Geoscience Reference
In-Depth Information
10
4
10
4
Refrigeration & air condit.
Blowing agent foam production
Aerosol propellant
Other
Refrigeration & air condit.
Blowing agent foam production
Aerosol propellant
Other
1000
1000
100
100
10
10
1
1
1980
1985
1990
1995
2000
2005
1980
1985
1990
1995
2000
2005
(a) Year
(b) Year
Figure 11.12.
Reported sales of CFC-11 and -12 between 1976 and 2003. Percentages are of the total for the
year. CFC, chlorofluorocarbon. From Alternative Fluorocarbons Environmental Acceptability Study (AFEAS)
(n.d.).
Hydrochlorofluorocarbons
(
HCFCs
)are another sub-
set of chlorocarbons. HCFCs are similar to CFCs,
except that HCFCs have at least one hydrogen atom.
The hydrogen atom allows HCFCs to be broken down
in the troposphere by reaction with OH(g), which does
not readily break down CFCs. Because HCFCs react
with OH(g) more readily than do CFCs, a smaller per-
centage of emitted HCFCs than CFCs reaches the strato-
sphere. Nevertheless, because HCFCs contain chlorine
and some HCFCs reach the stratosphere, HCFCs are
still a danger to stratospheric ozone. HCFC-22, first
produced in 1943, is the most abundant HCFC in the air
today. HCFC-22 has been used as a refrigerant, spray
can propellant, and blowing agent in foam production.
Other chlorocarbons include
carbon tetrachloride
[CCl
4
(g)],
methyl chloroform
[CH
3
CCl
3
(g)], and
methyl chloride
[CH
3
Cl(g)]. Carbon tetrachloride is
used as an intermediate in the production of CFCs and
HCFCs, and as a solvent and grain fumigant. Methyl
chloroform is used as a degreasing agent, a dry clean-
ing solvent, and an industrial solvent. Methyl chloride is
produced synthetically only in small quantities for use
in the production of silicones and tetramethyl lead inter-
mediates. Most methyl chloride in the air is produced
biogenically in the oceans.
Another chlorine-containing gas in the troposphere is
hydrochloric acid
[HCl(g)]. HCl(g) has larger natural
than anthropogenic sources. A natural source includes
evaporation of the chloride ion from sea spray and vol-
canic emissions. Although some anthropogenic emis-
sions of HCl(g) are from waste incineration, about 98
percent are from coal combustion (Saxena et al., 1993).
latter compounds are more efficient, molecule for
molecule, at destroying ozone. The primary source of
stratospheric bromine is
methyl bromide
[CH
3
Br(g)],
which is produced biogenically in the oceans and
emitted as a soil fumigant. Other sources of bromine
are a group of synthetically produced compounds
termed
halons
,which are used in fire extinguishers
and as fumigants. The most common halons are
H-
1301
[CF
3
Br(g)],
H-1211
[CF
2
ClBr(g)], and
H-2402
[CF
2
BrCF
2
Br(g)]. Methyl bromide and halons are
bro-
mocarbons
because they contain both bromine and
carbon.
11.5.1.4. Fluorine Compounds
Compounds that contain hydrogen, fluorine, and car-
bon but not chlorine or bromine are
hydrofluorocar-
bons
(HFCs). HFCs were produced in abundance only
recently as a replacement for CFCs and HCFCs. The
most abundantly emitted HFC to date has been
HFC-
134a
[CH
2
FCF
3
(g)]. Related to HFCs are
perfluoro-
carbons
(PFCs), such as perfluoroethane [C
2
F
6
(g)],
and
sulfur hexafluoride
[SF
6
(g)]. Because the fluo-
rine in HFCs and PFCs has little effect on ozone,
production of HFCs and PFCs may increase in the
future.
Unfortunately, because they strongly absorb thermal-
IR radiation, HFCs and PFCs enhance global warming.
This is illustrated in Table 11.2, which shows the
100-
year global warming potential (GWP)
of these and
other chemicals. The GWP gives the 100-year inte-
grated change in global atmospheric heating due to a
compound in the air relative to that of carbon dioxide
(Section 12.6.4). Table 11.2 indicates that CFCs, halons,
HFCs, and PFCs all have high GWPs, indicating that
they all contribute to atmospheric warming and will for
more than 100 years after their emission.
11.5.1.3. Bromine Compounds
Although chlorine-containing compounds are more
abundant than are bromine-containing compounds, the
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