Environmental Engineering Reference
In-Depth Information
The quantum yields related to DOM decrease exponentially with increasing
wavelength (Moran and Zepp
1997
; Vahatalo et al.
2000
; Gao and Zepp
1998
;
Sikorski and Zika
1993
; Ratte et al.
1998
). A generalized equation to find an expo-
nential relation between quantum yield and wavelength (Vahatalo et al.
2000
) can
be expressed below:
ϕ
λ
=
c
×
10
−
d
λ
(2.3)
where
c
(dimensionless) and
d
(nm
−
1
) are positive constants and
λ
is a wavelength
(nm). Different combinations of
c
and
d
can cover a wide range of exponential
relationships between quantum yield and wavelength.
2.1 Photoinduced Degradation of DOM in Natural Waters
Photoinduced degradation can decompose the DOM, estimated as dissolved
organic carbon (DOC) concentration, in natural waters. This has been verified in
photoexperiments conducted on waters by using direct natural sunlight or artificial
UV radiation in laboratory. Photoirradiation of the samples can gradually decrease
the DOC concentration as a function of integrated solar intensity (MJ m
−
2
) during
the irradiation period (Fig.
1
g-i). The initial DOC concentration, the changes in
DOC changes including relative percentages (%), as well as experimental condi-
tions in photoinduced degradation experiments are summarized in Table
1
(Morris
and Hargreaves
1997
; Ma and Green
2004
; Mostofa et al.
2007
; Bertilsson and
Allard
1996
; Amon and Benner
1996
; Mostofa et al.
2005
; Vähätalo and Wetzel
2004
; Miller and Moran
1997
; Mostofa K et al., unpublished; Mostofa and
Sakugawa unpublished data; Borisover et al.
2011
; Pullin et al.
2004
; Shiller et al.
2006
; Brooks et al.
2007
; Corin et al.
1996
; Winter et al.
2007
; Skoog et al.
1996
).
It is shown that DOC concentration photolytically decreases by approximately
21-36 % in stream waters during 12-13 days, 2-54 % in downstream river waters
during hours to 10 days, 6-41 % in lake waters during hours to 70 days, 31-36 %
in Estuarine waters during 71 days, 3-7 % in seawater during hours to 6 days of
irradiation (Table
1
). Photoirradiation can decompose 35 % of Nordic Reference
humic acid (NoHA) and 24 % of Nordic Reference fulvic acid (NoFA) extracted
from humus-rich pond water (Table
1
).
It is shown that DOC concentration in rivers photolytically decreases a little,
approximately 1-2 % in the surface layer (0 m), during 5.5-15.5 h of sunlight irra-
diation. The DOC losses decrease in deeper layers (<1 % at 6.5 and 24 m), as has
been observed in an in situ photo experiment conducted on rivers submerging at
different vertical depths (0, 6.5 and 24 m) in Lake Superior (Table
1
) (Ma and
Green
2004
). In lake waters the DOC mineralization is 22-23 % at the surface
layer (0 m), 23-24 % at 6.5 m and 4-9 % at 24 m depth, respectively, during the
5.5-15.5 h irradiation period. The results show that DOM mineralization gradually
decreases from surface (0 m) to deeper layers because of declining solar radiation
that penetrates into lake water.
