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Real-Time Studies of Iron Oxalate-Mediated Oxidation of Glycolaldehyde As a Model for Photochemical Aging of Aqueous Tropospheric Aerosols Daniel A

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Real-Time Studies of Iron Oxalate-Mediated Oxidation of Glycolaldehyde As a Model for Photochemical Aging of Aqueous Tropospheric Aerosols Daniel A Subscriber access provided by Caltech Library Services Article Real-Time Studies of Iron Oxalate-Mediated Oxidation of Glycolaldehyde as a Model for Photochemical Aging of Aqueous Tropospheric Aerosols Daniel A. Thomas, Matthew M. Coggon, Hanna Lignell, Katherine Ann Schilling, Xuan Zhang, Rebecca H. Schwantes, Richard C. Flagan, John H. Seinfeld, and Jesse Lee Beauchamp Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03588 • Publication Date (Web): 12 Oct 2016 Downloaded from http://pubs.acs.org on October 20, 2016 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 29 Environmental Science & Technology 1 Real-Time Studies of Iron Oxalate-Mediated 2 Oxidation of Glycolaldehyde as a Model for 3 Photochemical Aging of Aqueous Tropospheric 4 Aerosols 5 Daniel A. Thomas †,⊥, Matthew M. Coggon ‡,⊥,¶, Hanna Lignell ‡,§, Katherine A. Schilling ‡,∥, Xuan 6 Zhang §, Rebecca H. Schwantes §, Richard C. Flagan ‡,§, John H. Seinfeld ‡,§, and J. L. 7 Beauchamp †,* 8 †Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, 9 Pasadena, CA, 91125, USA 10 ‡Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, 11 CA, 91125, USA 12 § Environmental Science and Engineering, California Institute of Technology, Pasadena, CA, 13 91125, USA 14 Present Addresses 15 ¶ Cooperative Institute for Research in Environmental Sciences, Boulder, CO, 80309, USA 16 ∥ U. S. Army Criminal Investigation Laboratory, Fort Gillem, GA, 30297, USA 17 *To whom correspondence should be addressed: 18 Phone: 626-395-6525 19 Fax: 626-395-4912 ACS Paragon Plus Environment 1 Environmental Science & Technology Page 2 of 29 20 Email: [email protected] 21 Key Words: Fenton, Oxalate, Iron, Aerosol, Glycolaldehyde, Oxidation, Oxalic, Aqueous 22 Abstract 23 The complexation of iron (III) with oxalic acid in aqueous solution yields a strongly absorbing 24 chromophore that undergoes efficient photodissociation to give iron (II) and the carbon dioxide 25 anion radical. Importantly, iron (III) oxalate complexes absorb near-UV radiation (λ > 350 nm), 26 providing a potentially powerful source of oxidants in aqueous tropospheric chemistry. Although 27 this photochemical system has been studied extensively, the mechanistic details associated with 28 its role in the oxidation of dissolved organic matter within aqueous aerosol remain largely 29 unknown. This study utilizes glycolaldehyde as a model organic species to examine the oxidation 30 pathways and evolution of organic aerosol initiated by the photodissociation of aqueous iron (III) 31 oxalate complexes. Hanging droplets (radius 1 mm) containing iron (III), oxalic acid, 32 glycolaldehyde, and ammonium sulfate (pH ~ 3) are exposed to irradiation at 365 nm and 33 sampled at discrete time points utilizing field-induced droplet ionization mass spectrometry 34 (FIDI-MS). Glycolaldehyde is found to undergo rapid oxidation to form glyoxal, glycolic acid, 35 and glyoxylic acid, but the formation of high molecular weight oligomers is not observed. For 36 comparison, particle-phase experiments conducted in a laboratory chamber explore the reactive 37 uptake of gas-phase glycolaldehyde onto aqueous seed aerosol containing iron and oxalic acid. 38 The presence of iron oxalate in seed aerosol is found to inhibit aerosol growth. These results 39 suggest that photodissociation of iron (III) oxalate can lead to the formation of volatile oxidation 40 products in tropospheric aqueous aerosols. 41 42 ACS Paragon Plus Environment 2 Page 3 of 29 Environmental Science & Technology 43 44 45 Introduction 46 Tropospheric aqueous-phase chemistry plays a key role in the aging of dissolved organics in 47 the atmosphere. 1-5 Aqueous-phase processing may occur as a result of “dark” reactions such as 48 acid catalysis, hydration, and oligomerization 6, 7 or may result from photochemical excitation of 49 dissolved light-absorbing species. 8-10 One of the key components directing the aging of dissolved 50 organics is the availability of oxidative species such as the hydroxyl radical (OH) and hydrogen 51 peroxide (H 2O2), which may be present by direct generation in solution or by uptake from the 52 gas phase. 11 Many laboratory studies have investigated secondary organic aerosol (SOA) 12-15 53 production initiated by photolysis of dissolved H 2O2. There is increasing evidence, however, 54 that processes involving the photolysis of photoactive organic and organometallic compounds 55 may also be important sources of highly reactive aqueous oxidants. 9, 16-18 56 Transition metal ions present in cloudwater or aqueous aerosol are known to undergo photo- 57 initiated electron transfer processes that can be a significant source of oxidative species. 1, 19, 20 58 Iron, the most abundant transition metal ion in tropospheric particles, can be found in aerosol 59 particles originating from sea spray, 21 mineral dust, 20 and anthroprogenic emissions. 22 Measured 60 concentrations in fog waters range from micromolar or less in rural areas to tens of micromolar 61 in highly polluted environments. 23 As a result of these significant concentrations, iron can play 62 an important role in aqueous-phase tropospheric oxidation of organics. The reactions of 63 dissolved iron-hydroxy complexes, or the Fenton reactions, are a well-characterized source of 64 atmospheric oxidants. 24-26 The direct reaction of hydrogen peroxide with iron (III) yields ACS Paragon Plus Environment 3 Environmental Science & Technology Page 4 of 29 65 hydroxyl radicals in the dark Fenton reaction (1), whereas the photo-Fenton reaction yields 66 hydroxyl radicals by photolysis of the iron (III) hydroxy complex (2-3). ͦͮ ͧͮ • ͯ 67 Fe + HͦOͦ → Fe + OH + OH (1) ͧͮ ͦͮ ͮ 68 Fe + 2H ͦO ↔ Fe ʚOH ʛ + HͧO (2) Ζύ 69 Fe ʚOH ʛͦͮ → Fe ͧͮ + OH • (3) 70 Several groups have recently explored the impact of iron photochemistry on SOA formation. 71 Chu and co-workers studied the effects of iron (II) sulfate and iron (III) sulfate seed aerosol on 72 SOA formation at 50% relative humidity (RH) and found that iron (II) complexes inhibited SOA 73 formation, whereas iron (III) had little effect on SOA formation. 27 They attributed the effect of 74 iron (II) to the reduction of organic condensate by the iron species and subsequent disruption of 75 oligomerization processes. Nguyen and co-workers investigated the uptake and oxidation of 76 glycolaldehyde in aqueous aerosol containing hydrogen peroxide and iron complexes. 26 They 77 found that the photo-Fenton reaction significantly increased the degree of oxidation in aerosol 78 particles when compared with H 2O2 photolysis alone (ratio of O/C=0.9 with iron and H2O2 vs. 79 O/C=0.5 with H 2O2). 80 Although laboratory experiments have demonstrated that photo-Fenton chemistry can lead to 81 significant oxidation of organics, 28-30 the role of such processes in atmospheric aerosol is not 82 clear, as the reactions are highly dependent on pH, iron concentration, and the concentration of 83 other ligands that readily complex with iron. 25, 31, 32 Notably, the formation of stable complexes 84 between iron and dicarboxylate ligands can have a significant impact on the photochemical 85 reaction pathways. Complexation between the oxalate anion, one of the most abundant low- 86 molecular weight organic compounds found in aqueous aerosol, 22, 33-35 and iron has long been 87 known to generate significant yields of oxidative species by photochemical reduction of iron. 88 Initially studied as a chemical actinometer, 36, 37 seminal work by Zuo and Hoigné demonstrated ACS Paragon Plus Environment 4 Page 5 of 29 Environmental Science & Technology 89 that photolysis of iron-oxalate complexes could lead to the generation of oxidative species under 90 atmospherically relevant conditions. 25 91 The proposed pathway for the photochemical generation of oxidative species from iron (III) 92 oxalate complexes is shown in reactions 4-10 below. Depending on the pH, ionic strength, and + 93 concentration of iron and oxalate, three complexes are formed: Fe(C 2O4) , which has low 38 − 3− 94 photochemical reactivity, and Fe(C 2O4)2 and Fe(C 2O4)3 , both of which undergo 95 photochemical reduction of iron with high quantum yield. 31, 39 The mechanism of photochemical 3− 96 dissociation of Fe(C 2O4)3 has been investigated in detail, with two groups presenting evidence 97 for either intermolecular electron transfer (4a) or intramolecular electron transfer (4b) as the 40-43 − 98 primary reaction pathway.
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