Photobleaching of chromophoric dissolved organic matter in natural waters: kinetics and modeling |
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| Authors: | Rossana Del Vecchio Neil V Blough |
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| Institution: | 1. Earth System Science, University of California, Irvine, CA, USA;2. Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USA;3. Ecology, Evolution and Marine Biology, University of California, Santa Barbara, CA, USA;4. Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, MD, USA;5. Rosenstiel School of Marine and Atmospheric Science, University of Miami, FL, USA;1. University of Maryland Center for Environmental Science, PO Box 775, Cambridge, MD 21613, USA;2. Smithsonian Environmental Research Center, PO Box 28, Edgewater, MD 21037, USA;3. The City College of New York, The City University of New York, 160 Convent Ave, New York, NY 10031, USA;4. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai 200241, China;5. NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Code 616.1, Greenbelt, MD 20771, USA;1. Department of Marine Sciences, University of Georgia, Athens, GA 30602, USA;2. Research Group for Marine Geochemistry (ICBM-MPI Bridging Group), Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, 26129 Oldenburg, Germany;3. School of Oceanography, University of Washington, Seattle, WA 98195, USA;4. Romberg Tiburon Center, San Francisco State University, Tiburon, CA 94920, USA;5. Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA;6. Centro de Energia Nuclear na Agricultura, Universidade de São Paulo, Piracicaba, SP 13400–970, Brazil |
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| Abstract: | The effects of monochromatic and polychromatic UV and visible (VIS) radiation on the optical properties (absorption and fluorescence) of chromophoric dissolved organic matter (CDOM) were examined for a Suwannee River fulvic acid (SRFA) standard and for water from the Delaware and Chesapeake Bays. The primary (direct) loss of absorption and fluorescence occurred at the irradiation wavelength(s), with smaller secondary (indirect) losses occurring outside the irradiation wavelength(s). The efficiency of both direct and indirect photobleaching decreased monotonically with increasing wavelength. Exposure to polychromatic light increased the CDOM absorption spectral slope (S), consistent with previous field measurements. An analysis of the monochromatic photobleaching kinetics argues that a model based on a simple superposition of multiple chromophores undergoing independent photobleaching cannot apply; this conclusion further implies that the absorption spectrum of CDOM cannot arise solely from a simple superposition of the spectra of numerous independent chromophores. The kinetics of CDOM absorption loss with the monochromatic irradiation were employed to create a simple, heuristic model of photobleaching. This model allowed us to examine the importance of the indirect photobleaching losses in determining the overall photobleaching rates as well as to model the photobleaching of natural waters under polychromatic light fields. Application of this model to natural waters closely predicted the change in the CDOM spectral shape caused by photodegradation. The time scale of this process was consistent with field observations acquired during the summertime for coastal waters in the Middle Atlantic Bight (MAB). The results indicate that the ratio of the photodegradation depth to the mixed layer depth is a key parameter controlling the rate of the photobleaching in surface waters. |
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| Keywords: | CDOM Photobleaching Kinetics Modeling |
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