51
used as a weathering parameter. Since the curves over a given spectral range adjust
well to parabolic decay in a form akin to that of Eq. 2.1, logarithmic transformation
yields the spectral slope parameter in units of m
−1
.nm
−1
. The contribution of CDOM
to total DOC is variable depending on its chemical nature, the magnitude and periodicity of input to the coastal ocean and, due to its photolability, to the history of
light exposure of the source material.
Spectrophotometric parameterization favors the lower UV bands where absorption is greatest and, despite more energetic solar irradiance, stratospheric ozone
absorption significantly reduces radiant flux and thus the propensity for photodegradation. Parameters such as optical absorption at a specific wavelength or spectral
slope of the absorption decay curve across a predetermined wavelength range serve
for quantitative characterization. Absorbance at 300 nm of filtered samples (to
remove particulate matter) is commonly used as an index of CDOM abundance (Del
Castillo et al. 1999). However, for comparison to satellite ocean color instrument
data, the 412 nm and 443 nm bands are also frequently used. Submersible multispectral (3–9 bands) and hyperspectral (400–750 nm) absorption and attenuation
meters are commercially available, the latter taking readings in about 80 narrow
spectral bands. However, these instruments are extremely sensitive to temperature,
salinity, and fouling of the optical surface so their use is largely limited to research
applications.
CDOM fluorescence offers a practical alternative for operational CDOM
detection. High aromatic content of CDOM molecules imparts the capability for
fluorescence under UV irradiance allowing a more robust approach to CDOM determination. Excitation/emission matrix fluorescence spectroscopy, whereby both
excitation and emission (ex/em) are scanned across the UV-Vis spectrum using
laboratory benchtop instruments, coupled to parallel factor analyses, a statistical
technique for deconvolution of complex spectra, allows discrimination of discrete
CDOM fluorophores. Simpler, single or multiple-band submersible ex/em instruments used for operational CDOM detection are widely available. Band choices,
tuned to these fluorophores, range between 310–370 nm excitation and 450–470 nm
emission. These instruments generally exhibit an orthogonal arrangement of the
exciting radiation source and the emission sensing photodiode. Given the random
molecular structure of humic substances, no single CDOM molecule exhibits the
same fluorescence properties and no truly representative analytical standard exists.
A common organic chemical calibration proxy is quinine sulfate with ex/em peaks
at around 310 and 450 nm. Single design submersible fluorescence instruments now
available may be configured for measurement of most of the fluorophores of interest
to ocean observing including CDOM, here discussed, and petroleum hydrocarbons
and chlorophyll, discussed below.
Remote satellite detection and quantification of CDOM in the Orinoco River
Plume (Del Castillo et al. 1999) and the South Florida shelf (Müller-Karger et al.
2005) has been explored using empirical and semianalytical algorithms specific for
CDOM retrieval. Instrument resolution currently limits operational applicability
(Del Castillo 2005) to waters of very high CDOM content.
2.4 Electro-Optical Sensors for Measurement of Organic Matter in Seawater
used as a weathering parameter. Since the curves over a given spectral range adjust
well to parabolic decay in a form akin to that of Eq. 2.1, logarithmic transformation
yields the spectral slope parameter in units of m
−1
.nm
−1
. The contribution of CDOM
to total DOC is variable depending on its chemical nature, the magnitude and periodicity of input to the coastal ocean and, due to its photolability, to the history of
light exposure of the source material.
Spectrophotometric parameterization favors the lower UV bands where absorption is greatest and, despite more energetic solar irradiance, stratospheric ozone
absorption significantly reduces radiant flux and thus the propensity for photodegradation. Parameters such as optical absorption at a specific wavelength or spectral
slope of the absorption decay curve across a predetermined wavelength range serve
for quantitative characterization. Absorbance at 300 nm of filtered samples (to
remove particulate matter) is commonly used as an index of CDOM abundance (Del
Castillo et al. 1999). However, for comparison to satellite ocean color instrument
data, the 412 nm and 443 nm bands are also frequently used. Submersible multispectral (3–9 bands) and hyperspectral (400–750 nm) absorption and attenuation
meters are commercially available, the latter taking readings in about 80 narrow
spectral bands. However, these instruments are extremely sensitive to temperature,
salinity, and fouling of the optical surface so their use is largely limited to research
applications.
CDOM fluorescence offers a practical alternative for operational CDOM
detection. High aromatic content of CDOM molecules imparts the capability for
fluorescence under UV irradiance allowing a more robust approach to CDOM determination. Excitation/emission matrix fluorescence spectroscopy, whereby both
excitation and emission (ex/em) are scanned across the UV-Vis spectrum using
laboratory benchtop instruments, coupled to parallel factor analyses, a statistical
technique for deconvolution of complex spectra, allows discrimination of discrete
CDOM fluorophores. Simpler, single or multiple-band submersible ex/em instruments used for operational CDOM detection are widely available. Band choices,
tuned to these fluorophores, range between 310–370 nm excitation and 450–470 nm
emission. These instruments generally exhibit an orthogonal arrangement of the
exciting radiation source and the emission sensing photodiode. Given the random
molecular structure of humic substances, no single CDOM molecule exhibits the
same fluorescence properties and no truly representative analytical standard exists.
A common organic chemical calibration proxy is quinine sulfate with ex/em peaks
at around 310 and 450 nm. Single design submersible fluorescence instruments now
available may be configured for measurement of most of the fluorophores of interest
to ocean observing including CDOM, here discussed, and petroleum hydrocarbons
and chlorophyll, discussed below.
Remote satellite detection and quantification of CDOM in the Orinoco River
Plume (Del Castillo et al. 1999) and the South Florida shelf (Müller-Karger et al.
2005) has been explored using empirical and semianalytical algorithms specific for
CDOM retrieval. Instrument resolution currently limits operational applicability
(Del Castillo 2005) to waters of very high CDOM content.
2.4 Electro-Optical Sensors for Measurement of Organic Matter in Seawater