Environmental Engineering Reference
In-Depth Information
Mostofa et al.
2010
). As a consequence, the absorption of CDOM at specific wave-
lengths spectrum is found to considerably change for an array of waters. CDOM
absorption contributed in average to 66 and 40 % of the UV diffuse attenuation
coefficients in clear and turbid water, respectively (Belzile et al.
2002
).
3.1.4 Effect of Variation in Incident Light Wavelengths
and Solar Zenith Angle
The inherent and the apparent optical properties of natural waters are significantly
dependent on the angle of the light flux incident on the water surface. They vary
with solar altitude and with the proportion of diffuse and direct solar radiation
(Kirk
1984
,
1991
,
1994
; Morris
2009
; Morel and Bélanger
2006
). Remembering
the
a
and
b
coefficients of Eqs. (
2.5
-
2.8
), when the solar zenith angle increases
from 0° to 45° and finally to 89°, the ratio of the vertical attenuation coefficient to
the absorption coefficient (
K
d
,(
z
m
)/
a
) increases by 15 and 41 %, respectively, when
b
:
a
=
1, by 8 and 22 % when
b
:
a
=
5, and by 5 and 12 % when
b
:
a
=
10 (Kirk
1984
).
K
d
is thus rather insensitive to solar altitude in highly scattering waters
(high
b
), but a considerable effect of solar altitude is observed in clear oceanic
waters with low values of
b
:
a
. For all natural waters the shape of the volume scat-
tering function is such that there is much more scattering in a forward than in a
backward direction (Kirk
1984
). When the incident beam moves away from the
vertical, an increasing proportion of the more intense forward scattering becomes
upward rather than downward scattering, thereby increasing the irradiance reflec-
tance with decreasing solar altitude (increasing zenith angle) (Kirk
1984
).
3.1.5 Suspended Particulate Matter
Absorption and scattering play a major role in UV and PAR attenuation by
suspended particulate matter (SPM) such as phytoplankton pigments, algae,
living heterotrophs, mineral sediments and detritus (organic, inorganic and min-
eral constituents) (Zhang et al.
2009
, Kirk
1984
,
1991
,
1994
; Laurion et al.
2000
; Hayakawa and Sugiyama
2008
; Dupouy et al.
2010
; Uusikivi et al.
2010
;
Odriozola et al.
2007
; Vantrepotte et al.
2007
; Belzile et al.
2002
; Pierson et al.
2008
; Kratzer et al.
2008
; Devlin et al.
2009
; Hodoki and Watanabe
1998
; Smith
et al.
1999
; Stambler
2005
; Bowers and Binding
2006
; Binding et al.
2008
; Devlin
et al.
2008
; Foden et al.
2008
). The spectral absorption coefficients of particulate
matter (PM) are about twice higher in UV than in PAR wavelengths in the Baltic
Sea ice (Uusikivi et al.
2010
). Particulate absorption coefficients are appreciable
in magnitude, averaging up to 103 % of
a
CDOM
at 380 nm and reflecting sig-
nificant influence of both algal and detrital particles in lake and tributary waters
(Smith et al.
2004
). PM absorption spectra can include significant contribution by
mycosporine-like amino acids between 320 and 345 nm (Uusikivi et al.
2010
).
