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Proc Natl Acad Sci U S A

Title:		Halogen radicals contribute to photooxidation in coastal and estuarine waters
Author(s):		Parker KM; Mitch WA;
Address:		"Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305. Department of Civil and Environmental Engineering, Stanford University, Stanford, CA 94305 wamitch@stanford.edu"
Journal Title:		Proc Natl Acad Sci U S A
Year:		2016
Volume:		20160509
Issue:		21
Page Number:		5868 - 5873
DOI:		10.1073/pnas.1602595113
ISSN/ISBN:		1091-6490 (Electronic) 0027-8424 (Print) 0027-8424 (Linking)
Abstract:		"Although halogen radicals are recognized to form as products of hydroxyl radical ((*)OH) scavenging by halides, their contribution to the phototransformation of marine organic compounds has received little attention. We demonstrate that, relative to freshwater conditions, seawater halides can increase photodegradation rates of domoic acid, a marine algal toxin, and dimethyl sulfide, a volatile precursor to cloud condensation nuclei, up to fivefold. Using synthetic seawater solutions, we show that the increased photodegradation is specific to dissolved organic matter (DOM) and halides, rather than other seawater salt constituents (e.g., carbonates) or photoactive species (e.g., iron and nitrate). Experiments in synthetic and natural coastal and estuarine water samples demonstrate that the halide-specific increase in photodegradation could be attributed to photochemically generated halogen radicals rather than other photoproduced reactive intermediates [e.g., excited-state triplet DOM ((3)DOM*), reactive oxygen species]. Computational kinetic modeling indicates that seawater halogen radical concentrations are two to three orders of magnitude greater than freshwater (*)OH concentrations and sufficient to account for the observed halide-specific increase in photodegradation. Dark (*)OH generation by gamma radiolysis demonstrates that halogen radical production via (*)OH scavenging by halides is insufficient to explain the observed effect. Using sensitizer models for DOM chromophores, we show that halogen radicals are formed predominantly by direct oxidation of Cl(-) and Br(-) by (3)DOM*, an (*)OH-independent pathway. Our results indicate that halogen radicals significantly contribute to the phototransformation of algal products in coastal or estuarine surface waters"
Keywords:		Carbonates/chemistry *Estuaries Fresh Water/chemistry/microbiology Halogens/*chemistry Harmful Algal Bloom/radiation effects Hydroxyl Radical/*chemistry Iron/chemistry Kainic Acid/*analogs & derivatives/chemistry Kinetics Light Marine Toxins/*chemistry Ni;
Notes:		"MedlineParker, Kimberly M Mitch, William A eng Research Support, Non-U.S. Gov't Research Support, U.S. Gov't, Non-P.H.S. 2016/05/11 Proc Natl Acad Sci U S A. 2016 May 24; 113(21):5868-73. doi: 10.1073/pnas.1602595113. Epub 2016 May 9"