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Experimental determination of the photolysis of aqueous I as a source of atmospheric I -
2
Kosuke Watanabe, Shohei Matsuda, Carlos A Cuevas, Alfonso Saiz-Lopez, Akihiro Yabushita, and Yukio Nakano ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.9b00007 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 4, 2019
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ACS Earth and Space Chemistry
Experimental determination of the photolysis of aqueous I- as a source of atmospheric I2 Kosuke Watanabe 1, Shohei Matsuda 1, 2, 3, Carlos A. Cuevas 4, Alfonso Saiz-Lopez 4, Akihiro Yabushita 2, and Yukio Nakano 1, * 1
Department of Environmental Sciences, Tokyo Gakugei University, 4-1-1 Nukuikita-machi, Koganei-shi, Tokyo 184-8501, Japan
2
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
3
Materials Sciences Research Center, Japan Atomic Energy Agency, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 6795148, Japan
4
Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, 28006 Madrid, Spain
* Corresponding author. e-mail:
[email protected] Tel/Fax: +81-42-329-7518
Abstract The chemistry of iodine plays an important role in the oxidizing capacity of the global marine atmosphere. In this study, we experimentally determine the photolysis 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 (iodide()) and the photolytic quantum yields ( iodide()) of I-(aq) in the range of 290-500 nm were determined. The influence of pH and dissolved oxygen (DO) on the values of iodide() was also investigated. The emission of I2(g) into the atmosphere following the photolysis 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 photolysis of I-(aq) can increase the atmospheric iodine budget by up to ~8% over some oceanic regions.
Keywords iodine molecule, iodide ion, photolysis, molar absorption coefficient, photolytic quantum yield, halogen chemistry, global modelling.
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1. Introduction Reactive iodine compounds, such as iodine atoms (I) and iodine monoxide radicals (IO), are vital for tropospheric ozone loss and aerosol formation in the atmosphere.1,
2, 3, 4
Reactive iodine is the second most
important species family responsible for ozone depletion in the tropical marine boundary layer, with a contribution of (17-27%) to the total ozone loss in the region5. Iodine emissions in coastal areas can lead to the formation of fastgrowing molecular clusters, through the sequential addition of HIO3 followed by restructuring to I2O56. This mechanism could also occur in open ocean waters, where iodine is also emitted.7 Remarkably, a recent study has shown that atmospheric iodine concentrations in the Arctic have tripled since the mid-twentieth century.8 Despite the importance of these reactive iodine species in the atmosphere, their sources are not fully understood. Currently, organoiodine
compounds,
such
as
methyl
iodide
(CH3I),
diiodomethane,
chloroiodomethane,
and
bromoiodomethane, and inorganic species, such as hypoiodous acid (HOI) and molecular iodine (I2), are considered the main sources of atmospheric iodine.1 After these compounds are emitted from the ocean into the atmosphere, I atoms are released by photolysis.1 The reaction of CH3I with hydroxyl radicals can also release I atoms.9 Gaseous iodine molecules (I2(g)) are formed by the reaction of gaseous ozone molecules (O3(g)) with iodide ions in aqueous phase (I-(aq)) on the sea surface.10,
11, 12, 13, 14
In 2013, Carpenter et al. suggested that both iodine
molecules in aqueous phase (I2(aq)) and mainly gaseous hypoiodous acid (HOI(g)) could be emitted into the atmosphere.15 Another candidate is the generation of I2(g) following the formation of I2(aq) initiated by the photolysis of I-(aq). For decades, this reaction has been known to occur, and from the 1950s to 2000s, the reaction pathway and mechanism of this reaction and its impact on the atmosphere were studied by Grossweiner and Matheson16, Miyake and Tsunogai17, Jortner et al.18,
19, 20,
and Truesdale21. From those results, the following reactions could be
considered as the reaction pathway. I-(aq) + h(UV or Vis) I-*(aq)
(1)
I-*(aq) [I + e-](aq)
(2)
[I + e-](aq) + H+(aq) I(aq) + H(aq)
(3)
[I + e-](aq) + O2(aq) I(aq) + O2-(aq)
(4)
I(aq) + I-(aq)
I2-(aq)
(5)
2I2-(aq) I3-(aq) + I-(aq)
(6)
I(aq) + I2-(aq) I3-(aq)
(7)
2I(aq) I2(aq)
(8)
(9)
I3-(aq)
I2(aq) + I-(aq)
I2(aq) I2(g)
(10)
Here, [I + e-](aq) is both an iodine atom and a solvated electron in the same solvent cage. H+(aq), O2(aq), I(aq), I3-(aq), and I2-(aq) are hydrogen ion in aqueous phase, dissolved oxygen molecule, iodine atom, triiodide ion and diiodide ion, respectively. Additionally, the very small value of Henry’s law constant for I2, which is indicative of low solubility causes the I2(aq) formed through reactions 1-9 to be immediately released into the atmosphere according to reaction 2 ACS Paragon Plus Environment
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ACS Earth and Space Chemistry
10. This reaction pathway also indicates that either H+(aq) or O2(aq) must exist in aqueous phase for the generation of I2(g). Moreover, in reactions 1-10, the existence of organic matter could promote the increased formation of I2(g) as the photosensitizer.22 With respect to the quantitative research on the generation of I2(g) initiated by the photolysis of I-(aq), Miyake and Tsunogai17 reported that annual global emission of I2(g) was 400 Gg(I) year-1 based on the observation of the formation of I2(g) by the irradiation with UV lamp with filters that transmit 250-560 nm into sea water. However, Truesdale21 suggested the generation of I2(g) initiated by photolysis of I- (aq) has a relatively small effect on the source of the reactive iodine compounds in the atmosphere. Thus, the influence of the generation of I2(g) from this process on the source of the reactive iodine compounds in the atmosphere has yet to be fully understood. Raso et al.23 suggested that sunlit and artificially irradiated snowpack produced and released I2 due to snow enriched in iodide. In that work, they only tell that it was photochemical in nature and suggested hydroxyl radical formation and subsequent reaction with iodide. Therefore, this potentially important mechanism for the release of I2 from the photolysis of iodide from the oceans should be included in chemistry-climate models to assess its importance.5,
24
The photoabsorption of I-(aq) is the initial step in the generation of I2(g), and this step is considered to be the rate determining step under the conditions of the actual sea and atmosphere. The reaction rate of each step is as shown in Table S1 in the supporting information (SI). As shown in Table S1, the lifetime of the step of the reaction 1 is estimated to be 1 = 5.6 × 107 s by using the values of both the typical concentration of I-(aq) in the sea and the formation rates of I2(g) of which determining process is described below. This lifetime is larger than those of the other steps of the reaction 2-9. Therefore, the rate determining step under the conditions of the actual sea and atmosphere is considered to be the step of the photolysis of I-(aq) shown as reaction 1. The rate of I2(g) generation from the photolysis of I-(aq) can be theoretically evaluated using the following equation. d[I2(g)] dt
≈
d[I2(aq)] dt
=
1 [I(aq)]sea 2
∫
iodide()
Fsun() NA
iodide() d.
(11)
where d[I2(g)]/dt and d[I2(aq)]/dt are the formation rates of I2(g) and I2(aq), respectively. Because the generated I2(aq) is immediately released into the gas phase according to Henry’s law, d[I2(g)]/dt is considered to be virtually equivalent to d[I2(aq)]/dt. [I-(aq)]sea, iodide(), Fsun(), and NA are the typical concentrations of I-(aq) in the sea, the molar absorption coefficient of I-(aq), the solar actinic flux at the Earth’s surface and Avogadro’s number, respectively. iodide() is the pseudo-quantum yield of the photolysis of I-(aq) for the formation of I(aq), including the total influence by reactions 1-4. Because one ion of I-(aq), which absorbed one photon forms one atom of I(aq) by reactions 3 and 4 and then two atoms of I(aq) forms one molecule of I2(aq) by reactions 5-9, the absorption of one photon by I-(aq) produces half of the I2(aq). Thus, the right side of Eq. (11) is multiplied by 50%. The values of iodide() at less than 260 nm have been reported.25 However, those in the wavelength range from near ultraviolet to visible light, whose lights are included in the sunlight that hits the Earth’s surface have yet to be reported. Additionally, the values of iodide() have not been reported in those wavelength regions. Thus, the rate of I2(g) generation from the photolysis of I-(aq) has been impossible to theoretically estimate by using Eq. (11). In this study, we experimentally determined the values of iodide() and iodide() in the range of 290-500 nm and estimated the influence of I2(g) generation subsequent to the formation of I2(aq) initiated by the photolysis of I-(aq) on the source of reactive iodine compounds in the atmosphere. The values of iodide() are certainly changed by the 3 ACS Paragon Plus Environment
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potential of hydrogen (pH) and dissolved oxygen (DO) in solution, since H+(aq) and O2(aq) are involved in the reaction pathway of I2(g) generation initiated by the photolysis of I-(aq) as described in reactions 3 and 4. The dependence of the pH and DO on the values of iodide() was also investigated to estimate the values of iodide() in the actual sea environment. The influence of the generation and emission of I2(g) estimated from the values determined in the present work on the atmosphere was evaluated using a CAM-Chem global chemistry-climate model.
2. Experimental 2.1 Determination of iodide() Potassium iodide (KI) aqueous solutions were used as the sources of I-(aq). KI was obtained from commercial source (Kanto Chemical Co., Inc.: >99.5%) and used without further purification. To determine iodide() in the range of 290-500 and 500-510 nm, the absorption spectra of KI aqueous solutions with concentrations of 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.50, 0.60, 0.70, and 0.80 M in the range of 280-600 nm were measured using the 50cm optical cell. These concentrations of I-(aq) were much higher than the typical concentration of I-(aq) in the sea due to the detection limit of our experimental setup. KI aqueous solutions in these experiments were prepared using deionized water, which was bubbled with air for more than one hour until it was completely saturated with air, and thus also completely saturated with oxygen and carbon dioxide. Therefore, the pH and DO of these solutions were typically ~5.6 and ~7.8 mg L-1 at atmospheric partial pressures, respectively. These solutions were then poured into a hermetic optical cell composed of a 50-cm glass tube with a 2.5 cm diameter with synthetic quartz windows at the both sides of the tube. To obtain the length of the long optical pass, we made and used our own UV-vis spectrophotometer. The light emitted from the Xenon arc lamp (Ushio: OPM2-152XQ) was dispersed by the monochromator (Oriel: Cornerston 260 1/4 m) and used as monochromatic incident light to monitor the absorption at several wavelengths. The wavelength resolution of the light was better than 1.5 nm. The window of 50-cm optical cell filled with the solutions was irradiated with monochromatic incident light. The light transmitted from another side window of the cell was monitored with a photomultiplier tube (Hamamatsu: R11568). The signals from the photomultiplier tube were converted to digital signals by an analog-to-digital converter (Pico Technology: ADC-212/3) and transferred to a personal computer sweeping the wavelength of the monochromator. The monochromator was placed between a Xenon arc lamp and optical cell to suppress the formation of I3-(aq) by undispersed white light from the lamp. A schematic diagram of our experimental setup is shown in Figure S1 in SI. The stray light from the Xenon arc lamp also led to the formation of I3-(aq) in KI aqueous solutions. To minimize the influence of stray light, measurements were performed as quickly as possible. Additionally, experiments were performed under only red light (>590 nm) irradiation conditions to prevent unintended photolysis of I-(aq) by room light in the laboratory. Virtually no formation of I3-(aq) was observed during each measurement. To correct the temporal fluctuation of the light intensity from the Xenon arc lamp, the absorption spectra in the range of 280-600 nm for water in the 50-cm optical cell were monitored before and after measurement, and the spectrum obtained by taking the average of two spectra was then used as a background spectrum. The background of absorption level was also corrected using the absorbance of the background spectrum in the range of 500-510 nm, where the absorption of I-(aq) was presumed not to exist or was significantly lower than the detection limit under our experimental conditions. Experiments were conducted at (300 ± 2) K. 4 ACS Paragon Plus Environment
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2.2 Determination of iodide() For the determination of iodide(), which forms I(aq) in the range of 290-500 nm, the absorption spectra of I(aq)
in the range of 290-500 and 500-510 nm were measured after the irradiations of light emitting diodes (LEDs)
using the 50-cm optical cell filled with the KI aqueous solutions, whose concentrations were 0.50 M. The experimental setup was the same one used for the determination of iodide() (Figure S1 in SI). The center wavelengths of the bullet type LEDs (REVOX Inc.) used as the irradiation lights sources were 375, 405, 430, 450, and 470 nm with widths of 15, 12, 25, 20, and 22 nm, respectively, at half maximum. The voltages and currents of the LEDs during irradiation were measured by a digital multi-meter (IWATSU: VOAC7522), and the power consumed by the LEDs were determined. Due to the relationship between the consumed power of LEDs and their photon flux, measurement were performed using a Hatchard–Parker actinometer26,
27
in advance, the photon flux
from the LEDs could be estimated from the consumed power of LEDs measured by the digital multi-meter in each experiment. The procedure of measuring the photon flux of LEDs by the Hatchard–Parker actinometer is described in SI. Also, to investigate the influence of the difference among cations included in the solutions on iodide(), similar experiments were conducted using sodium iodide (NaI) (Kanto Chemical Co., Inc.: >99.5%), aqueous solutions instead of KI aqueous solutions as the sources of I-(aq). Additionally, to investigate the dependence of the concentration of I-(aq) on the rate of I2(g) generation from the photolysis of I-(aq), we conducted the experiments measuring the generation rate of I3-(aq) at several concentration of I-(aq) (0.10, 0.20, 0.30, 0.40, and 0.50 M). Once I-(aq) was irradiated with the LEDs, I3-(aq) was formed by the photolysis of I-(aq) and the sequential reactions as shown in reactions 1-7. Because the generated I3-(aq) has an absorption peak at 353 nm,25 the concentrations of I3-(aq) could be determined from the 353-nm absorbance of the solution and the reported molar absorption coefficient of I3-(aq) at 353 nm.25 To investigate the dependence of the pH and DO on the generation rates of I2(g) from the photolysis of I-(aq), the values of iodide() were also measured under several pH and DO conditions. In these experiments, pH buffer solutions (pH 7.0 and 8.0: NaH2PO4/NaOH, pH 10.0: NaHCO3/NaOH, pH 11.0: K2HPO4/NaOH, pH 13.0: NaCl/NaOH.) were used instead of the deionized water. The detailed procedures of preparation of each pH buffer solution is described in SI. These pH buffer solutions were either bubbled with N2 to completely remove O2(aq) or bubbled with air until saturation was reached. The DO of the solutions, which were bubbled with N2 or air were typically 0.8 and 7.8 mg L-1, respectively. The KI aqueous solutions were then prepared using these solvents. After preparation of KI solutions using these pH buffer solutions, the pH of these solutions decreased by 0.3-0.4. The other experimental procedures such as LED irradiation were performed in the same way as described above. Additionally, similar experiments were conducted using artificial seawater (pH 8.0, DO 7.0 mg L-1) as the solvent.
3. Modelling We used a global 3-D chemistry-climate model CAM-Chem (Community Atmospheric Model with chemistry, version 4.0) to assess the I2(g) ocean emission flux on a global scale. The model included a comprehensive chemistry scheme to simulate the evolution of trace gases and aerosols in the troposphere and the stratosphere.28 The model implements iodine and bromine chemistry schemes from previous studies,5,
29
including the photochemical
breakdown of bromo- and iodo-carbons emitted from ocean water30 and abiotic sources of HOI and I231. Model 5 ACS Paragon Plus Environment
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runs were performed in specified dynamic mode, to allow for a direct comparison between different simulations using the high-frequency meteorological input from a previous CAM-Chem simulation for the year 2000, which included a unique input meteorological dataset every 3-6 h. In this work, CAM-Chem has been configured with a horizontal resolution of 1.9º latitude by 2.5º longitude and 26 vertical levels, from the surface up to an altitude ~40 km. We ran two different scenarios in CAM-Chem, with and without the I2(g) release mechanism to assess the potential contribution of this proposed mechanism to the atmospheric iodine budget.
4. Results and discussion 4.1. Determination of iodide() Figure 1 shows absorption spectrum of I-(aq) in the range of 290-500 nm at pH 5.6, DO 7.8 mg L-1, and ambient temperature. The absorbance at 300, 325, 350, and 375 nm that were observed based on these measured absorption spectra as a function of the concentration of I-(aq) are shown in the inset of Figure 1. The lines in the inset of Figure 1 are Beer-Lambert (BL) plots obtained from linear least-squares fits for our data. From these BL plots, the values of iodide() at 300, 325, 350, and 375 nm were determined to be 0.015, 0.0077, 0.0053, and 0.0032 M-1 cm-1, respectively. To obtain the exact shape of absorption spectrum of I-(aq) in the range of 290-500 nm, all measured absorption spectra, which have the same shapes were then averaged. The absolute value of the averaged absorption shape was corrected using the estimated values of iodide() at 300, 325, 350 and 375 nm. As a result, the exact absorption spectrum of I-(aq) expressed in units of a molar absorption coefficient was obtained as shown in Figure 1. Additionally, the numerical data of iodide() are summarized in Table S2 and S3 in SI. The influence of the photolysis of I-(aq) in the atmosphere was estimated using this absorption spectrum of I-(aq) expressed in units of molar absorption coefficient. Some studies have reported that the oxygen iodide charge-transfer complex (O2-I-) can be formed once an iodide coexist with O2 in solutions.32,
33
Kim et al.32 reported that the absorption band of O2-I- could be observed around
280-330 nm using KI aqueous solution saturated with O2. On the other hand, Levanon and Navon33 reported that the absorption spectra of KI aqueous solutions under conditions of several concentrations of KI (0.2-1.0 M) and several O2 pressures (20-100 atm) and the very weak absorption peak attributable to O2-I- was observed at 264 nm under their experimental conditions. The intensity, shape, and spectral position of the absorption band attributable to O2-I- reported by Kim et al. was different from that found by Levanon and Navon, though the reason is unclear. To confirm the existence and influence of O2-I- under our experimental conditions, the absorption spectra of KI aqueous solutions prepared using deionized water bubbled with either N2 or O2 during more than one hour were measured under only red light (>590 nm) irradiation conditions to prevent unintended photolysis of I-(aq) by room light in the laboratory. These absorption spectra are shown in Figure S2 in SI. As a result, it is understood that these spectra are the same as the absorption spectrum of KI aqueous solution, which was prepared with the deionized water saturated by air. This result also showed that the absorption band at 280-330 nm reported by Kim et al. was not observed under our experimental conditions. Furthermore, Levanon and Navon also reported the upper limit of the equilibrium constant of O2 + I-
O2-I- and the molar absorption
coefficient of O2-I- at 264 nm to be K(O2-I-) ≤ 0.3 M-1 and (O2-I-)(264 nm) ≤ 290 cm-1 M-1, respectively. We estimate the maximum amount of the formation of O2-I-, [O2-I-]max under the experimental condition of present work by using our values of [I-(aq)] and [O2(aq)]. As a result, the value of [O2-I-]max under the experimental condition of present 6 ACS Paragon Plus Environment
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work obtained to be 3.18 × 10-5 M. By comparing [O2-I-]max with [I-(aq)] and (O2-I-)(264 nm) with the molar absorption coefficient of I-(aq) at 264 nm (I-(264 nm) = 30 cm-1 M-1), which is derived from the spectrum reported by Awtrey et al.25, it is supposed that the absorbance of O2-I- are significantly smaller than that of I-(aq). The supposed extremely low absorbance of O2-I- is consistent with the fact that the absorption of O2-I- was not observed in the present experiments as described above. This suggests that the O2-I- have an extremely small or no influence on the formation of I2(aq) initiated by the photolysis of I-(aq) in our experiment and even under actual atmospheric conditions. 4.2. Determination of iodide() The absorption spectra of 0.50 M of I-(aq) solutions at ambient temperature before and after the 375-nm LED light irradiation at 5-minute intervals are shown in Figure 2a as an example of the results. When the LED light irradiation time was increased, an increase in absorption around 353 nm, which is attributed to the formation of I3(aq)
was observed. This indicated that the 375-nm LED light irradiation was able to trigger the formation of I3-(aq)
through the photolysis of I-(aq) and the sequential reactions, as shown in reactions 1-7. The formed I3-(aq) also indicated that I2(aq) was able to be formed by the equilibrium reaction 9. The similar experiments with 405, 430, 450, and 470 nm of LEDs as the photolysis were performed. By using the increases of absorption around 353 nm obtained in those experiments and the reported molar absorption coefficient of I3-(aq) at 353 nm, the concentrations of I3-(aq) formed through the photolysis of I-(aq) and the sequential reactions could be determined. It is important to note that we confirmed in advance whether the generation of I3-(aq) occurred under dark conditions, which was quite small (~5.0 × 10-8 M) and only a slight increase (
