Uptake of Ozone to Deliquesced KI and Mixed KI/NaCl Aerosol Particles

Iodide oxidation by ozone at the surface of aqueous microdroplets

The oxidation of iodide by ozone occurs at the sea-surface and within sea spray aerosol, influencing the overall ozone budget in the marine boundary layer and leading to the emission of reactive halogen gases. A detailed account of the surface mechanism has proven elusive, however, due to the difficulty in quantifying multiphase kinetics. To obtain a clearer understanding of this reaction mechanism at the air-water interface, we report pH-dependent oxidation kinetics of I- in single levitated microdroplets as a function of [O3] using a quadrupole electrodynamic trap and an open port sampling interface for mass spectrometry. A kinetic model, constrained by molecular simulations of O3 dynamics at the air-water interface, is used to understand the coupled diffusive, reactive, and evaporative pathways at the microdroplet surface, which exhibit a strong dependence on bulk solution pH. Under acidic conditions, the surface reaction is limited by O3 diffusion in the gas phase, whereas under basic conditions the reaction becomes rate limited on the surface. The pH dependence also suggests the existence of a reactive intermediate IOOO- as has previously been observed in the Br- + O3 reaction. Expressions for steady-state surface concentrations of reactants are derived and utilized to directly compute uptake coefficients for this system, allowing for an exploration of uptake dependence on reactant concentration. In the present experiments, reactive uptake coefficients of O3 scale weakly with bulk solution pH, increasing from 4 × 10-4 to 2 × 10-3 with decreasing solution pH from pH 13 to pH 3.

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.

Reactive Uptake of Hydroperoxymethyl Thioformate to Sodium Chloride and Sodium Iodide Aerosol Particles

The oxidation products of dimethyl sulfide (DMS) contribute to the production and growth of cloud condensation nuclei (CCN) in the marine boundary layer. Recent work demonstrates that DMS is oxidized by OH radicals to the stable intermediate hydroperoxymethyl thioformate (HPMTF), which is both globally ubiquitous and efficiently lost to multiphase processes in the marine atmosphere. At present, there are no experimental measurements of the reactive uptake of HPMTF to aerosol particles, limiting model implementation of multiphase HPMTF chemistry. Using an entrained aerosol flow reactor combined with chemical ionization mass spectrometry (CIMS), we measured the reactive uptake coefficient (γ) of HPMTF to dry sodium chloride (NaCl), wet NaCl, and wet sodium iodide (NaI) particles to be (1.9 ± 1.3) × 10^-4, (1.6 ± 0.6) × 10^-3, and (9.2 ± 2.3) × 10^-1, respectively. While we did not directly measure the condensed-phase products of HPMTF reactive uptake in this experiment, the ionization products observed in the CIMS instrument provide mechanistic insight on the reaction mechanism of HPMTF with halides.

Reactive Uptake of Ozone to Simulated Seawater: Evidence for Iodide Depletion

The reaction of ozone with iodide in the ocean is a major ozone dry deposition pathway, as well as an important source of reactive iodine to the marine troposphere. Few prior laboratory experiments have been conducted with environmentally relevant ozone mixing ratios and iodide concentrations, leading to uncertainties in the rate of the reaction under marine boundary layer conditions. As well, there remains disagreement in the literature assessment of the relative contributions of an interfacial reaction via ozone adsorbed to the ocean surface versus a bulk reaction with dissolved ozone. In this study, we measure the uptake coefficient of ozone over a buffered, pH 8 salt solution replicating the concentrations of iodide, bromide, and chloride in the ocean over an ozone mixing ratio of 60-500 ppb. Due to iodide depletion in the solution, the measured ozone uptake coefficient is dependent on the exposure time of the solution to ozone and its mixing ratio. A kinetic multilayer model confirms that iodide depletion is occurring not only within ozone's reactodiffusive depth, which is on the order of microns for environmental conditions, but also deeper into the solution as well. Best model-measurement agreement arises when some degree of nondiffusive mixing is occurring in the solution, transporting iodide from deeper in the solution to a thin, diffusively mixed upper layer. If such mixing occurs rapidly in the environment, iodide depletion is unlikely to reduce ozone dry deposition rates. Unrealistically high bulk-to-interface partitioning of iodide is required for the model to predict a substantial interfacial component to the reaction, indicating that the Langmuir-Hinshelwood mechanism is not dominant under environmental conditions.

Iodide conversion to iodate in aqueous and solid aerosols exposed to ozone

The aqueous-phase and surface reactions of ozone (O₃) with iodide (I^-) in/on seawater have been recently found to be a strong atmospheric source of iodine. In addition, ozone also reacts with I^- in solid and aqueous sea-salt aerosol. However, the primary products of the heterogeneous reactions of ozone with I^- have not been clarified. In this paper, solid and aqueous KI aerosols have been exposed to ozone in an aerosol flow tube system and I^- and iodate (IO₃^-) concentrations have been measured by UV-Vis spectroscopy. The results of these experiments have been combined with a kinetic model to elucidate the primary products of the aqueous and surface reactions. The reaction of ozone with aqueous iodide has been inferred to originate different products depending on whether it occurs at the surface via O₃ adsorption (product I₂^-) or in the aqueous phase via O₃ solvation (product IO^-). The surface reaction of ozone with solid KI in the presence of water vapor forms KIO₃, and other species, which are likely to be gaseous. Although the reactions have been studied in aerosols, the results can be extrapolated to aqueous solutions as well.

Quantitative detection of iodine in the stratosphere

Oceanic emissions of iodine destroy ozone, modify oxidative capacity, and can form new particles in the troposphere. However, the impact of iodine in the stratosphere is highly uncertain due to the lack of previous quantitative measurements. Here, we report quantitative measurements of iodine monoxide radicals and particulate iodine (I_y,part) from aircraft in the stratosphere. These measurements support that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (I_y) is injected to the stratosphere. These high I_y amounts are indicative of active iodine recycling on ice in the upper troposphere (UT), support the upper end of recent I_y estimates (0 to 0.8 pptv) by the World Meteorological Organization, and are incompatible with zero stratospheric iodine injection. Gas-phase iodine (I_y,gas) in the UT (0.67 ± 0.09 pptv) converts to I_y,part sharply near the tropopause. In the stratosphere, IO radicals remain detectable (0.06 ± 0.03 pptv), indicating persistent I_y,part recycling back to I_y,gas as a result of active multiphase chemistry. At the observed levels, iodine is responsible for 32% of the halogen-induced ozone loss (bromine 40%, chlorine 28%), due primarily to previously unconsidered heterogeneous chemistry. Anthropogenic (pollution) ozone has increased iodine emissions since preindustrial times (ca. factor of 3 since 1950) and could be partly responsible for the continued decrease of ozone in the lower stratosphere. Increasing iodine emissions have implications for ozone radiative forcing and possibly new particle formation near the tropopause.

A kinetic model for ozone uptake by solutions and aqueous particles containing I- and Br-, including seawater and sea-salt aerosol

The heterogeneous interactions of gaseous ozone (O3) with seawater and with sea-salt aerosols are known to generate volatile halogen species, which, in turn, lead to further destruction of O3. Here, a kinetic model for the interaction of ozone (O3) with Br- and I- solutions and aqueous particles has been proposed that satisfactorily explains previous literature studies about this process. Apart from the aqueous-phase reactions X- + O3 (X = I, Br), the interaction also involves the surface reactions X- + O3 that occur via O3 adsorption on the aqueous surface. In single salt solutions and aerosols, the partial order in ozone and the total order of the surface reactions are one, but the apparent total order is second order because the number of ozone sites where reaction can occur is equal to the surficial concentration of X- ([X-]surf). In the presence of Cl-, the surface reactions are enhanced by a factor equal to , where and . Therefore, we have inferred that Cl- acts as a catalyst in the surface reactions X- + O3. The model has been applied to estimate ozone uptake by the reaction with these halides in/on seawater and in/on sea-salt aerosol, where it has been concluded that the Cl--catalyzed surface reaction is important relative to total ozone uptake and should therefore be considered to model Y/YO (Y = I, Br, Cl) levels in the troposphere.

A revisit of the interaction of gaseous ozone with aqueous iodide. Estimating the contributions of the surface and bulk reactions

The main source of atmospheric iodine is the heterogeneous reaction of aqueous iodide (I-) with ozone (O3), which takes place in surface seawater and probably in sea-salt aerosols. However, there are seemingly contradictory conclusions about whether this heterogeneous reaction occurs in the bulk of the aqueous phase, via O3 dissolution, or at the aqueous surface, via O3 adsorption. In this work, the ozone uptake coefficient has been calculated as a function of the concentration of aqueous iodide ([I-]aq) and gaseous ozone near the aqueous surface ([O3]gs) by estimating parameters of the resistor model using results of previous studies. The calculated uptake coefficients suggest that the aqueous-phase reaction dominates at low I- concentrations (about <10-4 mol L-1), regardless of [O3]gs, and also at sufficiently high [O3]gs (about >80 ppm), regardless of [I-]aq. In contrast, the surface reaction dominates at high [I-]aq (about >10-4 mol L-1) as long as [O3]gs is low enough (about <80 ppm). This trend is able to reconcile previous studies of this reaction, and is a consequence of several factors, including the high surface excess of both reactants ozone and iodide. Given the typical O3 concentrations in the troposphere and the possible I- concentrations and O3 solubilities in sea-salt aerosols, the surface reaction may compete with the aqueous-phase reaction in accumulation-mode aerosols, unlike in surface seawater, where the aqueous-phase reaction probably prevails. The rate constant of the surface reaction has been estimated as (3-40) × 10-13 cm2 molecule-1 s-1.

A polarizable continuum model for molecules at spherical diffuse interfaces

We present an extension of the Polarizable Continuum Model (PCM) to simulate solvent effects at diffuse interfaces with spherical symmetry, such as nanodroplets and micelles. We derive the form of the Green's function for a spatially varying dielectric permittivity with spherical symmetry and exploit the integral equation formalism of the PCM for general dielectric environments to recast the solvation problem into a continuum solvation framework. This allows the investigation of the solvation of ions and molecules in nonuniform dielectric environments, such as liquid droplets, micelles or membranes, while maintaining the computationally appealing characteristics of continuum solvation models. We describe in detail our implementation, both for the calculation of the Green's function and for its subsequent use in the PCM electrostatic problem. The model is then applied on a few test systems, mainly to analyze the effect of interface curvature on solvation energetics.

Impact of Enhanced Ozone Deposition and Halogen Chemistry on Tropospheric Ozone over the Northern Hemisphere

Fate of ozone in marine environments has been receiving increased attention due to the tightening of ambient air quality standards. The role of deposition and halogen chemistry is examined through incorporation of an enhanced ozone deposition algorithm and inclusion of halogen chemistry in a comprehensive atmospheric modeling system. The enhanced ozone deposition treatment accounts for the interaction of iodide in seawater with ozone and increases deposition velocities by 1 order of magnitude. Halogen chemistry includes detailed chemical reactions of organic and inorganic bromine and iodine species. Two different simulations are completed with the halogen chemistry: without and with photochemical reactions of higher iodine oxides. Enhanced deposition reduces mean summer-time surface ozone by ∼3% over marine regions in the Northern Hemisphere. Halogen chemistry without the photochemical reactions of higher iodine oxides reduces surface ozone by ∼15% whereas simulations with the photochemical reactions of higher iodine oxides indicate ozone reductions of ∼48%. The model without these processes overpredicts ozone compared to observations whereas the inclusion of these processes improves predictions. The inclusion of photochemical reactions for higher iodine oxides leads to ozone predictions that are lower than observations, underscoring the need for further refinement of the halogen emissions and chemistry scheme in the model.

Conversion of iodide to hypoiodous acid and iodine in aqueous microdroplets exposed to ozone

Halides are incorporated into aerosol sea spray, where they start the catalytic destruction of ozone (O3) over the oceans and affect the global troposphere. Two intriguing environmental problems undergoing continuous research are (1) to understand how reactive gas phase molecular halogens are directly produced from inorganic halides exposed to O3 and (2) to constrain the environmental factors that control this interfacial process. This paper presents a laboratory study of the reaction of O3 at variable iodide (I(-)) concentration (0.010-100 μM) for solutions aerosolized at 25 °C, which reveal remarkable differences in the reaction intermediates and products expected in sea spray for low tropospheric [O3]. The ultrafast oxidation of I(-) by O3 at the air-water interface of microdroplets is evidenced by the appearance of hypoiodous acid (HIO), iodite (IO2(-)), iodate (IO3(-)), triiodide (I3(-)), and molecular iodine (I2). Mass spectrometry measurements reveal an enhancement (up to 28%) in the dissolution of gaseous O3 at the gas-liquid interface when increasing the concentration of NaI or NaBr from 0.010 to 100 μM. The production of iodine species such as HIO and I2 from NaI aerosolized solutions exposed to 50 ppbv O3 can occur at the air-water interface of sea spray, followed by their transfer to the gas-phase, where they contribute to the loss of tropospheric ozone.

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.

Solvation structures of iodide on and below a surface of aqueous solution studied by photodetachment spectroscopy

We investigated solvation structures of I(-) on and below a surface of an aqueous solution by photodetachment spectroscopy. An aqueous solution of an alkali halide was introduced to the vacuum as a continuous liquid flow (liquid beam), and the liquid beam was irradiated with a UV laser pulse. The intensity of electrons emitted from the surface by the laser excitation was measured as a function of wavelength (photodetachment spectroscopy), and we obtained absorption spectrum of I(-) on and below the solution surface. From the absorption spectrum, we found that I(-) starts to appear on the solution surface as the bulk NaI concentration increases. Similar concentration dependence was observed for the KI solution. We also found that I(-) located inside the solution is pushed to the surface, when NaCl is added to the solution. These changes are explained in terms of the difference in the polarizability of halide ions.