Environmental Engineering Reference
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Further, away from the irradiation wavelength, the absorption and fluorescence decay at
similar rates causing a slight increase in the fluorescence quantum yield (I355) at
O
ex
355 nm) (Figure 8A) while closer to the irradiation wavelength the absorption
decays slower than the fluorescence causing a rapid drop in the fluorescence I355)
(Figures 8B, 8C).
Under laser-light exposure (O= 337 nm), the fluorescence loss depends on the
irradiation dose
74-75
. Under broadband light exposure, the fluorescence decays as a sum
of two exponentials
34,49
, with its decay being more rapid than the absorption loss
49
.
Experiments using simulated sunlight also show a decrease in the molecular weight of
the organic matter with a net photoproduction of smaller compounds
31,76
. A strong
correlation of absorbance loss with irradiation dose is also observed
48
. Overall, CDOM
optical properties are lost upon light exposure and the remnant CDOM shows modified
optical properties.
7. Modeling the CDOM photobleaching in natural waters
Detailed laboratory studies of CDOM photobleaching are necessary for
developing models capable of predicting the loss of CDOM due to photobleaching.
Very recently, such models have become available
49,77-78
and have been employed to
investigate the time scales and spectral dependence of the CDOM photobleaching.
Briefly, our model
49
employs laboratory data of CDOM photodegradation induced by
exposure to monochromatic light to estimate the CDOM loss that occurs under solar
irradiation (broad-band) during summertime stratification across the shelf in the MAB.
Results indicate that the crucial factor controlling the rate of CDOM photobleaching is
the ratio of the mixed layer to the photodegradation depth
49
. By decreasing this ratio,
the CDOM loss becomes more rapid.
Figure 9.
Scheme of CDOM photobleaching model (modified from Del Vecchio and Blough
49
).
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