Environmental Engineering Reference
In-Depth Information
well as of photosynthetically active radiation (PAR, 400-700 nm). CDOM absorb-
ance in aqueous solution is directly proportional to the path length (
l
) and the
concentration of the absorbing CDOM species (
c
). The
Beer's Law
states that
A
=
εlc
, where
ε
is a constant of proportionality called the molar absorption coef-
ficient. The attenuation of radiation in solution increases with increasing CDOM
absorbance.
As far as PAR radiation is concerned, CDOM in natural waters absorbs maximally
in the blue region of the spectrum and its absorption decreases exponentially with
increasing wavelength. CDOM spectra have been fitted to an exponential function by
Bricaud and his co-workers (Bricaud et al.
1981
) as described below (Eq.
2.1
):
A
CDOM
(λ) =
A
CDOM
(λ
O
)
E
−
S
(λ−λ
O
)
(2.1)
where
a
CDOM
(
λ
) and
a
CDOM
(
λ
o
) are the absorption coefficients at a specific wave-
length (
λ
) and at a reference wavelength (
λ
o
), respectively.
S
is the slope of the
absorption spectrum, determined using either a nonlinear least-squares fitting
routine over the wavelength range 290-700 nm or a linear least-squares fit of the
log-transformed absorption data, over the range from 290 nm to the instrument
detection limit (Vodacek et al.
1997
). It is shown that the
S
value is significantly
affected by the presence of different organic substances. Humic acids have a
lower
S
value than fulvic acids (Zepp and Schlotzhauer
1981
; Carder et al.
1989
;
Markager and Vincent
2000
. Jerlov (
1968
) suggested an
S
value of 15
μ
m
−
1
whilst Bricaud et al. (Bricaud et al.
1981
) showed
S
to vary between 10 and 20
μ
m
−
1
with a mean value of 14
μ
m
−
1
. The absorption coefficient is determined
using the following equation
A
CDOM
(λ) =
2. 303
×
A
CDOM
(λ)/
L
(2.2)
where
A
CDOM
(
λ
) is the absorbance (log
I
o
/
I
), and
l
is the path length (in meters).
The spectral slope of the absorption spectrum is widely used in remote sensing in
coastal and marine environments (Vodacek et al.
1995
; Hoge et al.
1995
).
2.4 Absorption Coefficients of CDOM and Particulate Matter
in Natural Waters
CDOM of both autochthonous and allochthonous origin generally exhibits UV
and visible absorption spectra with low absorbance at longer wavelengths, and
the absorbance increases with decreasing wavelength from 700 to 200 nm (Fig.
1
)
(Rochelle-Newall and Fisher
2002
; Wada et al.
2007
; Helms et al.
2008
; Hulatt et
al.
2009
; Hayase and Tsubota
1985
; Davies-Colley and Vant
1987
; Zhang et al.
2009
; Du et al.
2010
; Odriozola et al.
2007
; Bricaud et al.
2010
). Allochthonous
fulvic acids generally exhibit monotonous spectra whilst allochthonous humic
acids have a shoulder around 400 nm in aqueous media (Fig.
3
) (Ishiwatari
1973
; Lawrence
1980
; Zepp and Schlotzhauer
1981
; Hayase and Tsubota
1985
).
Dissolved mycosporine based amino acids (MAAs), released by dinoflagellates,
