Geoscience Reference
In-Depth Information
6.7.3 Fluorescence of CDOM and Passive Sensors
CDOM fluoresces under sunlight producing a broad emission spectrum that varies with
the source of the CDOM and with the environmental conditions. This solar-induced fluor-
escence can be strong enough to contribute significantly to irradiance reflectance (
R
) and
could affect the interpretation of remote sensing data. FCDOM may also fill Fraunhofer
lines suggesting a tool for detection of CDOM by remote sensors.
Several authors have evaluated the contribution of FCDOM to water leaving radiance
(L
W
, radiance flux coming out of the water in W m
-2
sr
-1
nm
-1
), and hence to
R
(ratio of
irradiance reflected from a surface to irradiance incident on a surface) (i.e., Spitzer and
Dirks,
1985
; Peacock et al.,
1990
; Hawes et al.,
1992
; Lee et al.,
1994
; Vodacek et al.,
1994
;
Haltrin et al., 1997; Zhou et al.,
2009
). Although these authors used different assump-
tions and different sources of CDOM in their studies, the consensus is that FCDOM only
becomes an important contaminant to
R
when CDOM is very high. How high depends on
the total optical properties of the water, and researchers should be cautions when working
in low-sediment, high-CDOM waters. Complications to satellite remote sensing measure-
ments are probably lower because high CDOM values required to influence
R
are typically
restricted to near-shore or inland water bodies. Ocean color remote sensing images nor-
mally mask an area along the coast depending on sensor resolution (i.e., ~1 km for NASA-
MODIS and SeaWiFS at nadir) to eliminate contamination from land reflectance. So pixels
more likely to be contaminated with FCDOM are likely masked during image processing.
The broad fluorescence spectrum of CDOM fills Fraunhofer lines presenting the pos-
sibility of using very sensitive remote and
in situ
sensors to study CDOM and chlorophyll
in ocean waters (Stoertz et al.,
1969
; Gee et al.,
1993
; Vodacek et al.,
1994
; Hu and Voss,
1998
). Although under some conditions has shown promise (i.e., Vodacek et al.,
1994
),
Natural FDOM is very faint (
f
~ 1%) and the emission spectrum is to broad to be useful for
passive remote sensing of organic matter.
6.7.4 Remote Sensing Summary
CDOM fluorescence measurements are a powerful tool to study dynamics of organic mat-
ter in the oceans. The application of fluorescence to remote sensing of CDOM is limited
to the use of airborne LIDAR systems that can measure
a
g
(
λ
) accurately. Contribution of
FCDOM to
R
and filling of Fraunhofer lines can be important only during special condi-
tions of high CDOM and low scatterers. FCDOM can be extremely useful in the validation
of remote sensing estimates of
a
g
(
λ
).
6.8 Summary
There are a number of commercially available fluorescence sensors and selecting the ideal
sensor for a particular application, while understanding the capabilities and limitations of
fluorometer design, is key to acquiring robust
in situ
CDOM fluorescence measurements.
