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Methyl Bromide
Ban
Prof. Peter Saunders
gives timely warning for maintaining the ban on a powerful
ozone depleter
Six years ago, 160 countries agreed in the
Montreal Protocol to phase out substances that are known to deplete the
ozone layer. The most important of these were the chlorofluorocarbons
(CFCs), which were being extensively used as refrigerants and as
propellants in aerosols. Also included was a plant pesticide, methyl
bromide, which developed countries are supposed to have phased out by 2005.
Developing countries were allowed a further ten years.
Methyl bromide (CH3Br) is
widely used for fumigating agricultural products (including wood) and to kill
the organisms in soil in which crops such as tomatoes and strawberries are
to be planted. It tends to be used more in warmer climates where soft
fruits are more susceptible to attack.
When methyl bromide reaches the stratosphere
it is split by ultra-violet radiation. This releases bromine, which is
approximately 40 times as effective as chlorine at breaking down ozone.
Only about a fifth of the methyl bromide that enters the atmosphere comes
from human activity, but it is such a powerful ozone depleter that banning
it makes an important contribution to restoring the ozone layer. It is also
highly toxic, and because it is volatile at 20 C, it is hard to prevent it
from drifting beyond the fields on which it is being applied or to escape
from greenhouses. When it enters the soil, it tends to penetrate to the
deeper soil layers where it breaks down only slowly, which makes it
dangerous to use in water collection areas.
Methyl bromide persists for less than a
year in the atmosphere, so measures taken now can have an immediate effect.
Following the signing of the Montreal Protocol, production of methyl
bromide has declined and so has the level in the atmosphere.
The Montreal Protocol allows exemptions
for “critical users”. This is a category that was intended to allow banned
substances to be used in small quantities in applications in which there is
no suitable alternative: CFCs are still permitted as propellants in
inhalers for asthmatics but the amounts involved are relatively small.
There is now, however, pressure to grant large-scale critical use
exemptions to farmers, especially in the USA, and so that raw wood can
still be used in packaging. If these are allowed, they could halt, or even
reverse, the decline in atmospheric methyl bromide.
A decision was expected in November but
the delegates to the meeting at the Nairobi headquarters of the United
Nations Environment Programme (UNEP) agreed they needed more time to
consider the issues. There is now to be an extraordinary meeting in
Montreal in March 2004. With only a year to go before the ban is meant to
come into force, any users who do not gain the exemptions they are hoping
for will have very little time to comply. So it’s likely to be a very
difficult meeting.
Sources
Montzka, S.A., J.H. Butler, B.D. Hall, D.J. Mondeel, and J.W. Elkins, A
decline in tropospheric bromine, Geophysical Research Letters, 30
(2003) 1826
Further reading can be found on the
following web sites:
US Environmental Protection Agency www.epa.gov/ozone/mbr/
Methyl Bromide Alternatives Outreach www.mbao.org/
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