Detecting CDOM Fluorescence Using High Spectrally Resolved Satellite Data: A Model Study - Towards an Interdisciplinary Approach in Earth System Science - page 110

concentrations are high and VRS signal is very weak, there is still signi cant

CDOM fl fluorescence signal. It follows, that the filling-in signal can be due to strong

CDOM fluorescence or strong VRS. However, the VRS is in general a stronger

process and

fl

fluorescence does not increase
strongly with increasing CDOM concentration, so in the regions of high CDOM we
see in fact weaker

filling-in originating from CDOM fl

filling-in due to absorption of CDOM and weakening of VRS.
As an example, we have run two simple DOAS retrievals for June and July 2009
on the SCIAMACHY (Bovensmann et al. 1999 ) data in the spectral regions of
those Fraunhofer lines (343

345.5 nm and 484 490 nm). For each retrieval we

- -

fitted only one reference spectrum, which was a sum of reference spectra of all the
PARAFAC components for a respective spectral window. In fact, due to spectral
similarities of the filling-in of Fraunhofer lines caused by different processes, the

fitted spectra correspond to the contribution from all inelastic processes. We
focused on the Atlantic area close to the river mouth of the Amazon, where loading
of the terrestrial CDOM into the ocean is generally very high, and where the highest
CDOM absorption in the surface waters of open ocean was found in a study by
Nelson and Siegel ( 2013 ). In Fig. 6 results of fit factors for the two spectral

windows are compared to the MODIS-Aqua absorption coef

cient for dissolved
and detrital particulate matter at 443 nm derived from the Garver-Siegel-Maritorena
algorithm (Garver and Siegel 1997 ; Maritorena et al. 2002 ). In case of using the
filling-in of H b line in the retrieval (Fig. 6 b), in the area of high CDOM concen-
tration, where we expect strong CDOM filling-in of

Fraunhofer lines is in fact weaker. It is due to weaker VRS, as described above. For
the CDOM

fl fluorescence, the retrieved

fluorescence retrieval in shorter wavelengths (Fig. 6 a), when prelimi-
narily examined, the spatial structure of the Amazon plum is not seen and the
results are completely dominated by RRS (which is stronger towards higher solar
zeniths angle hence higher latitudes). However,

fl

these results do not include

correction for Raman scattering contribution to filling-in of Fraunhofer lines, which

would be necessary for a solid comparison.

Fig. 6 Maps of retrieved CDOM fluorescence fit factors for 2 months (June and July 2009) in two
different spectral regions (a, b) compared with MODIS-Aqua absorption coef cient for dissolved
and detrital particulate matter a cdm at 443 nm ½m 1 (c)