Environmental Engineering Reference
In-Depth Information
CDOM absorption spectrum decays exponentially from the UV to the visible regime
(Figure 1).
12
1
10
8
6
4
2
3
2
0
300
350
400
450
500
550
600
Wavelength (nm)
Figure 1.
CDOM absorption spectra along the middle Atlantic Bight (MAB) during September 1998:
1. Delaware River at 39q41.33'N and 75q31.11'W (); 2. Delaware Bay mouth at 38q48.86'N
and 75q4.91'W (ยทยทยท); 3. Gulf Stream Western Edge at 37q6.95'N and 72q55.42'W ().
An exponential decay function has therefore been used to fit the decay of CDOM
absorbance with wavelength:
S
(
O
O
)
aa
(1)
where
a(
O
)
and
a(
O
0
)
are the absorption coefficients at the wavelength O and at the
reference O
0
11-15
. The absorption coefficient is obtained as follows:
L
(
O
)
(
O
)
e
0
0
(2)
where
A(
O
)
is the absorbance at the wavelength O and
L
is the cell path length in meters.
The spectral slope,
S
, indicates the rate at which the CDOM absorption coefficient
decreases with increasing wavelength and it has been used to discriminate among
CDOM of different nature.
S
increases with decreasing CDOM absorption coefficient,
aromatic content and molecular weight
16
. Moreover,
S
also increases from coastal and
fresh waters (~0.014 nm
-1
) to offshore locales (>0.02 nm
-1
)
13-15,17-19
. This trend may be
attributed to the replacement of terrestrial CDOM with marine CDOM or to
transformation of the terrestrial CDOM as it moves along the shelf
20
.
Since 1949, it is known that seawater fluoresces blue when hit by UV light
21
.
Because fluorescence spectroscopy is a more sensitive and easier tool to employ than
absorption spectroscopy, it is often preferred when examining the distribution of CDOM
in natural waters. Unfortunately, fluorescence is only representative of a portion of the
total chromophore pool and not of the whole CDOM. As true for absorption, CDOM
fluorescence spectra are broad and unstructured; the emission intensity decreases and its
maximum is red shifted with increasing excitation wavelength, indicative of the presence
of numerous emitting species or of interactions of a few chromophores that produce long
wavelength emission. For this reason, excitation/emission matrix spectra (EEMS) have
become a useful tool for studying this complex material, allowing for different classes
of fluorophores to be detected
22-25
. EEMS are obtained by collecting a series of emission
spectra at successive excitation wavelengths, and then merging them into three-
a
(
O
)
2
.
303
u
A
(
O
)
/
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