Environmental Engineering Reference
In-Depth Information
Table 8.5
Denomination and chemical makeup of halogenated hydrocarbons.
Prefix
Generic name
Atoms in the molecule
CFC
Chlorofluorocarbon
Cl, F, C
HCFC
Hydrochlorofluorocarbon
H, Cl, F, C
HBFC
Hydrobromofluorocarbon
H, Br, F, C
HFC
Hydrofluorocarbon
H, F, C
HC
Hydrocarbon
H, C
PFC
Perfluorocarbon
F, C
Halon
Br, Cl (in some but not all), F, H
(in some but not all), C
In CFCs and HCFCs, the first
C
is for chlorine (atomic symbol, Cl);
F
is for fluorine (atomic
symbol, F);
H
is for hydrogen (atomic symbol, H); and the final
C
is for carbon (atomic symbol, C)
PFCs, perfluorocarbons, have all bonds occupied by fluorine atoms.
Halons are a general term for compounds that contain C, F, Cl, H, and bromine (atomic
symbol, Br).
Adapted from “Numbering schemes for ozone-depleting substances and their substitutes,” n.d.
1930, a family of compounds containing chlorine has been introduced as chlorofluorocarbons
and hydrochlorofluorocarbons. These man-made chemicals have been used extensively in
refrigeration and air-conditioning equipment, as aerosol propellants, in foam blowing, and as
solvents. A similar family of compounds containing bromine instead of chlorine, or the
combination of chlorine, bromine, and fluorine are called halons (Table 8.5), and their main
use has been in fire-protection applications (“Numbering scheme for ozone-depleting
substances and their substitutes,” n.d.).
In the troposphere, chlorinated compounds are present all the time from both natural
sources and human activity (e.g., sea spray, volcanoes, and VOCs). However, most of these
chlorinated compounds are quickly transformed into inorganic species, such as HCl, and
washed out of the atmosphere by rain. With CFCs, HCFCs, and halons, the outcome is
different. Because these compounds are stable and water insoluble, they survive degrada-
tion in the troposphere and make it to the stratosphere where ultraviolet light strips chlo-
rine and bromine atoms from the molecules, forming free radicals. These free radicals then
react with ozone according to the following simplified chain reaction (Salomon, 1999;
Ghosh, 2002):
CFCl
+
UV light
→
Cl
•
+
CFCl
[8.7]
n
(n
−
1)
[8.8]
Cl
•
+→ +
O
ClO
•
O
3
2
[8.9]
ClO
•
+→ +
O
Cl
•
O
2
Other compounds, for instance nitrogen oxides, that are present in the air as a result of the
emission of nitric oxide compounds can also contribute to ozone destruction (Ghosh, 2002):
NO
+→
O
NO
+
O
[8.10]
3
2
2
NO
+→
O
NO
+
O
[8.11]
2
2
In addition, nitric oxide combines with chlorine monoxide radical (ClO
•
) to form gaseous
chlorine nitrate:
+
•
NO
ClO
→
ClONO
[8.12]
2
2
