without any added catalysts, oxidants, or irradiation. Specifically, we observe I3- formation within droplets, and I2 emission into the gas phase from NaI-containing droplets over a range of droplet sizes. The formation of both products is enhanced in the presence of electron scavengers, either in the gas phase or in solution, and it clearly follows a Langmuir-Hinshelwood mechanism, suggesting an interfacial process. These observations are consistent with iodide oxidation at the interface, possibly initiated by the strong intrinsic electric field present there, followed by well-known solution-phase reactions of the iodine atom. This interfacial chemistry could be important in many contexts, including atmospheric aerosols.

Influence of the Sea Surface Microlayer on Oceanic Iodine Emissions.

The influence of organic compounds on iodine (I2) emissions from the O3 + I– reaction at the sea surface was investigated in laboratory and modeling studies using artificial solutions, natural subsurface seawater (SSW), and, for the first time, samples of the surface microlayer (SML). Gas-phase I2 was measured directly above the surface of liquid samples using broadband cavity enhanced absorption spectroscopy. I2 emissions were consistently lower for artificial seawater (AS) than buffered potassium iodide (KI) solutions. Natural seawater samples showed the strongest reduction of I2 emissions compared to artificial solutions with equivalent [I–], and the reduction was more pronounced over SML than SSW. Emissions of volatile organic iodine (VOI) were highest from SML samples but remained a negligible fraction (<1%) of the total iodine flux. Therefore, reduced iodine emissions from natural seawater cannot be explained by chemical losses of I2 or hypoiodous acid (HOI), leading to VOI. An interfacial model explains this reduction by increased solubility of the I2 product in the organic-rich interfacial layer of seawater. Our results highlight the importance of using environmentally representative concentrations in studies of the O3 + I– reaction and demonstrate the influence the SML exerts on emissions of iodine and potentially other volatile species.

Experimental Determination of the Photooxidation of Aqueous I– as a Source of Atmospheric I2

The chemistry of iodine plays an important role in the oxidizing capacity of the global marine atmosphere. In this study, we experimentally determine the photooxidation parameters of iodide ions in aqueous phase (I–(aq)) and estimate the subsequent emission of gaseous iodine molecules (I2(g)) into the atmosphere. The values of the molar absorption coefficient (εiodide(λ)) and the photooxidative quantum yields (Φiodide(λ)) of I–(aq) in the range of 290–500 nm were determined. The influence of pH and dissolved oxygen (DO) on the values of Φiodide(λ) was also investigated. The emission of I2(g) into the atmosphere following the photooxidation of I–(aq) in deionized water solution (pH 5.6, DO 7.8 mg L–1) and artificial seawater solution (pH 8.0, DO 7.0 mg L–1) was estimated to be (2.2 × 10–8 × [I–(aq)]sea) and (1.8 × 10–8 × [I–(aq)]sea) mol L–1 s–1, respectively. Using a global chemistry-climate model, we estimated that the photooxidation of I–(aq) can increase the atmospheric iodine budget by up to ∼8% over some oceanic regions.

Correction to Ozone Formation Induced by the Impact of Reactive Bromine and Iodine Species on Photochemistry in a Polluted Marine Environment.

Reactive iodine and bromine species (RIS and RBS, respectively) are known for altering atmospheric chemistry and causing sharp tropospheric ozone (O3) depletion in polar regions and significant O3 reduction in the marine boundary layer (MBL). Here we use measurement-based modeling to show that, unexpectedly, both RIS and RBS can lead to enhanced O3 formation in a polluted marine environment under volatile organic compound (VOC)-limited conditions associated with high nitrogen oxide (NOX = [NO] + [NO2]) concentrations. Under these conditions, the daily average O3 mixing ratio increased to ∼44 and ∼28% for BrO and IO mixing ratios of up to ∼6.8 and 4.7 ppt, respectively. The increase in the level of O3 was partially induced by enhanced ClNO3 formation for higher Br2 and I2 emission flux. The increase in the level of O3 was associated with an increased mixing ratio of hydroperoxyl radical to hydroxyl radical ([HO2]/[OH]) and increased [NO2]/[NO] with higher levels of RBS and/or RIS. NOX-rich conditions are t...

Ozone Formation Induced by the Impact of Reactive Bromine and Iodine Species on Photochemistry in a Polluted Marine Environment.

Reactive iodine and bromine species (RIS and RBS, respectively) are known for altering atmospheric chemistry and causing sharp tropospheric ozone (O3) depletion in polar regions and significant O3 reduction in the marine boundary layer (MBL). Here we use measurement-based modeling to show that, unexpectedly, both RIS and RBS can lead to enhanced O3 formation in a polluted marine environment under volatile organic compound (VOC)-limited conditions associated with high nitrogen oxide (NOX = [NO] + [NO2]) concentrations. Under these conditions, the daily average O3 mixing ratio increased to ∼44 and ∼28% for BrO and IO mixing ratios of up to ∼6.8 and 4.7 ppt, respectively. The increase in the level of O3 was partially induced by enhanced ClNO3 formation for higher Br2 and I2 emission flux. The increase in the level of O3 was associated with an increased mixing ratio of hydroperoxyl radical to hydroxyl radical ([HO2]/[OH]) and increased [NO2]/[NO] with higher levels of RBS and/or RIS. NOX-rich conditions are typical of the polluted MBL, near coastlines and ship plumes. Considering that O3 is toxic to humans, plants, and animals and is a greenhouse gas, our findings call for adequate updating of local and regional air-quality models with the effects of activities of RBS and RIS on O3 mixing ratios in the polluted MBL.

Production of Molecular Iodine and Tri-iodide in the Frozen Solution of Iodide: Implication for Polar Atmosphere.

The chemistry of reactive halogens in the polar atmosphere plays important roles in ozone and mercury depletion events, oxidizing capacity, and dimethylsulfide oxidation to form cloud-condensation nuclei. Among halogen species, the sources and emission mechanisms of inorganic iodine compounds in the polar boundary layer remain unknown. Here, we demonstrate that the production of tri-iodide (I3(-)) via iodide oxidation, which is negligible in aqueous solution, is significantly accelerated in frozen solution, both in the presence and the absence of solar irradiation. Field experiments carried out in the Antarctic region (King George Island, 62°13'S, 58°47'W) also showed that the generation of tri-iodide via solar photo-oxidation was enhanced when iodide was added to various ice media. The emission of gaseous I2 from the irradiated frozen solution of iodide to the gas phase was detected by using cavity ring-down spectroscopy, which was observed both in the frozen state at 253 K and after thawing the ice at 298 K. The accelerated (photo-)oxidation of iodide and the subsequent formation of tri-iodide and I2 in ice appear to be related with the freeze concentration of iodide and dissolved O2 trapped in the ice crystal grain boundaries. We propose that an accelerated abiotic transformation of iodide to gaseous I2 in ice media provides a previously unrecognized formation pathway of active iodine species in the polar atmosphere.

Contribution of living and degrading kelp to coastal iodine fluxes

Laminaria digitata forms locally extensive and ecologically important kelp beds in northern temperate regions. Its sporophytes retain large quantities of iodine which, when released into seawater (as iodide, I−) and into air (as molecular iodine, I2), have the potential to contribute significantly to coastal iodine biogeochemical processes. Here, standing crop of a living L. digitata bed and biomass of degrading kelp deposited near the high tide mark were quantified in western Ireland (53°09′25″N, 09°06′58″W) in summer 2011. Measurements revealed an average of 17 living individuals m−2, accounting for 0.6 (kg dw) m−2, and the biomass of degrading kelp averaged 0.05 (kg dw) m−2. We further investigated the iodine-retention capacity of L. digitata and the effect of degradation. Under controlled conditions, I− release into seawater and I2 emission into air depended on the physiological state of L. digitata. Fresh and “healthy” phylloids absorbed I− from seawater with I2 emission rates reaching 27 nmol (g dw)−1 h−1. After 4 days of degradation, I2 emission rates were enhanced by a factor of 3.1. Over 29 days of submersed degradation, ~4.7 μmol I− (g dw)−1 were released into seawater, a process which could trigger iodine recycling in coastal areas. Combined field and laboratory data suggest that living L. digitata sporophytes possess an I2 emission capacity of ~16 μmol m−2 h−1, whilst emersed degrading kelp contributes 1–5 μmol m−2 h−1 to coastal atmospheric I2 during summer. This study provides evidence that not only living but also degrading kelp acts as driver of coastal iodine fluxes.

Emission of iodine-containing volatiles by selected microalgae species

Abstract. In this study we present the results of an emission study of different phytoplankton samples in aqueous media treated with elevated ozone levels. Halocarbon measurements show that the samples tested released bromoform and different iodocarbons, including iodomethane, iodochloromethane and diiodomethane. Iodide and iodate levels in the liquid phase were representative of concentrations of surface water in a natural environment. Measurement of volatile iodine (I2) emissions from two diatom samples (Mediopyxis helysia and Porosira glacialis) and the background sample (F/2 medium from filtered natural seawater) showed that the quantity of evolved I2 depends on the ozone concentration in the air. This behaviour was assumed to be caused by the oxidation reaction mechanism of iodide with ozone. The I2 emission flux agrees with model calculations at different iodide concentrations. The I2 emission of a natural plankton concentrate sample was, however, very low compared to other samples and showed no dependence on ozone. The reason for this was shown to be the low iodide concentration in the algal suspension, which seems to be the limiting factor in the oxidative formation of I2.

Modification of Ozone Deposition and I2 Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin

The reaction between gaseous ozone (O3) and aqueous iodide (I(-)) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O3 over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I2). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I2 by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O3 reactivity (λ) toward coastal marine DOC of ∼500 (420-580) s(-1), 2-5 times higher than that toward iodide at typical ocean concentrations (∼0.5-1 × 10(-7) M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I(-) solutions (1 × 10(-5) M) such that the relative reactivities of DOC and I(-) toward O3 (λDOC/λI) were in the expected range for natural seawater. The evolution of gaseous I2 and the loss of aqueous I(-) both reduced as DOC concentrations increased, with an overall suppression of I2 emissions of about a factor of 2 under conditions of λDOC/λI representative of open ocean waters (0.5-1). A kinetic model of the SML suggested that neither competition of DOC with I(-) for reaction with interfacial O3, nor direct loss of I2 and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I2 emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I2 from the aqueous to gas phase, as suggested by recent laboratory studies.

Molecular iodine (I2) emission from two Laminaria species (Phaeophyceae) and impact of irradiance and temperature on I2 emission into air and iodide release into seawater from Laminaria digitata

Kelps of the genus Laminaria accumulate iodine at high concentrations, but the iodine retaining capacity can be affected by emersion and physiological stress. In this study, I2 emission into the atmosphere from Laminaria digitata and Laminaria hyperborea was compared under controlled low irradiances and temperatures. The two species exhibited different I2 emission rates as blades of L. digitata emitted I2 at rates five times higher than those from newly-grown blades (current growth season) of L. hyperborea. I2 emission was not detectable from old blades (previous growth season) of L. hyperborea. Additionally, effects of irradiance and temperature on both I2 emission into air and net I− release into seawater where assessed for L. digitata while monitoring photo-physiological parameters as stress indicators. Irradiances between 30 and 120 μmol photons m−2 s−1 had only marginal effects on both I2 emission and I− release rates, but physiological stress, indicated by photoinhibition, was observed. The results suggest that the irradiances applied here were not stressful enough to impact on the iodine release. By contrast, at elevated temperatures (20 °C), photoinhibition was accompanied by an increase in I2 emission rates, but net I− release rates remained similar at 10–20 °C. High I2 emission rates into air and I− release into seawater observed from L. digitata underpin the fundamental function of this kelp as mediator of coastal iodine fluxes.

Spectroscopic Study of the I2 Formation from the Photolysis of Iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at Different Wavelengths

Emission spectra following the photolysis of iodomethanes (CHI3, CH2I2, CH3I, and CH2ICl) at 266 nm were recorded in a slow flow cell. In addition to emission from the electronically excited species including CH (A(2)Δ, B(2)Σ(-), and C(2)Σ(+)), C2 (d(3)Πg), and atomic iodine ((4)P(o)), a series of emission bands was observed in the 12,000-19,000 cm(-1) region. The dominant structure of these emission bands was verified as the I2 B(3)Π(+)(0,u)-X(1)Σ(+)g emission at the 532 nm excitation, and the observed I2 was formed from collisions between iodine atoms generated from the C-I bond dissociation in these iodomethanes. The I2 emission spectra following the photolysis of CH2I2 at different wavelengths were acquired, and the threshold energy for the first C-I bond cleavage was determined to be 208 ± 1 kJ mol(-1). We also obtained the emission spectra of pure I2 at several visible excitation wavelengths for comparison with those from the photolysis of iodomethanes, and a least-squares global fit of the observed I2 emission bands yields more accurate anharmonicity parameters for the vibrational structure in the I2 B-X transition.

The seaweeds &amp;lt;i&amp;gt;Fucus vesiculosus&amp;lt;/i&amp;gt; and &amp;lt;i&amp;gt;Ascophyllum nodosum&amp;lt;/i&amp;gt; are significant contributors to coastal iodine emissions

Abstract. Based on the results of a pilot study in 2007, which found high mixing ratios of molecular iodine (I2) above the intertidal macroalgae (seaweed) beds at Mweenish Bay (Ireland), we extended the study to nine different locations in the vicinity of Mace Head Atmospheric Research Station on the west coast of Ireland during a field campaign in 2009. The mean values of I2 mixing ratio found above the macroalgae beds at nine different locations ranged from 104 to 393 ppt, implying a high source strength of I2. Such mixing ratios are sufficient to result in photochemically driven coastal new-particle formation events. Mixing ratios above the Ascophyllum nodosum and Fucus vesiculosus beds increased with exposure time: after 6 h exposure to ambient air the mixing ratios were one order of magnitude higher than those initially present. This contrasts with the emission characteristics of Laminaria digitata, where most I2 was emitted within the first half hour of exposure. Discrete in situ measurements (offline) of I2 emission from ambient air-exposed chamber experiments of L. digitata, A. nodosum and F. vesiculosus substantially supported the field observations. Further online and time-resolved measurements of the I2 emission from O3-exposed macroalgal experiments in a chamber confirmed the distinct I2 emission characteristics of A. nodosum and F. vesiculosus compared to those of L. digitata. The emission rates of A. nodosum and F. vesiculosus were comparable to or even higher than L. digitata after the initial exposure period of ~20–30 min. We suggest that A. nodosum and F. vesiculosus may provide an important source of photolabile iodine in the coastal boundary layer and that their impact on photochemistry and coastal new-particle formation should be reevaluated in light of their longer exposure at low tide and their widespread distribution.

Enhancement of Gaseous Iodine Emission by Aqueous Ferrous Ions during the Heterogeneous Reaction of Gaseous Ozone with Aqueous Iodide

Gaseous I2 formation from the heterogeneous reaction of gaseous ozone with aqueous iodide in the presence of aqueous ferrous ion (Fe(2+)) was investigated by electron impact ionization mass spectrometry. Emission of gaseous I2 increased as a function of the aqueous FeCl2 concentration, and the maximum I2 formation with Fe(2+) was about 10 times more than without Fe(2+). This enhancement can be explained by the OH(-) scavenging by Fe(3+) formed from Fe(2+) ozonation to produce colloidal Fe(OH)3. This mechanism was confirmed by measurements of aqueous phase products using a UV-vis spectrometer and an electrospray ionization mass spectrometer. We infer that such a pH-buffering effect may play the key role in general halogen activations.

Iodine Emission in the Presence of Humic Substances at the Water’s Surface

Humic substances that preferentially adsorb at the air/water interfaces of water or aerosols consist of both fulvic and humic acid. To investigate the chemical reactivity for the heterogeneous reaction of gaseous ozone, O(3)(g), with aqueous iodide, I(-)(aq), in the presence of standard fulvic acid, humic acid, or alcohol, cavity ring-down spectroscopy was used to detect gaseous products, iodine, I(2)(g) and an iodine monoxide radical, IO(g). Fulvic acid enhanced the I(2)(g) production yield, but not the IO(g) yield. Humic acid, n-hexanol, n-heptanol, and n-octanol did not affect the yields of I(2)(g) or IO(g). We can infer that the carboxylic group contained in fulvic acid promotes the I(2)(g) emission by supplying the requisite interfacial protons more efficiently than water on its surface.

Modelling multi-phase halogen chemistry in the coastal marine boundary layer: investigation of the relative importance of local chemistry vs. long-range transport

Abstract. Measurements of significant concentrations of IO, I2 and BrO in a semi-polluted coast environment at Roscoff, in North-West France, have been made as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) campaign undertaken in September 2006. We use a one-dimensional column model, with idealised I2 emissions predicted using macroalgael maps and tidal data from the littoral area surrounding Roscoff, to investigate the probable causes for these observations. The coupled microphysical and chemical aerosol model simulates mixed-phase halogen chemistry using two separate particle modes, seasalt and non-seasalt, each comprising of eight size-sections. This work confirms the finding of a previous study that the BrO measurements are most likely caused by unknown, local sources. We find that the remote observations of IO and I2 are best replicated using the I2 recycling mechanism suggested by previous studies, but that such a mechanism is not wholly necessary. However in-situ measurements of I2 can only be explained by invoking an I2 recycling mechanism. We suggest that focussed observations of the changes in NOx and NOy concentrations, as well as changes in the nitrate fraction of the non-seasalt aerosol mode, in the presence of I2 bursts could be used to determine the atmospheric relevance of the predicted I2 recycling mechanism.

Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff

Abstract. Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship between geographically resolved regional emissions (based on maps of seaweed beds in the area and seaweed I2 emission rates previously measured in the laboratory) and in situ point and line measurements of I2 performed respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument sited on the shoreline and a long-path differential optical absorption spectroscopy (LP-DOAS) instrument sampling over an extended light path to an off-shore island. The modelled point and line I2 concentrations compare quantitatively with BBCRDS and LP-DOAS measurements, and provide a link between emission fields and the different measurement geometries used to quantify atmospheric I2 concentrations during RHaMBLe. Total I2 emissions over the 100 km2 region around Roscoff are calculated to be 1.7×1019 molecules per second during the lowest tides. During the night, the model replicates I2 concentrations up to 50 pptv measured along the LP-DOAS instrument's line of sight, and predicts spikes of several hundred pptv in certain conditions. Point I2 concentrations up to 50 pptv are also calculated at the measurement site, in broad agreement with the BBCRDS observations. Daytime measured concentrations of I2 at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I2 is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NOx chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I2 via the formation, transport and subsequent reactions of the IONO2 reservoir compound. The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields.

Spectroscopic studies of molecular iodine emitted into the gas phase by seaweed

Abstract. Time profiles of molecular iodine emissions from seven species of seaweed have been measured at high time resolution (7.5 s) by direct spectroscopic quantification of the gas phase I2 using broadband cavity enhanced absorption spectroscopy. Substantial differences were found between species, both in the amounts of I2 emitted when the plants were exposed to air and in the shapes of their emission time profiles. Two species of kelp, Laminaria digitata and Laminaria hyperborea, were found to be the most potent emitters, producing an intense burst of I2 when first exposed to air. I2 was also observed from Saccharina latissima and Ascophyllum nodosum but in lower amounts and with broader time profiles. I2 mixing ratios from two Fucus species and Dictyopteris membranacea were at or below the detection limit of the present instrument (25 pptv). A further set of experiments investigated the time dependence of I2 emissions and aerosol particle formation when fragments of L. digitata were exposed to desiccation in air, to ozone and to oligoguluronate stress factors. Particle formation occurred in all L. digitata stress experiments where ozone and light were present, subject to the I2 mixing ratios being above certain threshold amounts. Moreover, the particle number concentrations closely tracked variations in the I2 mixing ratios, confirming the results of previous studies that the condensable particle-forming gases derive from the photochemical oxidation of the plant's I2 emissions. This work also supports the theory that particle nucleation in the coastal atmosphere occurs in "hot-spot" regions of locally elevated concentrations of condensable gases: the greatest atmospheric concentrations of I2 and hence of condensable iodine oxides are likely to be above plants of the most efficiently emitting kelp species and localised in time to shortly after these seaweeds are uncovered by a receding tide.

UV emission of I2 from the ion-pair state following amplified spontaneous emission

This paper reports the results of processes resulting in D0(u) (+)-X (1)Sigma(g) (+) emission when a single rovibrational level of the E0(g) (+) state is prepared. Our study reveals that two kinds of processes populate the D0(u) (+) state; which one occurs depends on the experimental conditions. One process involves amplified spontaneous emission from the E0(g) (+) state. The other is collision-induced energy transfer in self-quenching. We distinguish these two processes from the time profiles of fluorescence signals. These processes give completely different vibrational distributions in the D0(u) (+) state from a given rovibrational level of the E0(g) (+) state. The discrepancy between our results and previous results for the E0(g) (+)-->D0(u) (+) relaxation is briefly discussed.

Formation of I2(B3Π) in the presence of O2(a1Δ)

The mechanism by which I2(B3Π0) is excited in the chemical oxygen-iodine laser was studied by means of emission spectroscopy. Using the intensity of the O2(b1Σ,υ′=0)→O2(X3Σ,υ″=0) band as a reference, I2(B3Π0) relative number densities were assessed by measuring the I2(B3Π0,υ′)→I2(X1Σ,υ″) emission intensities. Vibrationally excited singlet oxygen molecules O2(a1Δ,υ′=1) were detected using infrared emission spectroscopy. The measured relative density of O2(a1Δ,υ′=1) for the conditions of a typical oxygen-iodine laser medium amounted to ∼15% of the total O2(a1Δ) content. Mechanisms for I2(B3Π0) formation were proposed for both the I2 dissociation zone and the region downstream of the dissociation zone. Both pumping mechanisms involved electronically excited molecular iodine I2(A′3Π2u, A3Π1u) as an intermediate. It is proposed that in the dissociation zone the molecular iodine A′3Π2u and A3Π1u states are populated in collisions with vibrationally excited singlet oxygen molecules O2(a1Δ,υ′), whereas in the downstream region of the dissociation zone those intermediate states are populated by the atomic iodine recombination process. I2(B3Π0) is subsequently formed in collisions of I2(A′3Π2u, A3Π1u) with singlet oxygen. We also demonstrated that I2(B3Π0) does not participate measurably in the I2 dissociation process, and that energy transfer from O2(b1Σ) does not excite I2(B3Π0) to a significant degree.