Figure Q16-1. Halogen source gas changes. The rise in
effective stratospheric chlorine values in the 20th century has
slowed and reversed in the last decade (top panel). Effective
chlorine values combine the measured or projected abundances
of chlorine-containing gases with those of brominecontaining
gases in a way that properly accounts for the
greater effectiveness of bromine in depleting stratospheric
ozone. As effective chlorine decreases in the 21st century,
the potential for ozone depletion from halogen gases will also
decrease. The decrease in effective chlorine values is a result
of reductions in individual halogen source gas emissions.
The emissions decreased because of the Montreal Protocol,
which restricts production and consumption of manufactured
halogen gases. The changes in the atmospheric abundance
of individual gases are shown in the lower panels using a
combination of direct atmospheric measurements, estimates
of historical abundance, and future projections of abundance.
The increases of CFCs, along with those of CCl4 and
CH3CCl3, have either slowed significantly or reversed in the
last decade. HCFCs, which are being used as CFC substitutes,
will continue to increase in the coming decades. Some
halon abundances will also grow in the future while current
halon reserves are being depleted. Smaller relative
decreases are expected for CH3Br in response to restrictions
because it has substantial natural sources. CH3Cl has large
natural sources and is not regulated under the Montreal
Protocol. (See Figure Q7-1 for chemical names and formulas.
The unit “parts per trillion” is defined in the caption of
Figure Q7-1.)