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A Multicomponent Model of Chromophoric Dissolved Organic Matter Photobleaching¶§
Photochemistry and Photobiology, Jul/Aug 2004 by Goldstone, Jared V, Vecchio, Rossana Del, Blough, Neil V, Voelker, Bettina M
ABSTRACT
Light absorption by chromophoric dissolved organic matter (CDOM) plays a number of roles in natural waters, including both control of the underwater light field and the initiation of many photochemical reactions. A multicomponent analysis was used to describe the effects of UV and visible radiation on the optical absorption spectra of two natural water samples, a Suwannee River fulvic acid standard (SRFA) and a Delaware Bay water sample. This analysis used a constrained minimization technique to fit independent spectral components to the observed bleaching behavior of the water samples under monochromatic irradiation. Spectra derived from these fits were used to predict the bleaching behavior of both samples under polychromatic irradiation ([lambda] > 320 nm). This approach reproduces the kinetics and spectral behavior of polychromatic photobleaching very well at times
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Abbreviations: CDOM, chromophoric dissolved organic matter; SRFA, Suwannee River fulvic acid.
© 2004 American Society for Photobiology 0031-8655/04 $5.00+0.00
INTRODUCTION
Light absorption by the chromophore-containing constituents of dissolved organic matter plays a number of roles in natural waters. Absorption of ultraviolet wavelengths has important ecological consequences (1,2), and the transformation of UV and visible radiation to heat can influence the thermal structure of aquatic systems (3). In addition, light absorption by chromophoric dissolved organic matter (CDOM) in surface waters has a significant effect on the remote sensing of ocean color (3-8). Another important process is the formation of photoproducts, such as dissolved inorganic carbon (9), carbon monoxide (10-13), low-molecular weight carbonyl compounds (14-16) and various reactive oxygen species (17,18). Light absorption also alters the bioavailability of CDOM (1,19-25). The various photochemical reactions initiated by light absorption eventually lead to the destruction of the chromophores and thus to the loss of CDOM light absorption (photobleaching) (26-30).
One method of investigating constituents contributing to the absorption spectra of CDOM is to model the bleaching behavior of CDOM under both monochromatic and polychromatic irradiation. This technique has been used in studies of photochemical and photobiological systems to examine the wavelength dependence of different phenomena (31). A system in which the effects of different monochromatic wavelengths can be easily summed to produce the effects under polychromatic irradiation is a linear system. Both CO and CO2 photoproduction from CDOM appear to be describable as linear systems (12,31-33).
Bleaching spectra of marine CDOM have been commonly described at various observation wavelengths by biexponential decay equations in which each different observation wavelength must be described by a different set of exponential decay functions and amplitude factors (28-30). This approach is purely observational, and no previous attempt has been made to approach a thorough theoretical understanding of photobleaching. A possible solution is the description of CDOM absorption spectra as a superposition of multiple chromophores. A significant issue that must be addressed within this theoretical description of CDOM spectra is the bleaching of absorption at wavelengths far from the irradiation wavelengths ("off axis") observed during photobleaching experiments (34).
The goals of this study are two-fold. First, we describe a chemically reasonable model of CDOM photobleaching under monochromatic irradiations on the basis of the linear superposition of independent chromophores. Second, we examine the feasibility of applying this linear model to the photobleaching of CDOM observed under polychromatic irradiations. We intended this to be a general, practical model of CDOM photobleaching that might be applicable in certain systems. However, we have found that polychromatic irradiations of CDOM cannot be adequately reproduced with the same model, suggesting that CDOM photo-bleaching is not describable as a linear system.
MATERIALS AND METHODS
Theory
An approach to modeling photobleaching rates is possible if CDOM behaves as a mixture of independent chromophores bleaching to products that do not absorb light in the spectral range being observed. This model may hold true even if the different components are chemically bonded to each other, as long as they absorb light independently and the photobleaching of one chromophore does not affect the photobleaching of others. The important assumption that there is no production of intermediate products with different bleaching rates cannot be tested readily with the model presented here; it is an issue that should be addressed in future studies.
The variable a^sub T^([lambda]^sub obs^,t) is thus the experimental observations of bleaching of total absorption as a function of irradiation time. The goal of the modeling, then, is to find the component spectra a^sub i^([lambda]^sub obs^,0) and rate coefficients k^sub i^ which best fit the experimental data. Note that Eq. 6 implies that the bleaching rate coefficient of the ith component chromophore, k^sub i^([lambda]^sub j^), must be independent of observation wavelength [lambda]^sub obs^ and directly proportional to the chromophore's molar extinction coefficient at the irradiation wavelength, [epsilon]^sub i^([lambda]^sub i^), the quantum yield of photobleaching, [phi]^sub i^([lambda]^sub j^), and the irradiance at wavelength [lambda]^sub j^, I^sub 0^([lambda]^sub j^) (see Eq. 2).
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