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Thus, changes in the source of marine iodocarbon
species in the atmosphere may signii cantly inl u-
ence CCN concentration (O'Dowd et al. 2002 ), with
implications for global radiative forcing and cli-
mate. During the 2006 mesocosm experiment, the
time-integrated mean concentration of CH 3 I was
44% lower in the high-CO 2 mesocosms ( p CO 2 ~750
μatm) than in the control mesocosms ( p CO 2 ~280
μatm), and 35% and 27% lower for C 2 H 5 I and CH 2 I 2 ,
respectively (Hopkins et al. 2010 ). The current l ux
of iodine from the oceans to the atmosphere, using
a globally averaged marine atmosphere surface-
mixed-layer height of 300 cm, is 1.4 × 10 3 atoms cm -3
s -1 ( O'Dowd et al. 2002). With a mean 42% reduction
in iodocarbons in the future high-CO 2 world (the
mean decrease in iodocarbon concentrations
observed during mesocosm experiments) and assu-
ming no changes to other parameters (e.g. sea-
surface temperature, mixed-layer depth, wind
speed), the l ux would decrease to 8.3 × 10 2 atoms
cm -3 s -1 . Such a decrease in net input of particles into
the aerosol population would result in a compara-
ble percentage decrease in CCN (O'Dowd et al .
2002). As around 10% of new particles survive to
CCN sizes (O'Dowd et al . 2002 ), this would corre-
spond to an approximate 4.2% decrease in available
CCN in the clean marine atmosphere. It is difi cult
to quantify the impact that this decrease would
have on radiative forcing. The previous discussion
on the problems of attempting to quantify the
impact of DMS-derived aerosols is similarly appli-
cable to iodine-derived aerosols. Additionally,
understanding is lacking in a number of areas, rang-
ing from the initial production and consumption of
organohalogens by phytoplankton and bacteria in
seawater, to the inl uence of I-derived particles on
the present-day climate. Further research is required
to enable quantii cation of the climatic impacts of a
decrease in the production of marine biogenic iodo-
carbons as a result of ocean acidii cation.
important to acknowledge that the data are of a lim-
ited and somewhat contradictory nature; so far,
only a small number of studies have been per-
formed, and of those studies, there is little consist-
ency in the observed responses of trace gases to
ocean acidii cation. Furthermore, as almost all of
these studies have been carried out at the mesocosm
facilities in Bergen in early summer during a phyto-
plankton bloom, the data are very specii c to one
region, season, and situation, with very little diver-
sity in experimental conditions. For these reasons,
and also due to the complex atmospheric chemistry
processes described in Sections 11.2.2 and 11.3.2, it
is problematic to make global extrapolations from
the available data. There is also a lack of under-
standing of the underlying mechanisms responsible
for the observed responses, with little information
on how and why ocean acidii cation may affect
trace gas concentrations. In addition, only DMS and
organohalogens have been considered, a small rep-
resentation of a large number of important gases
produced in the surface oceans. Therefore, in order
to advance our understanding, there are a number
of areas that warrant further research:
11.4.1
Marine trace gases
The studies reported so far have only assessed the
response of DMS/P and organohalogens to ocean
acidii cation. The oceans are a vital source (and in
some cases, sink) of other atmospherically and cli-
matically important trace gases. Alkyl nitrates
(RONO 2 ), including methyl nitrate (MeONO 2 ) and
ethyl nitrate (EtONO 2 ), are a major source of oxi-
dized nitrogen to the remote marine atmosphere.
They form a signii cant part of the 'odd nitrogen'
(NO y ) reservoir and play an important role in
regulating tropospheric ozone (Neu et al. 2008 ).
Oxygenated volatile organic compounds (OVOCs)
are a group of trace gases that include ketones (e.g.
acetone), aldehydes (e.g. acetaldehyde), and alco-
hols (e.g. methanol, propanol). The occurrence of
these compounds in the troposphere can strongly
inl uence the oxidative capacity and ozone-forming
potential of the atmosphere through the production
of HO x (OH and HO 2 ) free radicals (Singh et al.
2001). Non-methane hydrocarbons such as isoprene
(2-methyl-1,3-butadiene) are thought to be produced
11.4
Conclusions and future research
needs
In this chapter, an assessment has been made of the
currently available information on the effects of
ocean acidii cation on the production of atmospher-
ically important marine trace gases. Firstly, it is
 
 
 
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