Halons. The atmospheric abundances of Halon-1211
and Halon-1301 are significant fractions of all brominecontaining
source gases (see Figure Q7-1) and continue to
grow despite the elimination of production in developed
nations in 1994 (see Figure Q16-1). The growth in abundance
continues because substantial reserves are held in fireextinguishing
equipment and are gradually being released,
and production and consumption are still allowed in developing
nations. Release of stored halons could keep atmospheric
halon abundances high well into the 21st century.
Methyl chloride and methyl bromide. Both methyl
chloride (CH3Cl) and methyl bromide (CH3Br) are unique
among halogen source gases because a substantial fraction
of their sources are associated with natural processes (see
Q7). The average atmospheric abundance of methyl chloride,
which is not regulated under the Montreal Protocol,
is expected to remain nearly constant throughout this century.
At century’s end, methyl chloride is expected to
account for a large fraction of remaining effective stratospheric
chlorine because the abundances of other gases,
such as the CFCs, are expected to be greatly reduced (see
Figure Q16-1). The abundance of methyl bromine, which
is regulated under the Protocol, is projected to decrease in
the first decades of this century as a result of production
phaseouts in developed and developing countries. In the
remaining decades of the century, methyl bromide abundances
are expected to be nearly constant