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ocean acidii cation was observed during laboratory
incubations of natural plankton assemblages from
UK coastal waters (Hopkins 2010).
Hopkins et al. (2010) also reported the response of
a number of bromine-containing organohalogens
(bromocarbons) to ocean acidii cation during the
2006 mesocosm experiment (Fig. 11.5). The tempo-
ral development of bromoform (CHBr 3 ), dibro-
momethane (CH 2 Br 2 ), and dibromochloromethane
(CHBr 2 Cl) was substantially different from that of
the iodocarbons, and these gases tended to show
some increase in concentrations in response to high
p CO 2 . Mean concentrations were elevated under
high p CO 2 , particularly CHBr 2 Cl which was statisti-
cally higher during the whole experiment. However,
as considerable differences in bromocarbon concen-
trations were apparent from the start of the experi-
ment, and as the temporal development of
bromocarbons did not rel ect the growth and decline
of the bloom, it is not clear whether these differ-
ences can be attributed to an effect of p CO 2 .
play a number of important roles which may be
affected by a decrease in their sea-to-air l ux, and
are discussed below.
11.3.2.1 Oxidative capacity of the atmosphere
Ozone (O 3 ) is a highly oxidizing gas that performs a
number of important roles in the atmosphere. In the
stratosphere (~25 km above the surface of the earth)
it absorbs solar ultraviolet-B (UV-B) radiation, thus
protecting the earth's living organisms from its
harmful effects (Solomon 1999). By contrast, ele-
vated concentrations of O 3 in the troposphere (the
surface to 10 km) are not benei cial to life, adversely
affecting plant, animal, and human health, and act-
ing as a potent GHG (Ramaswamy et al. 2001 ).
Therefore, a clear understanding of the processes
that control O 3 levels in the lower atmosphere is
vital for projections of future O 3 levels to be made.
The production and use of anthropogenic chlo-
rol uorocarbons (CFCs) has resulted in depletions
to stratospheric O 3 allowing increased levels of
UV-B to reach the earth's surface (Solomon 1999).
Not only harmful to plant and animal life, an
increase in the penetration of UV-B results in an
enhancement in the chemical activity of the tropo-
sphere, with implications for a number of processes
( Tang et al. 1998 ).
In polluted air, such as in the Northern
Hemisphere, an increase in UV-B leads to an increase
in tropospheric O 3 through the photo-oxidation of
pollutants such as CH 4 , NO x , and volatile organic
compounds (VOCs; Crutzen 1974 ; Tang et al. 1998 ).
This process leads to an increase in hydroxyl (OH)
radicals. These radicals exert an important control
on the oxidative capacity of the atmosphere and are
effective atmospheric cleansers, promoting the
removal of GHGs and other pollutants (Tang et al.
1998). In clean, remote air, an increase in UV-B sim-
ply results in a decrease in tropospheric O 3 through
photolysis in the presence of water vapour. This
leads to an increase in OH radicals and an enhance-
ment of the atmosphere's oxidative capacity (Tang
et al. 1998 ).
Upon entering the atmosphere, marine iodocar-
bons undergo rapid photolysis to produce free radi-
cals (I, IO, Br, BrO) which act as effective catalytic
O 3 -depleting species (Chameides and Davis 1980;
Solomon et al. 1994 ; Davis et al. 1996 ). Due to weak
11.3.2
Atmospheric and climatic implications
The oceans are naturally enriched in iodine, a result
of volcanism earlier in the earth's history. Most
marine iodine (>96%) is present in the thermody-
namically stable form of iodate which is reduced to
iodide (I - ) in surface-ocean waters by the activity of
bacteria and phytoplankton (Elderi eld and
Truesdale 1980). Hence, in the euphotic zone the
dissolved iodine pool may be dominated by iodide
by up to 50% (Wong 1991). Iodide is subsequently
taken up by seaweed and phytoplankton and can
be released as volatile iodocarbons (CH 3 I, CH 2 I 2 ,
etc.). These gases undergo sea-air exchange, and
through photochemical reactions, iodine and its
associated oxidized radicals (IO, OIO) are released
to, or formed in, the atmosphere. As described
above, decreased seawater concentrations of all
measured iodocarbons were found during a meso-
cosm phytoplankton bloom experiment in 2006
( Hopkins et al . 2010), and this result is supported by
observations from laboratory incubations of UK
coastal plankton assemblages (Hopkins 2010). These
i ndings suggest that ocean acidii cation can have
an impact on the net production of these gases.
Once released to the atmosphere, these compounds
 
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