Geoscience Reference
In-Depth Information
iodine bonds, iodocarbons are very photochemi-
cally active, with an atmospheric lifetime of a few
minutes to days (Solomon et al. 1994 ). This gener-
ally limits their O 3 -depleting capacity to the tropo-
sphere, where a signii cant impact on O 3 levels is
possible under certain conditions (Davis et al. 1996 ).
Additionally, in regions that experience strong
atmospheric convection, such as the tropics, iodo-
carbons can be rapidly transported to the upper
troposphere and lower stratosphere, and can con-
tribute to O 3 depletion at these levels (Solomon et al.
1994 ).
The regulation of the oxidative capacity of the
atmosphere is clearly complex and is controlled by
a number of processes, some of which have under-
gone signii cant anthropogenic perturbations. The
impact that a decrease in marine emissions of vola-
tile iodocarbons to the troposphere will have is
therefore difi cult to quantify, but may result in a
decrease in tropospheric O 3 destruction. This would
reduce the atmosphere's capacity to remove this
potent GHG and air pollutant, enhancing global
warming and contributing to negative impacts on
human health and plant growth.
vational and modelling studies (O'Dowd et al. 2002 ;
Pechtl et al. 2007 ). The inl uence of Br and Cl oxides
on DMS chemistry has also received some atten-
tion. In a modelling study, von Glasow et al. ( 2002 )
found that when atmospheric halogen chemistry
was included, DMS oxidization in the MBL
increased by about 63%. Therefore, the potential
climate impact of both DMS- and halocarbon-
derived new particles/CCN is closely related.
Future modelling studies on the impacts of ocean
acidii cation on marine biogeochemistry and cli-
mate feedbacks need to consider the synergistic
impacts of changes in the net production of these
gases. A combined decrease in both DMS and iodo-
carbons would result in a decrease in two of the
steps involved in new particle formation in the
MBL and lead to an overall positive feedback to
global warming.
The work of O'Dowd et al . ( 2002 ) was based on
studies of new particle bursts at Mace Head, Eire,
over dense beds of kelp. Therefore, it is most appli-
cable to coastal regions with signii cant seaweed
cover, and it becomes problematic to directly extrap-
olate such observations to the open oceans, result-
ing in uncertainty in the signii cance of any climatic
impact. The situation in the open ocean is less
clear, as particle burst events are less frequent and
lower in intensity than their coastal analogues.
Consequently, O'Dowd et al . ( 2002 ) expanded their
observational and experimental work by simulat-
ing the process using a marine aerosol model.
Further modelling work by Pechtl et al . ( 2007 ) con-
i rmed the importance of iodine oxides in both pri-
mary particle formation and secondary growth of
particles in the clean marine atmosphere. These
simulations suggest that concentrations of iodocar-
bons over the open ocean may be high enough to
inl uence marine particle production. Thus, pelagic
open-ocean production of iodocarbons may exert a
signii cant inl uence on climate through the produc-
tion of new particles and CCN. In order to fully
understand and quantify the role of phytoplankton,
and achieve an understanding of the possible glo-
bal climatic impacts, further knowledge of produc-
tion and consumption of halocarbons and DMS by
phytoplankton and bacteria in surface seawater, as
well as the process of particle formation in the
atmosphere, is required.
11.3.2.2 New particle formation in the marine
boundary layer
The formation of new particles in the MBL from
volatile iodocarbon precursors originating from
marine macroalgae and kelp beds has been demon-
strated by both observational and experimental
studies (Makela et al. 2002 ; O'Dowd et al. 2002 ), sug-
gesting that in coastal regions biogenic iodocarbons
may exert a signii cant impact on local, and more
speculatively global, radiative forcing.
Studies have shown that the oxidation of DMS in
the MBL is connected to this process (O'Dowd et al.
2002). Oxidation of DMS represents the i rst step in
the production of new particles, resulting in the
production of small (~1 nm), thermodynamically
stable clusters. In order to achieve an increase in
particle number concentration, these stable clusters
must rapidly grow to a size of about 3-4 nm to
avoid colliding with larger pre-existing particles
and being captured (Kulmala et al. 2000 ). The role,
in this second process, of condensable iodine
vapours (CIVs) produced by the photolysis of CH 2 I 2
in the presence of O 3 has been coni rmed by obser-
 
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