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Recent observations of the rapid,
episodic formation of large numbers (> 106 cm-3)
of ultra-fine aerosol particles in the marine boundary layer (MBL) at
seaweed-rich coastal areas [O'Dowd and Hoffmann, 2005] and the
identification of iodine within the particles [Makela et al., 2002] are
most readily explained by the following scenario; the release of
iodine-containing gases (i.e. I2 and CH2I2)
from macro-algae during day-time low-tide periods brings about the
photolytic production of reactive iodine atoms which are rapidly oxidised
by O3. This leads to the formation of gas-phase iodine oxide
species such as IO and OIO which, through further reaction / polymerisation
routes, lead to the homogeneous nucleation of IOP's.
Figure 1 Laminaria Digitata (iodine-rich seaweed species)
Despite recent laboratory studies, two major unresolved questions remain
regarding (i) the precise chemical composition and (ii) growth behaviour in
the presence of condensable vapours (primarily water, sulphuric acid, and
organics), of this secondary aerosol in the marine atmosphere. Both of
these factors are likely to play a defining role in determining whether
these particles have any significant global impact on climate through
either direct (scattering and absorption of solar radiation) or indirect (enhancement
of cloud condensation nuclei or CCN levels) radiative processes. Therefore,
a thorough knowledge of particle composition and properties requires a
description of how they interact with other gases (i.e. H2O, H2SO4,
and organic acids in particular) in the marine boundary layer (MBL).
Figure 2 Some potential chemical pathways, currently under study in our
group, for solid (s) iodine oxide particles (IOPs) in the MBL.
We are currently carrying out studies to resolve these unknowns – see Laboratory
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