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18 Where does the bromine come from?




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This article is from the Ozone Depletion: Stratospheric Chlorine and Bromine FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

18 Where does the bromine come from?

a.) Methyl Bromide

The largest source of stratospheric Bromine is methyl bromide,
CH3Br. It is also the most poorly characterized source. Much of it is
naturally produced in the oceans, but a significant portion (30-60%,
according to [Khalil et al.) is manmade; it is widely used as a
fumigant. Methyl bromide is also produced during biomass burning,
which can be either natural or anthropogenic [Mano and Andreae]. The
1994 assessment from the World Meteorological Organization [WMO 1994]
estimates the major sources as:

Oceans: 60-160 ktons/yr
Fumigation: 20-60 ktons/yr
Biomass burning: 10-50 ktons/yr .

This assessment estimates the atmospheric lifetime of methyl bromide
to be 0.8-1.7 years (best estimate 1.3 years) and its ozone depletion
potential to be about 0.6 . However, recent laboratory and field
experiments [Shorter et al.] indicate that large amounts of methyl bromide
are consumed by soil bacteria. This would push the atmospheric lifetime
down to the lower limit of 0.8 years, and reduce the ozone depletion
potential to 0.4; it may also require adjustments in the estimated sources.

Methyl bromide is also produced in the combustion of leaded gasolines,
which use ethylene dibromide as a scavenger. One estimate for the methyl
bromide emissions from this source gave 9-22 ktons/yr, but another
estimate gave only 0.5-1.5 ktons/yr.

b.) Halons

Another important Bromine source is the family of "halons", widely
used in fire extinguishers. Like CFC's these compounds have long
atmospheric lifetimes (65 years for CF3Br) and very little is lost in
the troposphere. [WMO 1994]. Halons are scheduled for phase-out
under the Montreal Protocol, and their rate of increase in the
atmosphere has slowed by a factor of three since 1989. (Before then
halon concentrations were increasing by 15-20% _per year_.)

 

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