Volatile Halomethanes
Robert M. Moore and Charles Geen
Volatile organohalogens have an important influence on atmospheric chem-
istry. In particular, halogenated methanes provide a source of atmospheric
chlorine and bromine radicals, which can affect ozone concentrations. There
has been shown to be a strong correlation between elevated atmospheric concen-
trations of brominated substances and sudden tropospheric ozone depletion
events, especially during the Arctic spring. There are also indications that bro-
minated compounds play a significant role in stratospheric ozone destruction.
It is important to distinguish between natural and anthropogenic inputs of
these compounds. In attempting to understand the ozone depletion events in
the Arctic boundary layer, there has been considerable interest in examining
the natural Arctic marine origins of volatile bromine compounds (bromoform
in particular) as potentially significant sources of gaseous halogens to the atmo-
sphere. Our research group has previously found evidence of an ice-algal source
of bromoform and dibromomethane in the Arctic. Evidence also exists in north-
ern waters for a phytoplankton source of volatile iodine compounds, which
can play a role in the biogeochemical cycling of iodine and in atmospheric
chemistry.
Our objective in participating in the AOS-94 cruise was to extend the data-
base of measurements of distributions and concentrations of brominated, chlor-
inated and iodinated methanes in the Arctic Ocean. This would contribute to
the goal of determining the natural origins of these compounds; in fact, this
expedition was the culmination of a five-year, Arctic-wide study of the sources
of such substances. Measurements were also made of the trace gas concentrations
in seawater that had been allowed to equilibrate with the atmosphere; these
are needed for calculating fluxes of the gases between the ocean and the atmo-
sphere.
Almost 500 seawater samples were analyzed by gas chromatography dur-
ing the cruise. Vertical profiles from many stations provided further general
oceanographic knowledge of water column distributions of the halomethanes
and generally showed the concentrations of the halocarbon gases of interest to
be higher in the surface waters and to decrease with depth. However, since our
Robert Moore is with the Department of Oceanography at Dalhousie University at Halifax, Nova Scotia,
Canada. Charles Geen is with Bovar-Concord Environmental in Toronto, Ontario, Canada.
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