Geoscience Reference
In-Depth Information
Because it is not known what the
fl
fluorescent components are, the molar absor-
bance, quantum ef
ciency and concentration cannot be calculated correctly. Hence,
the terms of the PARAFAC model are only proportional to the molar absorbance,
quantum ef
ciency and concentration. We decided to scale the matrices in order to
obtain values that are realistic and comparable to the
first approach we used for
CDOM modeling. Fluorescence quantum yields of CDOM were previously reported
to range from 0.005 up to 0.025 (Green and Blough
1994
). The values obtained by
Hawes (
1992
), used in the
first approach of modeling CDOM
fl
uorescence, vary
within these limits (0.009 to 0.019). In order to obtain
fluorescence yields of the
same order for the PARAFAC components, we scaled matrix c by a factor of 0.001,
after which the
fl
fluorescence yields varied between 0.009 and 0.024.
The absorption of the PARAFAC components is calculated as a product of
matrices b and d. We decided to scale these matrices so that we obtain absorption
similar to the
fl
first modeling approach. In case of humic and fulvic acids, we set
their concentrations to 1 and 10 g m
−
3
, and the sum of 1 g m
−
3
of fulvic and 1 g m
−
3
of humic acids lead to the absorption at 443 nm of 0.086 m
−
1
. To be consistent, we
scaled the product of matrices so that the
final absorption of all components at
443 nm would be similar. In fact, it is only component 1 that absorbs at this
wavelength region, and its absorption after applied scaling equaled 0.088 m
−
1
at
445 nm. The scaled matrices were used to calculate R and then the form of spectral
volume inelastic scattering function
b
was derived (Eq.
3
). Just like in case of the
first modeling approach, we also simulated scenarios for ten times higher con-
centrations of PARAFAC components. It is important to bear in mind, that we
could only scale matrices to realistic values, but
their correct values remain
unknown.
3 Results and Discussion
With the methods described above, the reference spectra were calculated for fulvic
and humic acids (according to Hawes
1992
) and for seven PARAFAC components
identi
rgensen et al. (
2011
). We investigated the spectral characteristics of
different CDOM components, particularly in terms of their possible application in
the satellite DOAS retrieval technique. In particular, we looked into the narrow
sharp features resulting from
ed by J
ø
filling-in of Fraunhofer lines.
We present reference spectra for fulvic and humic acids and PARAFAC com-
ponents, calculated according to Eq.
2
, in Fig.
3
a
-
c. The noticeable sharp and
narrow features correspond to
filling-in of Franuhofer lines or telluric absorption
lines. Among the amino-acid like PARAFAC components (Fig.
3
c),
fluorescence of
component 2 is noticeably stronger than the others at wavelengths above 310 nm.
This component was identi
fl
ø
ed as trypthophan-like (J
rgensen et al.
2011
). Com-
ponents 3 and 7 have very weak
fl
fluorescence, and component 5 did not produce
any
fluorescence signal. In general, amino acid-like components have much smaller
(up to three orders of magnitude) reference spectra than the humic-like (Fig.
3
b).
fl
Search WWH ::
Custom Search