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Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone Elizabeth A

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Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone Elizabeth A View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Kentucky University of Kentucky UKnowledge Chemistry Faculty Publications Chemistry 10-1-2013 Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone Elizabeth A. Pillar University of Kentucky, [email protected] Marcelo I. Guzman University of Kentucky, [email protected] Jose M. Rodriguez NASA Goddard Space Flight Center Right click to open a feedback form in a new tab to let us know how this document benefits oy u. Follow this and additional works at: https://uknowledge.uky.edu/chemistry_facpub Part of the Analytical Chemistry Commons, Atmospheric Sciences Commons, Environmental Chemistry Commons, Environmental Sciences Commons, Inorganic Chemistry Commons, and the Physical Chemistry Commons Repository Citation Pillar, Elizabeth A.; Guzman, Marcelo I.; and Rodriguez, Jose M., "Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone" (2013). Chemistry Faculty Publications. 17. https://uknowledge.uky.edu/chemistry_facpub/17 This Article is brought to you for free and open access by the Chemistry at UKnowledge. It has been accepted for inclusion in Chemistry Faculty Publications by an authorized administrator of UKnowledge. For more information, please contact [email protected]. Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone Notes/Citation Information Published in Environmental Science & Technology, v. 47, issue 19, p. 10971-10979. © 2013 American Chemical Society This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Digital Object Identifier (DOI) http://dx.doi.org/10.1021/es401700h This article is available at UKnowledge: https://uknowledge.uky.edu/chemistry_facpub/17 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article pubs.acs.org/est Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone † † ‡ Elizabeth A. Pillar, Marcelo I. Guzman,*, and Jose M. Rodriguez † University of Kentucky, Department of Chemistry, Lexington, Kentucky 40506, United States ‡ NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, United States *S Supporting Information ABSTRACT: Halides are incorporated into aerosol sea spray, where they ff start the catalytic destruction of ozone (O3) over the oceans and a ect 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 − − μ O3 at variable iodide (I ) concentration (0.010 100 M) 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. ■ INTRODUCTION known that marine macroalgae in coastal regions produce I2, Increasing levels of greenhouse gases in the atmosphere over rather than iodine-containing organic compounds, which is fi transferred to the atmosphere and photoactivated to release the past 160 years have led to a signi cant climate perturbation 15,16 1 iodine atoms. However, measurements over the open in the present time scale. Ozone, O3(g), near the tropopause is 17 an important greenhouse gas. Thus, a better understanding of ocean cannot be explained by this mechanism of O3 loss. the anthropogenic effects on climate requires the full Perhaps, a major contribution to the loss of tropospheric ozone over the open ocean originates from inert halide ions (e.g., Br− characterization of the pathways of production and destruction − 18,19 − 20 and I ) or oxyanions (e.g., IO3 ), which are somehow of tropospheric O3(g). Ozone molecules also play a key role in converted into molecular halogens (e.g., Br2 and I2), and tropospheric chemistry, absorb infrared radiation, and have • • • ff reactive halogen species, RHS (e.g., Br , BrO , and IO ). RHS adverse e ects in air quality and public health. The major ∼ deplete the level of O3(g)( 50 ppbv) during early spring in the source of ozone in the lower atmosphere is the oxidation of 5,21−23 Artic troposphere. The production of I2(g) and HOI(g) organic compounds and carbon monoxide in the presence of − nitrogen oxides occurring over the continents, followed by a from submicromolar levels of aqueous I reacting with O3(g) in ≈ 24 minor contribution from stratospheric influx.2,3 Ozone is the top micrometer surface layer of the ocean (pHocean 8) functions as a source of IO• radicals independent of the bulk transported from continental regions to the marine boundary 23 layer (MBL), where major ozone loss occurs, primarily due to pH. The process of sea spray formation generates negatively photolysis in the presence of water. However, the ozone loss 25 calculated with all known losses, excluding halogens, accounts charged droplets that contain dissolved halides salts and 4 establishes a negative current into the atmospheric boundary for only about 50% of the observed loss. 26,27 Similarly to the stratospheric destruction of ozone by layer. We have previously shown how the fractionation of − 28,29 chlorine and bromine, halogen species emitted from surface halides species occurring at the air water interface during waters, can contribute to the global decrease of ozone levels.4 aerosol formation depends mainly on the concentration of The destruction of tropospheric O (g)5,6 occurs at air−water 3 − and air-ice interfaces, mediated by heterogeneous reactions7 10 Received: April 19, 2013 − catalyzed by organic species.11 13 The activation of halogens, Revised: August 26, 2013 such as Br2 and I2, to participate in the photochemical depletion Accepted: August 29, 2013 of tropospheric ozone remains poorly understood.14 It is well- Published: August 29, 2013 © 2013 American Chemical Society 10971 dx.doi.org/10.1021/es401700h | Environ. Sci. Technol. 2013, 47, 10971−10979 Environmental Science & Technology Article species present in surface waters that is strongly biased by the min−1) produced in a spark discharge ozone generator (Ozone 18 size of the ions. However, the remaining question to be Solutions) fed with O2(g). The ultrahigh purity gases used were answered is: How do those aerosolized inert halide salts are from Scott-Gross. UV absorption spectrophotometry allows the 14,30 fi 37 later oxidized to become RHS in the air? Thus, it is crucial quanti cation of O3(g) diluted in N2(g) in a 10-cm path to identify the mechanisms of production of halogen oxides in length cuvette (Supporting Information), which is transported − the tropical MBL.1,31 34 Electrospray ionization mass spec- to a stainless steel tube, positioned 32 mm far from the trometer (ESI-MS) was recently used at high temperature (350 entrance cone and 12 mm from the needle tip carrying the ° fi C) to propose that interfacial reactivity patterns exist in solution. Furthermore, a nal 61-times dilution with the N2(g) microdroplets.35,36 However, experiments at lower temperature nebulizing gas (12.0 L min−1) occurs in the chamber. are needed together with a detailed analysis of the direct Therefore, the experiments yield fundamental mechanistic reaction products and intermediates for the reaction of iodide information covering the low [O (g)] found in the troposphere. 3 − at variable ozone concentration to model the loss of ozone. In The encounter of O3(g) with the microdroplets containing I this article, we investigate a widespread effect of halides produces oxidized species. The overall time from the formation reactions with ozone to produce I2 and reactive species at the of droplets, transport through the O3(g) plume, and ion − ° air water interface at 25 C to simulate the nonphotochemical detection is <1 ms. However, the contact time between O3(g) processes occurring in the MBL. We report the enhanced and the plume of microdroplets is τ ≈ 1 μs (Supporting dissolution of O3 in aerosolized microdroplets with increasing Information). In summary, the experimental conditions were: sodium halide (X− =Br−,I−) concentration (10 nM−100 μM). Drying gas temperature, 25 °C; nebulizer voltage, −2.5 kV; In addition, we study the cycle of ozone destruction on cone voltage, −70 V; and nebulizer pressure, 70 psi. The microdroplets by freshly produced iodine species, such as HIO, solvent background was subtracted from the mass spectrum of ff fi ≥ HIO2, and HIO3, the e ect of polysorbate 20 surfactant to the sample acquired at xed time
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