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Optical Constituents of the Ocean

Colored Dissolved Organic Matter

Page updated: Mar 15, 2010
Principal author: Emmanuel Boss
 
Colored or Chromophoric Dissolved Organic Matter

(CDOM) is an important optical constituent in water often dominating absorption in the blue. It is based on the absorption or fluorescence by material passing through a given filter (most often with pore size of $ 0.2\mu m$). As such, it is an absorption (or fluorescence)-weighted sum of the different dissolved materials in the water. Note that most of the material comprising DOM does not absorb or fluoresce and that there exists inorganic dissolved materials that also absorb (e.g. iron oxides, nitrate) though it is believed that fluorescence is due solely to organic materials. From this discussion it follows CDOM is thus not necessarily a good proxy for DOM, particularly in the open ocean. Nevertheless CDOM has been found to be a useful tracer of water masses, as well as indicator of different biogeochemical processes (see Coble (2007), for a recent review article on the link between the optical and chemical properties of DOM). Sample preparation and methodology of measurement are important to obtain accurate CDOM measurement. See Nelson and Coble (2009), for a recent analysis of methodology.

CDOM absorption:

CDOM spectrum is the visible is most often described by an exponentially decreasing function (e.g. Jerlov (1966):

$\displaystyle a_g(\lambda)= a_g(\lambda_0)exp^{-s(\lambda-\lambda_0)} \; [m^{-1}].$ (1)

where $ s$ is referred to as the spectral slope and $ \lambda_0$ a reference wavelength. A theoretical explanation for this shape has been hypothesized by Shifrin (1988) as arising from a superposition of resonances of different molecular $ \pi$-bonds in the long organic molecules comprising CDOM. Single bonds, which are most abundant, will absorb short wavelength radiation while resonance of multiple bond, less abundant, absorb longer wavelength radiation. Since numerically there many more short bonds, the spectra is higher at short wavelengths. This explanation is consistent with the observation that small values of the spectral slope of CDOM, $ s$, are associated with higher molecular weight materials (e.g. Carder et al. (1989), Yacobi et al. (2003)). For visible wavelength the most common values of $ s$ appear to be near 0.014 $ nm^{-1}$, varying in the visible from 0.007 to 0.026 $ nm^{-1}$ (e.g. Table 1 in Twardowski et al. (2004).

While this is the most frequent model of CDOM absorption, other models have been suggested that may provide better fit to data (even when taking into account that fits improve as more free parameters are available in the fit, e.g. Twardowski et al. (2004). In particular, often a constant is added to the exponential fit:

$\displaystyle a_g(\lambda)= a_g(\lambda_0)exp^{-s(\lambda-\lambda_0)} + Const. \; [m^{-1}].$ (2)

What this constant represent is not clear. In some cases it is supposed to account for scattering by the dissolved component, however there is no reason to believe such scattering would be spectrally flat (see Bricaud et al. (1981) for in-depth discussion). It may account for bubbles in the sample.

Another model that has been found to work even better than the exponential model is the power-law model (e.g. Twardowski et al. (2004).

$\displaystyle a_g(\lambda)= a_g(\lambda_0) \left( \frac{\lambda}{\lambda_0} \right)^{-s} \; [m^{-1}].$ (3)

Given that molecular absorption is often symmetric function for a given chromophore, frequency domain fit has been suggested (e.g. Shwarz et al. (2002)). Those are based on Gaussian or Lorentzian functions with the visible domain being the tail-end of the distribution. Trying to fit together UV and visible bands is complicated by the abosrption of UV light by dissolved salts which are not part of DOC.

Elastic scattering by CDOM CDOM contribution to scattering by seawater is somewhat controversial. By definition colloids are part of DOM and, if abundant enough, could contribute significantly to scattering (particularly to backscattering, see Stramski and Wozniak (2005)) by sea water. However, there is no observational evidence that CDOM contribute significantly to scattering (see Dall'Olmo et al. (2009) for recent measurements). Thus, currently, CDOM contribution to scattering is most often neglected.

Inelastic scattering by CDOM One of the primary methods to quantify CDOM is through fluorescence. Since not all dissolve material that absorbs fluoresces, this material is often denoted as FDOM. In general absorption and fluorescence covary, however their ratio can vary by orders of magnitude between different locations. The fluorescence of CDOM in the field is often limited to a single excitation/emission band pair. With lab instrumentation two-dimensional excitation-emission spectra (EEMS) are measured and used to characterize the FDOM based on the size and presence of known excitation-emission peaks (see, for example, table 1 in Coble (2007)).