Impact of halides on the photobleaching of dissolved organic matter

Understanding absorbance photobleaching of marine dissolved organic matter (DOM) is important because DOM chromophores impact oceanic primary productivity by affecting the depth of the photic zone, absorb UV radiation and affect ocean color used in remote sensing. However, the fundamental mechanisms...

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Bibliographic Details
Published in:Marine chemistry 2009-06, Vol.115 (1), p.134-144
Main Authors: Grebel, Janel E., Pignatello, Joseph J., Song, Weihua, Cooper, William J., Mitch, William A.
Format: Article
Language:English
Subjects:
Dissolved organic matter
Earth sciences
Earth, ocean, space
Exact sciences and technology
External geophysics
Geochemistry
Halides
Hydroxyl radical
Marine
Mineralogy
Photobleaching
Physical and chemical properties of sea water
Physics of the oceans
Reactive halogen species
Silicates
Water geochemistry
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Summary:Understanding absorbance photobleaching of marine dissolved organic matter (DOM) is important because DOM chromophores impact oceanic primary productivity by affecting the depth of the photic zone, absorb UV radiation and affect ocean color used in remote sensing. However, the fundamental mechanisms which account for this bleaching are largely unknown. Controlled laboratory studies demonstrated that the presence of seawater concentrations of chloride and bromide ions enhanced absorbance photobleaching reaction rates by ~ 40%, regardless of DOM source or the presence or absence of carbonate ions. In contrast, halide ions generally did not affect fluorescence bleaching rates. Variations in ionic strength did not alter the enhancement in absorbance photobleaching by halide ions. Accordingly, the enhancement in absorbance photobleaching was specific to halide ions, rather than a generalized salinity effect. We confirmed the formation of hydroxyl radical (HO •) in illuminated samples, and its significant scavenging in the presence of halide salts. Gamma-radiolysis experiments and associated modeling indicated that a small component (~ 12%) of the photobleaching enhancement by halides was consistent with the hypothesis that halide scavenging of HO • will form reactive halogen radicals that target electron-rich chromophores within DOM more selectively than HO •. The mechanism responsible for the major component of absorbance photobleaching rate enhancement by halides remains unresolved.
ISSN:0304-4203
1872-7581
DOI:10.1016/j.marchem.2009.07.009