Report No. 24
Trends of Controlled
National Oceanic and Atmospheric Administration
Environmental Research Laboratories
Atmospheric trends of chemicals, chlorofluorocarbons (CFCs)
and chlorinated solvents, controlled under the Montreal Protocol to
Reduce Substances that Deplete the Ozone Layer.
Almost all production of these major CFCs and chlorinated solvents
ended on January 1, 1996, in the developed countries.
All mixing ratios are reported as parts-per-trillion (ppt)
in dry air. Upper panel: The mixing ratios from weekly flask pairs that
started in 1977 are reported as monthly means for these major CFCs.
The atmospheric growth rate of CFC-12 is decreasing with time
as a result of voluntary and mandated emission reductions under the
Montreal Protocol. CFC-12
was used in pre-1993 auto air conditioners, as aerosol propellant,
and in refrigerators. The
accumulation of CFC-11 in the atmosphere peaked during 1993-1994.
CFC-11 was used as a cell-blowing agent for manufacture of
foams, in large building air conditioning, and refrigeration.
Lower panel: Atmospheric trends reported as monthly means (ppt) of the chlorinated
solvents of methyl chloroform (CH3CCl3) and
carbon tetrachloride (CCl4)
measured by in situ gas chromatography, and flask pair means reported
for CFC-113 (CCl3F-CClF3)
collected from weekly flask pairs measured by gas chromatography and
mass spectrometry. Note
that CH3CCl3 is dropping rapidly with time.
It was used as a metal degreaser in manufacturing.
CCl4 is decreasing in mixing ratio at a rate of
0.8% yr-1. It
was used in dry cleaning and as a feedstock to produce the CFCs.
CFC-113 has leveled off and is slowly dropping in mixing ratio
with time. It was used
as a solvent degreaser in the manufacture of circuit boards.
(This figure was created by James W. Elkins and used data provided
by Thayne M. Thompson, Andrew D. Clarke, Stephen A. Montzka, and James