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Ocean Optics Web Book

Absorption

Definitions

Page updated: Jul 27, 2013
Principal author: Collin Roesler
 

Introduction

When light penetrates the ocean, photons are either absorbed or scattered. While scattering redirects the angle of the photon path, absorption removes the photons permanently from the path. Unlike scattering which is weakly spectrally dependent, absorption is highly spectrally dependent. The magnitude and spectral features depend upon the concentration and composition of the particulate and dissolved constituents and water itself. The inherent optical properties (IOPs) are conservative properties and therefore the magnitude of the absorption coefficient varies linearly with the concentration of the absorbing material. Theoretically, the absorption coefficient can be expressed as the sum of the absorption coefficients of each component:

$\displaystyle a(\lambda) ~=~ \Sigma_{i~=~1,N} a_{i}(\lambda) \\ $ (1)

Where N is the total number of components and a_i indicates the i^th component, and all absorption is spectral, indicated by (&lambda#lambda;).

Practically, however, it is not possible to measure the absorption properties of each individual absorbing component and thus the individual components can be grouped into similarly absorbing constituents based upon similarity in their optical properties and/or analytically-based groupings:

$\displaystyle a(\lambda) ~=~ a_{w}(\lambda) ~+~ a_{phyt}(\lambda ~+~ a_{NAP}(\lambda ~+~ a_{CDOM}(\lambda \\ $ (2)

where subscriptions w, phyt, NAP and CDOM indicate water, phytoplankton, non-algal particles, and colored dissolved organic matter, respectively.

Generic Component Absorption Spectra

Absorption by water (Fig. 1) is weak in the blue and strong in the red and varies with temperature and salinity.

Figure: Fig. 1. Absorption spectrum for pure water (data from Pope and Frye 1992).
Image 411b037b4cbaac9f963ce1fb89458908

Absorption by particles is separated into phytoplankton and non-algal particles (NAP) using spectrophotometry and an extractive technique. Phytoplankton absorption (Fig. 2) demonstrates the most spectral variations of any of the components due to the individual pigment absorption spectra but in general exhibits peaks in the blue and red regions of the spectrum due to the ubiquitous presence of chlorophyll a.

Figure: Fig. 2. Generic phytoplankton absorption spectrum for mixed algal composition (modified from Roesler et al. 1989).
Image fc8b2ab700d3074ede11f5e58e6f0ea4

Non-algal particle absorption (Fig. 3) is strongest in the blue, decreasing approximately exponentially to the red. This component, operationally-defined, includes living zooplankton and bacteria, as well as the non-pigmented parts of phytoplankton (cell walls, membranes etc), detrital material as well as inorganic particles.

Figure: Fig. 3. Generic non-algal particle absorption spectrum for mixed composition.
Image ec0764caaf98804a767567afd5e8a989

Colored dissolved organic matter (CDOM) absorption (Fig. 4) is very similar to that of NAP due in part to the similarity in composition (organic material), but generally exhibits a steeper exponential slope. This material is operationally separated from NAP by filtration; CDOM is measured on the filtrate passing through 0.2mm or 0.7 mm nominal pore sized filters.

Figure: Fig. 4. Generic colored dissolved organic matter (CDOM) absorption spectrum for mixed composition.
Image c52b6a2a18f037bab7b3536d045be04f

For oligotropic environments with very low concentrations of suspended and dissolved material, the absorption coefficient is dominated by water (Figure 5A) and the wavelength of minimum absorption is in the blue, hence the blue color of the seawater. For eutrophic and/or coastal environments with high concentrations of suspended and dissolved material (Figure 5B), the absorption coefficient is dominated by that material and the wavelength of minimal absorption shifts to the green, lending green color to that environment.

Figure: Fig. 5. Component and total absorption spectra for A. clear open ocean waters where water dominates the absorption and B. eutrophic coastal waters where particulate and dissolved organic matter dominate the blue and green portions of the spectrum.
Image 93659370015107cb31453cd9ed77a92f