F) and
F12 (CCl
F
), while F113 (CCl
FCClF
) and carbon
tetrachloride (CCl
) have only recently been measured. In this
review, for brevity the four compounds are referred to collectively
as chlorofluorocarbons (CFCs), even though CCl
does not contain
a fluorine atom. Analysis of CFCs is done on board ship using an
electron capture gas chromatography system [e.g. for F11 and F12
see Gammon et al., 1982; Bullister and Weiss, 1983 and
1988; for F113 see Wisegarver and Gammon, 1988; for CCl
see Fogelqvist, 1985; Krysell and Wallace, 1988]. The
compound CCl
may have a small natural background <0.01 pmol/kg
[picomoles (10
moles) per kilogram] [e.g. Wallace et al.,
1994]. The compounds F11, F12 and F113 are presently believed to
be stable in the oceans. The compound CCl
undergoes temperature
dependent hydrolysis [ Jeffers and Wolfe, 1989], which limits
its usefulness when sea surface temperatures exceed 18
C.
Surface ocean CFC concentrations are governed by gas exchange
and their solubilities. Solubilities have been measured in the
laboratory as a function of temperature and salinity (for F11 and
F12 see Warner and Weiss, [1985]; for F113 see Bu and
Warner, [1995]; for CCl
see Hunter-Smith et al., [1983]).
The oceans equilibrate with the atmosphere fairly rapidly [
Broecker and Peng, 1974]. However, during periods of warming,
supersaturations have been observed [e.g. Warner, 1988],
while undersaturations of 25-40% have been observed in the high
latitudes [e.g., Bullister, 1984; Wallace and Lazier,
1988].
Atmospheric concentrations of the CFCs have been measured
directly since 1977 at a network of stations around the world as
part of the Atmospheric Lifetime Experiment and Global Atmospheric
Gases Experiment. For the period prior to 1977, atmospheric
concentrations are reconstructed using industrial release data.
Figure 2 shows the reconstructed and directly measured atmospheric
CFC concentrations (for F11 and F12 see Bullister, [1984];
Warner, [1988]; Smethie et al., [1988]; for CCl
see Krysell and Wallace, [1988]; for F113 see Smethie,
[1994]). Early on, atmospheric concentrations of CFCs increased
exponentially. However, in the past twenty years concentrations of
all the CFC compounds have been increasing at a slower rate as they
are being phased out. The CFCs have only a small (5-10%)
hemispheric gradient [ Singh et al., 1983], because of their
long atmospheric residence times.