Halogencontaining Compounds

Atmospheric halogen-containing compounds are referred to by a variety of names:

Halocarbons—a general term referring to halogen-containing organic compounds Chlorofluorocarbons{CFCs)—the collective name given to a series of halocarbons containing carbon, chlorine, and fluorine atoms

Hydrochlorofluorocarbons (HCFCs)—halocarbons containing atoms of hydrogen, in addition to carbon, chlorine, and fluorine Hydrofluorocarbons (HFCs)—halocarbons containing atoms of hydrogen, in addition to carbon and fluorine Perhalocarbons—halocarbons in which every available carbon bond contains a halogen atom (compounds saturated with halogen atoms) Halons—bromine-containing halocarbons, especially used as fire extinguishing agents

The earliest interest in halogens in the atmosphere arose from seasalt as a source of gaseous halogens (Eriksson 1959). Synthetic halocarbons have been known for the past century. Chlorofluorocarbons (CFCs) were first synthesized in the late nineteenth century, and their properties as refrigerants were recognized over 60 years ago. Halocarbons achieved widespread industrial use as refrigerants, propellants, and solvents.

As a group of atmospheric chemicals, halogen-containing compounds have a wide variety of anthropogenic and natural sources. They are produced by biological processes in the oceans, from seasalt, from biomass burning, and from industrial synthesis. Their atmospheric lifetimes vary considerably depending on their mechanism of removal, ranging from a few days to several centuries.

Table 2.15 lists atmospheric halogenated organic species with global average concentrations, atmospheric burdens, lifetimes, sources, and sinks. Of the exclusively human-made organic halogenated species, the chlorofluorocarbons are used as refrigerants (CFC-12, HCFC-22), blowing agents (CFC-11, HCFC-22), and cleaning agents (CFC-113). Methyl chloroform (CH3CC13), methylene chloride (CH2C12), and tetrachloroethene (C2C14) are used as degreasers and as dry cleaning and industrial solvents. Methyl bromide (CH3Br) is a widely used agricultural and space fumigant. All the monomethyl halides listed in Table 2.15 have natural sources. Methyl chloride (CH3C1) and CH3Br are also products of biomass burning. Atmospheric levels of CFC13 and CF2C12 are shown in Figure 2.5.

Lovelock (1971) first detected SF6 and CFC13 in the atmosphere using the electron-capture detector. In landmark work in atmospheric chemistry for which they received the 1995 Nobel Prize in Chemistry, Molina and Rowland (1974) showed that CFCs that are immune to removal in the troposphere could decompose photolytically in the stratosphere to release CI atoms capable of catalytic destruction of stratospheric ozone. The very lack of chemical reactivity that makes chlorofluorocarbon molecules so intrinsically useful also allows them to survive unchanged in most commercial applications and eventually to be released to the atmosphere in their original gaseous form. The usual tropospheric sinks of oxidation, photodissociation, and wet and dry deposition are ineffective with the chlorofluorocarbons. The only important sink for CFC13 and CF2C12 is photodissociation in the midstratosphere (25-40 km). These same CFCs that lead to stratospheric ozone depletion are efficient absorbers of infrared radiation and potentially important greenhouse gases.

There is a sharp demarcation in atmospheric behavior between fully halogenated halocarbons and those containing one or more atoms of hydrogen. Halocarbons containing at least one hydrogen atom, such as CF2HC1, CHC13, and CH3CC13, are effectively broken down in the troposphere by reaction with the hydroxyl radical before they can reach the stratosphere. Atmospheric lifetimes of these species range from months to decades.

TABLE 2.15

Atmospheric Halogens

1998 Mixing

Lifetime

Compound

Gcncric Name

Ratio (ppl)

(yr)

Sources"

Sinks''

CFClj

CFC-11

268

45

A

Sirai./iv

CF2CI2

CFC-12

533

100

A

Strat./iv

CFjCICFCIJ

CFC-113

84

85

A

Sirat./iv

CF2CICF2C1

CFC-114

15

300

A

Strat./iv

CC14

Carbon

102

35

A

Stral./iv

tetrachloride

CHjCCl,

Methyl

69

4.8

A

Trop. OH

chloroform

CH3C!

Methyl chloride

500

1.5

N(0),BB

Trop. OH

CF2HC1

HCFC-22

132

11.9

A

Trop. OH

CHjBr

Methyl bromide

9-10

0.8

N(0)A,BB

Trop. OH

cf3Br

H-1301

2.5

65

A

Strat.fiv

cf4

Pe rfl uorom eth ane

80

50.000

A

Mcso./iv

sf6

Sulfur

4.2

3200

A

Meso.

hexafluoride

electrons

cf,chci2

HCFC-123

1.4

A

Trop. OH

cfjchfci

HCFC-I24

5.9

A

Trop. OH

CH3CFC12

HCFC-!4ib

10

9.3

A

Trop. OH

ch3cf2ci

HCFC-142b

11

19

A

Trop. OH

cf3cf2chci2

HCFC-225ca

2.5

A

Trop. OH

CC1F2CF2CHC1F HCFC-225cb

6.6

A

Trop, OH

chci3

Chloroform

0,55

A,N(0)

Trop, OH

ch2ci2

Methylene chloride

0.41

A

Trop. OH

CF,CF2C]

CFC-115

7

1700

A

Strat. O('D)

c2c14

Tetrac h lor oethene

0.4

A

Trop. OH

"A — anthropogenic; N(O) = natural (oceanic); BB = biomass burning.

''Sir/.'V = photolysis in stratosphere; Trop. OH = hydroxy! radical reaction in troposphere; Meso, electrons = mesosphere electron impact; Strat. Oi'D) = reactions in stratosphere with excited atomic oxygen. Sources: IPCC (2001) and Singh (1995).

"A — anthropogenic; N(O) = natural (oceanic); BB = biomass burning.

''Sir/.'V = photolysis in stratosphere; Trop. OH = hydroxy! radical reaction in troposphere; Meso, electrons = mesosphere electron impact; Strat. Oi'D) = reactions in stratosphere with excited atomic oxygen. Sources: IPCC (2001) and Singh (1995).

Chlorofluorocarbons The term hydrochlorafluorocarbons is the collective name given to a series of chemicals with varying number of carbon, hydrogen, chlorine, and fluorine atoms. The somewhat arcane system of numbering these compounds was proposed by the American Society of Heating and Refrigeration Engineers in 1957. For the simpler hydrochlorofluorocarbons, the numbering system may be summarized as follows:

1, The first digit on the right is the number of fluorine (F) atoms in the compound,

2, The second digit from the right is one more than the number of hydrogen (H) atoms in the compound.

3, The third digit from the right, plus one. is the number of carbon (C) atoms in the compound. When this digit is zero (i.e., only one carbon atom in the compound), it is omitted from the number.

1950

1950

0.00

1950

1960

1970

1980

1990

FIGURE 2.5 Global mean CFC-l 1 <CFC13) and CFC-12 (CF2C12) tropospheric abundance from 1950 to 1998 based on smoothed measurements and emission models (IPCC 2001). The radiative forcing of each CFC is shown on the right axis (see Chapter 23).

1950

0.00

1960

1970

1980

1990

2000

FIGURE 2.5 Global mean CFC-l 1 <CFC13) and CFC-12 (CF2C12) tropospheric abundance from 1950 to 1998 based on smoothed measurements and emission models (IPCC 2001). The radiative forcing of each CFC is shown on the right axis (see Chapter 23).

4. The number of chlorine (CI) atoms in the compound is found by subtracting the sum of the fluorine and hydrogen atoms from the total number of atoms that can be connected to the carbon atoms.

CC12F2 C2C12F4 CHC1F2 CCIjF CFC-12 CFC1-114 CFC-22 CFC-l 1

2.5.1 Methyl Chloride (CH^Cl)

Methyl chloride supplies about 0.5 ppb of chlorine to the stratosphere. Prior to 1996, the atmospheric input of CH3C1 was assumed to be a result of biological processes in the ocean. Tropical terrestrial sources, biomass burning and tropical plants, are now believed to be important. The dominant sink of CHSC1 is reaction with the OH radical in the

1980

1990

1960 1970

1980

1990

-Jo.oo 2000

1960 1970

TABLE 2.16 Global Budget of Methyl Chloride (CH3C1)

Sources, Ggyr

Oceans

Biomass burning Tropical plants Fungi

Salt marshes

Wetlands

Coal combustion

Incineration

Industrial

Rice

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