Ion-induced iodic acid nucleation

<p>Aside from capable of influencing atmospheric oxidation capacity, iodine species are known to contribute to particle formation processes. Iodine particle formation was commonly believed to be important in coastal regions only, e.g. Mace Head, but emerging evidence shows that it also plays an important role in Arctic regions.</p><p> </p><p>Although the nucleation mechanisms have been proposed to involve mainly iodine oxides, recent field observations suggest that HIO<sub>3</sub> plays a key role in the cluster formation processes. Despite these advances, experiments with atmospherically relevant vapor concentrations are lacking and the time evolution of charged cluster formation processes has never been detected at the molecular level to validate the mechanisms observed in the field.</p><p> </p><p>In this study, we carried out iodine particle formation experiments in the CLOUD chamber at CERN. The precursor vapor (I<sub>2</sub>) and oxidation products were carefully controlled at concentrations relevant to those in marine boundary layer conditions. Natural galactic cosmic rays were used to produce ions in the chamber which further initiated ion-induced nucleation processes. An atmospheric pressure interface time-of-flight mass spectrometer was used to trace the time evolution of charged iodine clusters which revealed HIO<sub>3</sub> as the major contributor.</p>

Are iodic acid measurements by chemical ionization unambiguous?

<p>Field observations of IO<sup>3−</sup> and HIO<sup>3−</sup>-containing cluster anions by chemical ionization–atmospheric pressure interface–time-of-flight mass spectrometry (CI-API-ToF-MS) have been reported1. These observations, which employ nitrate (NO<sup>3−</sup>) reagent ions for reaction with the analytes, have been interpreted as resulting from atmospheric gas-phase iodic acid (HOIO<sub>2</sub>) and molecular cluster formation via HOIO<sub>2 </sub>addition steps. CI-API-ToF-MS chamber measurements with alternative ionization schemes have also reported signals that could be attributed to gas-phase HOIO and HOIO<sub>2</sub>. However, well-established chemical kinetics and thermochemistry do not indicate any straightforward route to gas-phase iodine oxyacids and HOIO2 particle formation in the atmosphere. This does not only hinder the ability of chemical models for linking iodine emissions and particle formation, but also calls into question the interpretation of these CI-API-ToF-MS measurements. It has been proposed that water plays an important role in generating gas phase and HOIO<sub>2</sub>-containing molecular clusters, but recent flow tube experiments have established extremely low upper limits to the rate constants of possible reactions between iodine oxides (IO<sub>x</sub> and I<sub>x</sub>O<sub>y</sub>) and water. In this presentation, we discuss experimental and theoretical kinetics and thermochemistry of proposed routes to gas-phase HOIO and HOIO<sub>2</sub> in the atmosphere as well as potential ion-molecule reactions turning iodine oxides into IO<sup>3-</sup> ions in the CI-API-ToF-MS inlet. We show that there is an important ambiguity in the interpretation of IO<sup>3- </sup>and other signals observed with CI instruments as a result of barrierless reactions between I<sub>x</sub>O<sub>y</sub> and the reagent ions. Experiments for solving this ambiguity and reconciling conflicting results are proposed.</p>

Theoretical Study on the Mechanisms of Catalytic Hydration of Diiodine Trioxide in Marine Regions

Diiodine trioxide (I2O3) is one of the most common iodine oxides in the marine boundary layer (MBL). Both theoretical and experimental studies have confirmed that they can be quickly formed and are relatively stable under dry conditions. However, there is no report on the field observation of I2O3, which means that I2O3 is likely to be lost in the actual marine atmosphere. But the specific loss pathways and mechanisms are still unclear. Considering that the humidity in the marine regions is generally high and the loss of I2O3 will be affected by some substances in the marine atmosphere, water (H2O, W) and iodic acid (HIO3, IA) were selected as a catalyst to investigate the catalytic hydration mechanisms of I2O3 at DLPNOCCSD(T)//ωB97X-D/aug-cc-pVTZ + aug-cc-pVTZ -PP (for iodine) level of theory. The results show that hydration of I2O3 presents a high energy barrier, but IA can reduce it to 3.76 kcal/mol. Therefore, in the marine atmosphere, I2O3 can be hydrolyzed under the catalysis of IA, and cannot directly participate in the new particle formation process.

Determination of the absorption cross sections of higher-order iodine oxides at 355 and 532 nm

Iodine oxides (IxOy) play an important role in the atmospheric chemistry of iodine. They are initiators of new particle formation events in the coastal and polar boundary layers and act as iodine reservoirs in tropospheric ozone-depleting chemical cycles. Despite the importance of the aforementioned processes, the photochemistry of these molecules has not been studied in detail previously. Here, we report the first determination of the absorption cross sections of IxOy, x=2, 3, 5, y=1–12 at λ=355 nm by combining pulsed laser photolysis of I2∕O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration. The oxides selected for absorption cross-section determinations are those presenting the strongest signals in the mass spectra, where signals containing four iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, σ355nm,IO= (1.2±0.1) ×10-18 cm2, which is found to be in good agreement with the most recent literature. The results obtained are σ355nm,I2O3<5×10-19 cm2 molec.−1, σ355nm,I2O4= (3.9±1.2)×10-18 cm2 molec.−1, σ355nm,I3O6= (6.1±1.6)×10-18 cm2 molec.−1, σ355nm,I3O7= (5.3±1.4)×10-18 cm2 molec.−1, and σ355nm,I5O12= (9.8±1.0)×10-18 cm2 molec.−1. Photodepletion at λ=532 nm was only observed for OIO, which enabled determination of upper limits for the absorption cross sections of IxOy at 532 nm using OIO as an actinometer. These measurements are supplemented with ab initio calculations of electronic spectra in order to estimate atmospheric photolysis rates J(IxOy). Our results confirm a high J(IxOy) scenario where IxOy is efficiently removed during daytime, implying enhanced iodine-driven ozone depletion and hindering iodine particle formation. Possible I2O3 and I2O4 photolysis products are discussed, including IO3, which may be a precursor to iodic acid (HIO3) in the presence of HO2.

A gas-to-particle conversion mechanism helps to explain atmospheric particle formation through clustering of iodine oxides

Emitted from the oceans, iodine-bearing molecules are ubiquitous in the atmosphere and a source of new atmospheric aerosol particles of potentially global significance. However, its inclusion in atmospheric models is hindered by a lack of understanding of the first steps of the photochemical gas-to-particle conversion mechanism. Our laboratory results show that under a high humidity and low HOx regime, the recently proposed nucleating molecule (iodic acid, HOIO2) does not form rapidly enough, and gas-to-particle conversion proceeds by clustering of iodine oxides (IxOy), albeit at slower rates than under dryer conditions. Moreover, we show experimentally that gas-phase HOIO2 is not necessary for the formation of HOIO2-containing particles. These insights help to explain new particle formation in the relatively dry polar regions and, more generally, provide for the first time a thermochemically feasible molecular mechanism from ocean iodine emissions to atmospheric particles that is currently missing in model calculations of aerosol radiative forcing.

Adsorption properties of iodine on fused silica surfaces when evaporated from tellurium in various atmospheres

The evaporation of iodine containing species from tellurium has been investigated together with their adsorption behavior on a fused silica surface. In inert gas, the formation of two species was observed with adsorption enthalpies of around − 90 to − 100 and − 110 to − 120 kJ/mol, respectively. For reducing environments, a single species identified as monatomic iodine was observed with an adsorption enthalpy around − 95 kJ/mol. In oxidizing conditions, species with low adsorption enthalpies ranging from − 65 to − 80 kJ/mol were observed. Presumably, these are iodine oxides as well as oxo-acids of iodine (HIOx). The results of the thermochromatography experiments performed here prove the usefulness of the employed production method for carrier-free iodine isotopes and enhance the understanding of the evaporation and deposition behavior of iodine under various chemical conditions.

Determination of the absorption cross-sections of higher order iodine oxides at 355 nm and 532 nm

Iodine oxides (IxOy) play an important role in the atmospheric chemistry of iodine. They are initiators of new particle formation events in the coastal and polar boundary layer and act as iodine reservoirs in tropospheric ozone-depleting chemical cycles. Despite the importance of the aforementioned processes, the photochemistry of these molecules has not been studied in detail previously. Here, we report the first determination of the absorption cross sections of IxOy, x = 2, 3, 5, y = 1–12 at λ = 355 nm by combining pulsed laser photolysis of I2/O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration. The oxides selected for absorption cross section determinations are those presenting the strongest signals in the mass spectra, where signals containing 4 iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, σ355 nm, IO = (1.2 ± 0.1) × 10–18 cm2, which is found to be in good agreement with the most recent literature. The results obtained are: σ355 nm, I2O3

Atmospheric iodine chemistry from molecular level to 0D/3D simulations: applications to Fukushima nuclear accident

&lt;p&gt;In the case of a hypothetical nuclear accident, fission products are released into the environment. Simulation tools are commonly used to predict the radiological consequences on populations. After the Fukushima accident, significant differences have been observed between measured and modeled concentrations for iodine 131. This can be attributed to the high reactivity of iodine in the atmosphere not considered in the current dispersion crisis tools.&lt;/p&gt;&lt;p&gt;To address this, a new gas-phase mechanism of atmospheric iodine chemistry was developed containing 248 reactions. In parallel, missing thermokinetic data were determined by molecular-scale simulations for iodous and iodic acids. The 0D simulation results showed a partial and rapid transformation of these iodinated gaseous compounds. The influence of several parameters (air quality, quantity and nature of iodine released) was evaluated. For all simulations, iodine is quickly found in the form of iodine oxides and nitroxides or gaseous iodinated organic compounds. The latter may be the cause of iodinated aerosols formation and deposition.&lt;/p&gt;&lt;p&gt;Results from the 3D chemistry-climate model CAM-Chem will be compared to iodine Fukushima deposits measurements. Implications for atmospheric chemistry (air quality and climate) will be discussed.&lt;/p&gt;

The first steps of iodine gas-to-particle conversion as seen in the lab: constraints on the role of iodine oxides and oxyacids

&lt;p&gt;The photooxidation of gas phase iodine-bearing molecules emitted by marine biota leads to intense particle nucleation events in the coastal and polar marine boundary layer&lt;sup&gt;1-3&lt;/sup&gt;. The ubiquity of iodine in the marine atmospheric environment&lt;sup&gt;4-7&lt;/sup&gt; has suggested that this may be a previously unrecognized global source of new aerosol particles&lt;sup&gt;8&lt;/sup&gt;. Atmospheric modeling is required in order to evaluate the importance of this process, but a substantial lack of understanding of the gas-to-particle conversion mechanism is hindering this effort, especially regarding the gas phase chemistry of the nucleating molecules (iodine oxides&lt;sup&gt;9&lt;/sup&gt;&lt;sup&gt;,&lt;/sup&gt;&lt;sup&gt;10&lt;/sup&gt; and/or oxyacids&lt;sup&gt;7&lt;/sup&gt;) and the formation kinetics of molecular clusters. To address this problem, we have conducted new flow tube laboratory experiments where pulsed laser photolysis or continuous broad-band photolysis of I&lt;sub&gt;2&lt;/sub&gt;/O&lt;sub&gt;3&lt;/sub&gt; mixtures&amp;#160; in air are used to generate iodine radicals in the presence of atmospherically representative mixing ratios of water vapor. The molecular reactants and the resulting molecular products are detected by time-resolved VUV laser photo-ionization time-of-flight mass spectrometry. High-level quantum chemistry and master equation calculations and gas kinetics modelling are used to analyse the experimental data. In this presentation we discuss our results and their implications for the interpretation of field meassurements and for the implementatiion of an iodine oxide particle formation mechanism in atmospheric models.&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;1. Hoffmann, T., O'Dowd, C. D. &amp; Seinfeld, J. H. Iodine oxide homogeneous nucleation: An explanation for coastal new particle production. Geophys. Res. Lett. &lt;strong&gt;28&lt;/strong&gt;, 1949-1952 (2001).&lt;/p&gt;&lt;p&gt;2. McFiggans, G. et al. Direct evidence for coastal iodine particles from Laminaria macroalgae - linkage to emissions of molecular iodine. Atmos. Chem. Phys. &lt;strong&gt;4&lt;/strong&gt;, 701-713 (2004).&lt;/p&gt;&lt;p&gt;3. O'Dowd, C. D. et al. Marine aerosol formation from biogenic iodine emissions. Nature &lt;strong&gt;417&lt;/strong&gt;, 632-636 (2002).&lt;/p&gt;&lt;p&gt;4. Prados-Roman, C. et al. Iodine oxide in the global marine boundary layer. Atmos. Chem. Phys. &lt;strong&gt;15&lt;/strong&gt;, 583-593, doi:10.5194/acp-15-583-2015 (2015).&lt;/p&gt;&lt;p&gt;5. Sch&amp;#246;nhardt, A. et al. Simultaneous satellite observations of IO and BrO over Antarctica. Atmos. Chem. Phys. &lt;strong&gt;12&lt;/strong&gt;, 6565-6580, doi:10.5194/acp-12-6565-2012 (2012).&lt;/p&gt;&lt;p&gt;6. Mahajan, A. S. et al. Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment. Atmos. Chem. Phys. &lt;strong&gt;10&lt;/strong&gt;, 27227-27253 (2010).&lt;/p&gt;&lt;p&gt;7. Sipil&amp;#228;, M. et al. Molecular-scale evidence of aerosol particle formation via sequential addition of HIO3. Nature &lt;strong&gt;537&lt;/strong&gt;, 532-534, doi:10.1038/nature19314 (2016).&lt;/p&gt;&lt;p&gt;8. Saiz-Lopez, A. et al. Atmospheric Chemistry of Iodine. Chem. Rev. &lt;strong&gt;112&lt;/strong&gt;, 1773&amp;#8211;1804, doi:DOI: 10.1021/cr200029u (2012).&lt;/p&gt;&lt;p&gt;9. G&amp;#243;mez Mart&amp;#237;n, J. C. et al. On the mechanism of iodine oxide particle formation. Phys. Chem. Chem. Phys. &lt;strong&gt;15&lt;/strong&gt;, 15612-15622, doi:10.1039/c3cp51217g (2013).&lt;/p&gt;&lt;p&gt;10. Saunders, R. W., Mahajan, A. S., G&amp;#243;mez Mart&amp;#237;n, J. C., Kumar, R. &amp; Plane, J. M. C. Studies of the Formation and Growth of Aerosol from Molecular Iodine Precursor. Z. Phys. Chem. &lt;strong&gt;224&lt;/strong&gt;, 1095-1117 (2010).&lt;/p&gt;