formation of a variety of LMW organic compounds,
the release of biologically available forms of nitrogen, and the loss of CDOM absorption. Through
direct photochemical reactions and reactions with
the ROS, the speciation of metal ions is also affected.
These photochemical intermediates and products
are produced at relatively low efficiencies. About
98–99% of the photons absorbed by CDOM are
released as heat, while another B1% are re-emitted
as fluorescence. These percentages (or fractions) of
absorbed photons giving rise to particular photoresponses are known as quantum yields ðFÞ. The F
for the production of H 2 O 2 and O 2
À (the two reduced
oxygen species produced with highest efficiency), are
approximately one to two orders of magnitude
smaller than those for fluorescence, ranging from
B0.1% at 300 nm to B0.01% at 400 nm. The F for
other intermediates and products range even lower,
from B0.01% to 0.000 0001% (see below). The F
for most of the intermediates and products created
from the CDOM are highest in the UV-B and UV-A,
and fall off rapidly with increasing wavelength;
yields at visible wavelengths are usually negligible
(see for example, Figure 3).
The hydroxyl radical, a very powerful oxidant,
can be produced by the direct photolysis of nitrate
and nitrite (eqns [I]–[III]).
NO 3
À þ hv-O
À þ NO 2
½I
NO 2
À þ hv-O
À þ NO
½II
O
À þ H 2 O-OH þ OH
À
½III
The F values for these reactions are relatively high,
about 7% for nitrite and about 1–2% for nitrate.
However, because of the relatively low concentrations of these compounds in most marine surface
waters, as well as their low molar absorptivities in
the ultraviolet, the fraction of light absorbed is generally small and thus fluxes of OH from these sources
also tend to be small. Recent evidence suggests that
OH, or a species exhibiting very similar reactivity, is
produced through a direct photoreaction of the
CDOM; quinoid moieties within the CDOM may be
responsible for this production. Quantum yields are
low, B0.01%, and restricted primarily to the ultraviolet. In estuarine and near-shore waters containing
higher levels of iron, the production of OH may also
occur through the direct photolysis of iron–hydroxy
complexes or through the Fenton reaction (eqn [IV]).
Fe
2þ þ H 2 O 2 -Fe
3þ þ OH þ OH
À
½IV
Compounds that do not absorb light within the
surface solar spectrum are also subject to photochemical modification through indirect or ‘sensitized’
photoreactions. In this case, the ROS or intermediates produced by direct photoreactions of a lightabsorbing constituent such as CDOM can react
secondarily with the nonabsorbing compounds.
DMS and COS, two trace gases of some importance
to the atmosphere, are thought to be destroyed and
created, respectively, by sensitized photoreactions in
marine surface waters.
Lake Valkea-kotiten
Sea water
River water
Suwanee
Houghton
Kinoshe
Okefenokee
CO 2
CO
COS
300 320 340 360 380 400 420 440
Wavelength (nm)
2.0
1.5
1.0
0.5
0.0
3.0
2.0
1.0
0.0
4.0
2.0
0.0
Φ (
× 10
3
)
Φ (
× 10
4
)
Φ (
× 10
7
)
Figure 3 Wavelength dependence of the quantum yields (F)
for the photochemical production of CO 2 , CO, and COS. Data
have been replotted from those dependencies originally reported
in Va ¨ ha ¨ talo et al. (2000), Valentine and Zepp (1993), and Weiss
et al. (1995) for CO 2 , CO, and COS, respectively.
92 PHOTOCHEMICAL PROCESSES
the release of biologically available forms of nitrogen, and the loss of CDOM absorption. Through
direct photochemical reactions and reactions with
the ROS, the speciation of metal ions is also affected.
These photochemical intermediates and products
are produced at relatively low efficiencies. About
98–99% of the photons absorbed by CDOM are
released as heat, while another B1% are re-emitted
as fluorescence. These percentages (or fractions) of
absorbed photons giving rise to particular photoresponses are known as quantum yields ðFÞ. The F
for the production of H 2 O 2 and O 2
À (the two reduced
oxygen species produced with highest efficiency), are
approximately one to two orders of magnitude
smaller than those for fluorescence, ranging from
B0.1% at 300 nm to B0.01% at 400 nm. The F for
other intermediates and products range even lower,
from B0.01% to 0.000 0001% (see below). The F
for most of the intermediates and products created
from the CDOM are highest in the UV-B and UV-A,
and fall off rapidly with increasing wavelength;
yields at visible wavelengths are usually negligible
(see for example, Figure 3).
The hydroxyl radical, a very powerful oxidant,
can be produced by the direct photolysis of nitrate
and nitrite (eqns [I]–[III]).
NO 3
À þ hv-O
À þ NO 2
½I
NO 2
À þ hv-O
À þ NO
½II
O
À þ H 2 O-OH þ OH
À
½III
The F values for these reactions are relatively high,
about 7% for nitrite and about 1–2% for nitrate.
However, because of the relatively low concentrations of these compounds in most marine surface
waters, as well as their low molar absorptivities in
the ultraviolet, the fraction of light absorbed is generally small and thus fluxes of OH from these sources
also tend to be small. Recent evidence suggests that
OH, or a species exhibiting very similar reactivity, is
produced through a direct photoreaction of the
CDOM; quinoid moieties within the CDOM may be
responsible for this production. Quantum yields are
low, B0.01%, and restricted primarily to the ultraviolet. In estuarine and near-shore waters containing
higher levels of iron, the production of OH may also
occur through the direct photolysis of iron–hydroxy
complexes or through the Fenton reaction (eqn [IV]).
Fe
2þ þ H 2 O 2 -Fe
3þ þ OH þ OH
À
½IV
Compounds that do not absorb light within the
surface solar spectrum are also subject to photochemical modification through indirect or ‘sensitized’
photoreactions. In this case, the ROS or intermediates produced by direct photoreactions of a lightabsorbing constituent such as CDOM can react
secondarily with the nonabsorbing compounds.
DMS and COS, two trace gases of some importance
to the atmosphere, are thought to be destroyed and
created, respectively, by sensitized photoreactions in
marine surface waters.
Lake Valkea-kotiten
Sea water
River water
Suwanee
Houghton
Kinoshe
Okefenokee
CO 2
CO
COS
300 320 340 360 380 400 420 440
Wavelength (nm)
2.0
1.5
1.0
0.5
0.0
3.0
2.0
1.0
0.0
4.0
2.0
0.0
Φ (
× 10
3
)
Φ (
× 10
4
)
Φ (
× 10
7
)
Figure 3 Wavelength dependence of the quantum yields (F)
for the photochemical production of CO 2 , CO, and COS. Data
have been replotted from those dependencies originally reported
in Va ¨ ha ¨ talo et al. (2000), Valentine and Zepp (1993), and Weiss
et al. (1995) for CO 2 , CO, and COS, respectively.
92 PHOTOCHEMICAL PROCESSES