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rates from ultraviolet-B (UVB), UVA and photosynthetically active radiation
(PAR), and loss rates from temperature-dependent and temperature-independent
processes (Rusak et al.
2010
). The retrieved model terms confirm that H
2
O
2
is pro-
duced by both UVB and UVA radiation. These results demonstrate that changes
in solar radiation reaching the study site are closely correlated with the observed
seasonal pattern in H
2
O
2
concentrations in the water (Rusak et al.
2010
).
4.2 Production and Decay Affected by Contents
and Molecular Nature of DOM
The production and decay of H
2
O
2
and ROOH in natural waters are signifi-
cantly affected by the total contents and molecular nature of DOM (Mostofa and
Sakugawa
2009
; Scully et al.
1995
). An increase in standard Suwannee River
Fulvic Acid (SRFA) contents in aqueous solution increases the photoinduced pro-
duction of H
2
O
2
, but the production of ROOH decreases with an increase in SRFA
concentration. It is suggested that the photoinduced generation of H
2
O
2
depends
on the total contents of DOM components in natural waters. It is demonstrated that
the production rates of H
2
O
2
are greatly different for a variety of waters, and the
production rates for various standard organic substances are also widely variable
(Table
2
). The photoinduced generation of H
2
O
2
by natural waters and standard
organic substances is generally much higher at short irradiation times (60 min),
after which it often decreases. Such an effect has been observed in upstream
waters as well as in aqueous solutions of Suwannee River Fulvic Acid (SRFA),
Suwannee River Humic Acid (SRHA), tryptophan, DSBP and DAS1, during photo
experiments carried out with a solar simulator (Fig.
1
a, b) (Mostofa and Sakugawa
2009
). The production of H
2
O
2
and its disappearance for prolonged irradiation
times has suggested two important phenomena. Firstly, H
2
O
2
is initially generated
as a consequence of the excitation of highly efficient functional groups of organic
substances. These groups are effectively excited and transformed by solar radia-
tion, after which the effectiveness of the functional groups to yield H
2
O
2
gradually
decreases. This effect, combined with consumption processes, causes a decrease of
H
2
O
2
concentration at the end of the long-term irradiation period. Secondly, H
2
O
2
produced upon irradiation is photolytically converted to HO
•
, which can degrade
the parent organic substances and yields a variety of photoproducts in the aqueous
solution (Southworth and Voelker
2003
; Kramer et al.
1996
; Legrini et al.
1993
;
von Sonntag et al.
1993
; Corin et al.
1996
; Schmitt-Kopplin et al.
1998
; Wang et
al.
2001
; Leenheer and Croué
2003
). These results suggest that the photoinduced
generation of H
2
O
2
and ROOH depends on the molecular nature of DOM com-
ponents in natural waters. The relationship between DOC concentration and pro-
duction rates of H
2
O
2
(Fig.
10
) shows that the rate is higher for upstreams and
groundwater, and increases non-linearly with an increase of DOC concentration
in rivers (Fig.
10
) and lakes (Scully et al.
1996
). It can be considered that the
highly reactive DOM is photolytically and rapidly degraded into photoproducts in
