and atmosphere of the Pacific Ocean. The source of
the significant discrepancy between these two estimates has yet to be resolved. Depending on the answer, CO emitted to the atmosphere from the oceans
could play a significant role in controlling OH levels
in the marine troposphere.
Carbonyl sulfide (COS) is produced primarily in
coastal/shelf waters, apparently by the CDOM-photosensitized oxidation of organosulfur compounds.
UV-B light is the most effective in its formation, with
F decreasing rapidly from B6 Â 10
À7 at 300 nm to
B1 Â 10
À8 by 400 nm (Figure 3). The principal sinks
of seawater COS are release to the atmosphere and
hydrolysis to CO 2 and H 2 S. Accounting for perhaps
as much as one-third of the total source strength, the
photochemical production of COS in the oceans is
probably the single largest source of COS to the atmosphere, although more recent work has revised
this estimate downward. Smaller amounts of carbon
disulfide (CS 2 ) are also generated photochemically
in surface waters through CDOM sensitized reaction(s); F values decrease from B1 Â 10
À7 at
313 nm to 5 Â 10
À9 at 366 nm. The CS 2 emitted to
the atmosphere can react with OH to form additional COS in the troposphere. Although it was
previously thought that the oxidation of COS in the
stratosphere to form sulfate aerosol could be important in determining Earth’s radiation budget and
perhaps in regulating stratospheric ozone concentrations, more recent work suggests that other sources contribute more significantly to the background
sulfate in the stratosphere.
Dimethyl sulfide (DMS), through its oxidation to
sulfate in the troposphere, acts as a source of cloud
condensation nuclei, thus potentially influencing the
radiative balance of the atmosphere. DMS is formed
in sea water through the microbial decomposition of
dimethyl sulfonioproprionate (DMSP), a compound
believed to act as an osmolyte in certain species of
marine phytoplankton. The flux of DMS to the atmosphere is controlled by its concentration in surface
sea waters, which is controlled in turn by the rate of
its decomposition. Estimates indicate that 7–40% of
the total turnover of DMS in the surface waters of
the Pacific Ocean is due to the photosensitized destruction of this compound, illustrating the potential
importance of this pathway in controlling the flux of
DMS to the atmosphere.
In addition to these compounds, the photochemical production of small amounts of nonmethane hydrocarbons (NMHC) such as ethene,
propene, ethane, and propane has also been reported. Production of these compounds appears to
result from the photolysis of the CDOM, with F
values of the order of 10
À7
–10
À9
. The overall
emission rates of these compounds to the atmosphere
via this source are negligible with respect to global
volatile organic carbon emissions, although this
production may play some role in certain restricted
locales exhibiting stronger source strengths, or in the
marine environment remote from the dominant terrestrial sources.
The photolysis of nitrate and nitrite in sea water
produces nitrogen dioxide (NO 2 ) and nitric oxide
(NO), respectively (eqns [I] and [II]). Previous work
indicated that the photolysis of nitrite could act as a
small net source of NO to the marine atmosphere
under some conditions. However, this conclusion
seems to be at odds with estimates of the steady-state
concentrations of superoxide and the now known
rate constant for the reaction of superoxide with
nitric oxide (6.7 Â 10
9 M
À1 s
À1
) to form peroxynitrite in aqueous phases (eqn [V]).
O
À
2 þ NOÀ OONO
½V
The peroxynitrite subsequently rearranges in part to
form nitrate (eqn [VI]).
À OONO-NO 3
À
½VI
Even assuming a steady-state concentration of O 2
À
(10
À12 M) that is about two orders of magnitude
lower than that expected for surface sea waters
(B10
À10 M), the lifetime of NO in surface sea waters
would be only B150 s, a timescale too short for
significant exchange with the atmosphere except for
a thin surface layer. Moreover, even in this situation,
the atmospheric deposition of additional HO 2 radicals to this surface layer (to form O 2
À ) would be
expected to act as an additional sink of the NO (flux
capping). It appears that most if not all water bodies
exhibiting significant steady-state levels of O 2
À
, produced either photochemically or thermally, should
act as a net sink of atmospheric NO and probably of
NO 2 as well. Further, although less is known about
the steady-state levels of peroxy radicals in sea
waters due largely to their unknown decomposition
routes, their high rate constants for reaction with
NO (1–3 Â 10
9 M
À1 s
À1
) indicate that they should
also act as a sink of NO. In fact, methyl nitrate,
a trace species found in sea waters, may in part
be produced through the aqueous phase reactions
(eqns [VII] and [VIII]) with the methylperoxy
radical (CH 3 OO) generated through a known
photochemical reaction of CDOM (or through atmospheric deposition) and the NO arising from
the photolysis of nitrite (or through atmospheric
deposition).
94 PHOTOCHEMICAL PROCESSES
the significant discrepancy between these two estimates has yet to be resolved. Depending on the answer, CO emitted to the atmosphere from the oceans
could play a significant role in controlling OH levels
in the marine troposphere.
Carbonyl sulfide (COS) is produced primarily in
coastal/shelf waters, apparently by the CDOM-photosensitized oxidation of organosulfur compounds.
UV-B light is the most effective in its formation, with
F decreasing rapidly from B6 Â 10
À7 at 300 nm to
B1 Â 10
À8 by 400 nm (Figure 3). The principal sinks
of seawater COS are release to the atmosphere and
hydrolysis to CO 2 and H 2 S. Accounting for perhaps
as much as one-third of the total source strength, the
photochemical production of COS in the oceans is
probably the single largest source of COS to the atmosphere, although more recent work has revised
this estimate downward. Smaller amounts of carbon
disulfide (CS 2 ) are also generated photochemically
in surface waters through CDOM sensitized reaction(s); F values decrease from B1 Â 10
À7 at
313 nm to 5 Â 10
À9 at 366 nm. The CS 2 emitted to
the atmosphere can react with OH to form additional COS in the troposphere. Although it was
previously thought that the oxidation of COS in the
stratosphere to form sulfate aerosol could be important in determining Earth’s radiation budget and
perhaps in regulating stratospheric ozone concentrations, more recent work suggests that other sources contribute more significantly to the background
sulfate in the stratosphere.
Dimethyl sulfide (DMS), through its oxidation to
sulfate in the troposphere, acts as a source of cloud
condensation nuclei, thus potentially influencing the
radiative balance of the atmosphere. DMS is formed
in sea water through the microbial decomposition of
dimethyl sulfonioproprionate (DMSP), a compound
believed to act as an osmolyte in certain species of
marine phytoplankton. The flux of DMS to the atmosphere is controlled by its concentration in surface
sea waters, which is controlled in turn by the rate of
its decomposition. Estimates indicate that 7–40% of
the total turnover of DMS in the surface waters of
the Pacific Ocean is due to the photosensitized destruction of this compound, illustrating the potential
importance of this pathway in controlling the flux of
DMS to the atmosphere.
In addition to these compounds, the photochemical production of small amounts of nonmethane hydrocarbons (NMHC) such as ethene,
propene, ethane, and propane has also been reported. Production of these compounds appears to
result from the photolysis of the CDOM, with F
values of the order of 10
À7
–10
À9
. The overall
emission rates of these compounds to the atmosphere
via this source are negligible with respect to global
volatile organic carbon emissions, although this
production may play some role in certain restricted
locales exhibiting stronger source strengths, or in the
marine environment remote from the dominant terrestrial sources.
The photolysis of nitrate and nitrite in sea water
produces nitrogen dioxide (NO 2 ) and nitric oxide
(NO), respectively (eqns [I] and [II]). Previous work
indicated that the photolysis of nitrite could act as a
small net source of NO to the marine atmosphere
under some conditions. However, this conclusion
seems to be at odds with estimates of the steady-state
concentrations of superoxide and the now known
rate constant for the reaction of superoxide with
nitric oxide (6.7 Â 10
9 M
À1 s
À1
) to form peroxynitrite in aqueous phases (eqn [V]).
O
À
2 þ NOÀ OONO
½V
The peroxynitrite subsequently rearranges in part to
form nitrate (eqn [VI]).
À OONO-NO 3
À
½VI
Even assuming a steady-state concentration of O 2
À
(10
À12 M) that is about two orders of magnitude
lower than that expected for surface sea waters
(B10
À10 M), the lifetime of NO in surface sea waters
would be only B150 s, a timescale too short for
significant exchange with the atmosphere except for
a thin surface layer. Moreover, even in this situation,
the atmospheric deposition of additional HO 2 radicals to this surface layer (to form O 2
À ) would be
expected to act as an additional sink of the NO (flux
capping). It appears that most if not all water bodies
exhibiting significant steady-state levels of O 2
À
, produced either photochemically or thermally, should
act as a net sink of atmospheric NO and probably of
NO 2 as well. Further, although less is known about
the steady-state levels of peroxy radicals in sea
waters due largely to their unknown decomposition
routes, their high rate constants for reaction with
NO (1–3 Â 10
9 M
À1 s
À1
) indicate that they should
also act as a sink of NO. In fact, methyl nitrate,
a trace species found in sea waters, may in part
be produced through the aqueous phase reactions
(eqns [VII] and [VIII]) with the methylperoxy
radical (CH 3 OO) generated through a known
photochemical reaction of CDOM (or through atmospheric deposition) and the NO arising from
the photolysis of nitrite (or through atmospheric
deposition).
94 PHOTOCHEMICAL PROCESSES