DOM Background-Absorption Spectroscopy
Colin A. Stedmon (taken from Ph.D. thesis introduction chapter (2004)
The absorption of light by organic compounds is dependent on their chemical nature. The presence of different functional groups and different atomic bonds will lead to the absorption of radiation in different regions of the electromagnetic spectrum. Absorption spectroscopy is often used in chemistry and biology to measure the concentration of known substances with documented absorption properties (e.g. chlorophyll concentrations in algal extracts). As DOM in natural waters consists of a complex mixture of compounds, its absorption spectrum represents the sum of the different overlapping absorption peaks. An example of absorption spectra of CDOM taken from a range of environments is shown in Figure 4.
Figure 4. Light absorption by the coloured fraction of DOM: Example of CDOM absorption spectra from different localities. Note the spectrum from peatlands in Finland has been divided by 10.
Modelling CDOMs absorption spectra
CDOM absorbs light strongly in the UV and blue region (<450 nm.) and its absorption decreases with increasing wavelength (Figure 4). When present in high concentrations, CDOM therefore gives water a yellow-brown colour. The shape of the absorption curve is often characterised by this exponential equation,
alambda=a0 exp (S(400-lambda)) eq. 1
Where alambda is the absorption at wavelength lambda and S is the spectral slope coefficient, describing the gradient of the exponential curve (Jerlov, 1968; Bricaud et al., 1981). The absorption at a chosen wavelength is used as a quantitative measure of CDOM (e.g. a375). As a large majority of the compounds present are still not identified, we are unable to report the presence of distinct organic compounds via absorption spectroscopy. However by quantifying the characteristics of the absorption curve in the UV and visible regions of the spectrum, we are able to trace relative changes in the composition and size of the coloured fraction of the DOM pool. As DOM originating from differing sources (e.g. terrestrial run-off from different catchments and product from marine algae) differs in chemical composition, the absorption properties are also seen to vary. Studies have shown that relations can be drawn between the physical properties of DOM and its optical behaviour. Increases in the spectral slope values (S) can be associated with decreases in the aromatic character due to the fact that extended aromatic systems tend to absorb at larger wavelengths. It has also been suggested that absorption in the 500-600 nm region can be due to intramolecular interactions between organic molecules in close proximity to each other (Blough & Del Vecchio, 2000 and references therein).
The majority of the CDOM pool is thought to be comprised of two fractions, fulvic acids (soluble at all pH’s) and humic acids (insoluble at low pH), which are collectively called humic substances. Isolation and measurement of their characteristics has shown that there are some noticeable differences. The two fractions differ in aromaticity, with fulvic acids being less aromatic than humic acids (Harvey et al 1983). Carder et al. (1989) found that marine fulvic and humic acids isolates also had differing S values. Marine fulvic acids had S values that were twice as large as that of humic acids and that S of fulvic acids decreased with increasing molecular weight. So a degree of the variations in S values observed in the natural environment can be explained by varying relative proportions of humic and fulvic fractions.
Specific absorption
The specific absorption coefficient (a*lambda) is calculated by normalising the absorption to the dissolved organic carbon (DOC) concentration and is also used as a descriptor of CDOM’s quality. a*lambda is correlated to the average molecular weight of the chromophores present in CDOM (Chin et al., 1992). Exposure to degradation processes (e.g. photodegradation) cause it to decrease (bleaching), mirroring an overall decrease in average molecular weight.
This text is taken from my Ph.D. thesis introduction chapter (2004).