Dinc Ibrahim. Refrigeration systems and applications 2th edition

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Refrigerants 71

2.5 Refrigerant–Absorbent Combinations

The refrigerant–absorbent combinations (so-called working ﬂuids) are basically used in absorp-

tion refrigeration and heat pump systems. Inorganic and organic groups are major sources of the

refrigerants and absorbents. Some organic groups for refrigerants are amines, alcohols, halogens, and

HCs, and for absorbents, alcohols, ethers, alcohol-ethers, amides, amines, amine-alcohols, esters,

ketones, acids, or aldehydes can be used.

Two well-known examples are ammonia–water and water–lithium bromide. In some literature,

the absorbent is also called the solvent. The absorbent should have a greater chemical afﬁnity

for the refrigerant than that indicated by the ordinary law of solubility. Very little heat is released

when the freons, nitrogens, or certain other gases are dissolved in water. However, water has a high

chemical afﬁnity for ammonia, and considerable heat is evolved during absorption. For example,

at 15

◦

C one unit of water can absorb approximately 800 units of ammonia. Thus the quantity of

heat released in absorption is a crude measure of the chemical afﬁnity.

In practical absorption–refrigeration applications, besides ammonia–water and water–lithium bro-

mide combinations, various refrigerant–absorbent combinations have been considered such as:

• ammonia/calcium chloride,

• ammonia/strontium chloride,

• ammonia/heptanoyl,

• ammonia/triethanolamine,

• ammonia/glycerin,

• ammonia/silicon oil,

• ammonia/lithium nitrate,

• ammonia/lithium bromide,

• ammonia/zinc bromide,

• ammonia/dimethyl ether tetraethylene glycol (DMETEG),

• ammonia/dimethyl formamide (DMF),

• methyl amine/water,

• methyl chloride/tetraethylene glycol,

• R12/dimethyl acid amide,

• R12/cyclohexanone,

• R21/dimethyl ester,

• R22/DMETEG,

• R22/DMF, and

• R22/dimethyl acid amide.

The interest in ﬁnding new refrigerants and working ﬂuids brought R134a as an alternative

refrigerant with DMETEG and DMF forefront for absorption refrigeration and heat pump

systems.

A desirable refrigerant–absorbent combination should have the property of high solubility at

conditions in the absorber but should have low solubility at conditions in the generator. In absorp-

tion refrigeration and heat pump systems, the following summarizes the desirable properties and

inﬂuences of the absorbents (Dincer, 1997):

• negligible vapor pressure at the generator, compared to the vapor pressure of the refrigerant at

37.5

◦

C, inﬂuencing rectiﬁer losses and operating cost;

• good temperature, pressure, and concentration relations (absorbent should remain liquid through-

out the cycle, and the relations must be in conformity with practical condenser, absorber, and

generator temperatures and pressures);

72 Refrigeration Systems and Applications

• high stability, inﬂuencing the ability to withstand the heating operation at the maximum temper-

atures encountered in the generator;

• low speciﬁc heat, inﬂuencing heat-transfer requirements;

• low surface tension, inﬂuencing heat transfer and absorption; and

• low viscosity, inﬂuencing heat transfer and power for pumping.

Also, refrigerant–absorbent combinations are expected to meet the following requirements:

• solubility (high solubility of the refrigerant at the temperatures of the cooling medium, e.g., air

or water, and at a pressure corresponding to the vapor pressure of the refrigerant at 5

◦

C, plus

low solubility of the refrigerant in the absorbent at generator temperatures and at a pressure

corresponding to the vapor pressure of the refrigerant at the temperature of the cooling medium);

• stability (refrigerant and absorbent must be incapable of any nonreversible chemical action with

each other within a practical temperature range, for example, from −5 to 120

◦

C); and

• superheating and supercooling (inﬂuencing operation).

The properties of the combinations relate to the liquid and/or vapor state, as encountered in normal

operation of absorption systems using such combinations, and to the crystallization boundary of

the liquid phase, where applicable. The property data can be classiﬁed as follows:

• vapor–liquid equilibria,

• crystallization temperature,

• corrosion characteristics,

• heat of mixing,

• liquid-phase densities,

• vapor–liquid-phase densities,

• speciﬁc heat,

• thermal conductivity,

• viscosity,

• stability,

• heat mass transfer rates,

• entropy,

• refractive index,

• surface tension,

• toxicity, and

• ﬂammability.

2.6 Stratospheric Ozone Layer

Here, we explain some very important deﬁnitions and names before going into details.

UV radiation is a portion of the electromagnetic spectrum with wavelengths shorter than visible

light. The sun produces UV, which is commonly split into three bands known as UVA, UVB, and

UVC.

• UVA. This is a band of UV radiation with wavelengths from 320–400 nm produced by the sun

and is not absorbed by ozone. This band of radiation has wavelengths just shorter than visible

violet light.

• UVB. This is a band of UV radiation with wavelengths from 280–320 nm produced by the

sun. UVB is a kind of UV light from the sun (and sun lamps) that has several harmful effects,

particularly effective at damaging DNA. It is a cause of melanoma and other types of skin cancer.

It has also been linked to damage to some materials, crops, and marine organisms. The ozone

layer protects the earth against most UVB coming from the sun. It is always important to protect

Refrigerants 73

oneself against UVB, even in the absence of ozone depletion, by wearing hats, sunglasses, and

sunscreen. However, these precautions will become more important as ozone depletion worsens.

• UVC. This is a band of UV radiation with wavelengths shorter than 280 nm. Despite being

extremely dangerous, it is completely absorbed by ozone and normal oxygen (O

Stratosphere is a region of the atmosphere above the troposphere and extends from about

15 to 50 km in altitude. As a matter of fact, in the stratosphere, temperature increases with altitude

because of the absorption of UV light by oxygen and ozone. This creates a global inversion layer

which impedes vertical motion into and within the stratosphere – since warmer air lies above colder

air in the upper stratosphere, convection is inhibited. The word stratosphere is related to the word

stratiﬁcation or layering.

Troposphere is a region of the atmosphere closest to the earth and extends from the surface up

to about 10 km in altitude, although this height varies with latitude. Almost all weather takes place

in the troposphere. Mt Everest, the highest mountain on earth, is only 8.8 km high. Temperatures

decrease with altitude in the troposphere. As warm air rises up, it gets cooled, falling back to

the earth. This process, known as convection, means that there are huge air movements that mix the

troposphere very efﬁciently.

Ozone is a gas composed of three atoms of oxygen, known as a bluish gas, that is harmful

to breathe. Nearly 90% of the earth’s ozone is situated in the stratosphere and is referred to

as the ozone layer . Ozone absorbs a band of UVB that is particularly harmful to living organisms.

The ozone layer prevents most UVB from reaching the ground.

Ozone layer is a region of the stratosphere containing the bulk of atmospheric ozone. The ozone

layer lies approximately 15–40 km above the earth’s surface, in the stratosphere. The ozone layer is

between 2 and 5 mm thick in the stratosphere under normal temperature and pressure conditions and

its concentration varies depending on the season, hour of the day, and location. The concentration

is greatest at an altitude of about 25 km near the equator and at about an altitude of 16 km near the

poles. The ozone comes mostly from the photodisassociation of oxygen by UV radiation of very

short wavelength (i.e., 200 µm).

Column ozone is the ozone between the earth’s surface and outer space. Ozone levels can be

described in several ways. One of the most common measures is the amount of ozone in a vertical

column of air. The Dobson unit (DU) is a measure of column ozone, which is described in the next

paragraph. Other measures include partial pressure, number density, and concentration of ozone,

and can represent either column ozone or the amount of ozone at a particular altitude.

Dobson unit (DU) is a measure of column ozone levels. If 100 DU of ozone were brought to the

earth’s surface, it would form a layer of 1 mm thickness. In the tropics, ozone levels are typically

between 250 and 300 DU year-round. In temperate regions, seasonal variations can produce large

swings in ozone levels. For instance, measurements in St Petersburg have recorded ozone levels

as high as 475 DU and as low as 300 DU (Wayne, 1991). These variations occur even in the

absence of ozone depletion. Ozone depletion refers to reductions in ozone below normal levels

after accounting for seasonal cycles and other natural effects. A DU is convenient for measuring

the total amount of ozone occupying a column overhead. If the ozone layer over the United States

were compressed to 0

◦

C and 1 atm pressure, it would be about 3 mm thick. So, 0.01 mm thickness

at 0

◦

C and 1 atm is deﬁned to be 1 DU; this makes the ozone layer over the United States measures

∼300 DU. In absolute terms, 1 DU is about 2.7 × 10

mol/cm

(Wayne, 1991). In total, there are

about 3 billion metric tons, or 3 × 10

g, of ozone in the earth’s atmosphere; about 90% of this is

in the stratosphere. In absolute terms, ozone is distributed at about 10

mol/cm

at 15 km, rising

to nearly 10

at 25 km, then falling to 10

at 45 km; and in relative terms, ozone is distributed at

about 0.5 parts per million by volume (ppmv) at 15 km, rising to ∼8 ppmv at ∼35 km and falling

to ∼3 ppmv at 45 km (Wayne, 1991).

In the past, ozone measurements were made from the ground utilizing an accurately cali-

brated Dobson instrument and, more recently, other types. The ﬂuctuation in the concentration

74 Refrigeration Systems and Applications

was extremely large depending on the season and the seasonal activities of weather due to varying

solar activity, some of which was of an apparently stochastic nature with the reason yet unexplained.

2.6.1 Stratospheric Ozone Layer Depletion

The stratospheric ozone layer depletion is a chemical destruction beyond natural reactions and is

known as one of the global environmental problems (Figure 2.1). It has been shown that this issue

is mainly caused by the ozone depletion substances (ODSs). Stratospheric ozone is constantly being

created and destroyed through natural cycles. Various ODSs, however, accelerate the destruction

processes, resulting in less than normal ozone levels. Depletion of this layer by ODS will lead

to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential

damage to some marine organisms, plants, and plastics.

The ozone-depleting substances are the compounds that contribute to stratospheric ozone

depletion. ODSs include CFCs, HCFCs, halons, methyl bromide, carbon tetrachloride, and methyl

chloroform. ODSs are generally very stable in the troposphere and only degrade under intense UV

light in the stratosphere. When they break down, they release chlorine or bromine atoms, which

then deplete ozone.

Three decades ago, Rowland and Molina ﬁrst launched a theory that CFCs and some other

anthropogenic trace gases in the atmosphere may act to deplete the stratospheric ozone layer by

catalytic action of free chlorine. They predicted very rapid reduction of ozone concentration despite

having ozone measurements almost steady for nearly 50 years, without any serious consideration.

This theory brought in a new phase in the modeling of stratospheric chemistry and gave rise

to renewed activities in the ﬁeld. In fact, the most signiﬁcant point that makes the conditions

quite complicated is the natural air movement in all directions, air having nearly 40 different

compounds giving several hundred possible reactions. That is why the models were extremely

complex. The reduction in mean ozone level was estimated in the range between 0 and 10%,

depending on the assumptions.

Stratosphere

Troposphere

Ozone depletion reactions

Earth's surface

Cosmic radiation

Photodissociation

Figure 2.1 A schematic representation of stratospheric ozone depletion.

Refrigerants 75

In the 1930s, Chapman described the following reactions: ozone is created in the upper strato-

sphere by short-wavelength UV radiation (less than ∼240 nm) when it is absorbed by oxygen

molecules (O

), which dissociate to give oxygen (O) atoms. These atoms combine with other

oxygen molecules and make ozone as follows (Rowland, 1991; Dincer, 1997):

+ UV → 2O and O + O

→ O

Sunlight with wavelengths ranging between 240 and 320 nm is absorbed by ozone, which then

falls apart to give an O atom and an O

molecule. Ozone is transformed back into oxygen if an O

atom comes together with an O

as follows:

+ sunlight → O + O

and O + O

→ 2O

This cycle seems to combine with many actions, particularly catalytic destructive actions. An

example of ozone depletion is as follows:

R + O

→ RO + O

and RO + O → R + O

where R may be nitrogen or hydroxide or chlorine radicals.

CFCs are compounds with at least one chlorine, one ﬂuorine, and one carbon atom in their

molecule. Chlorine from the CFCs has been understood to lead to the depletion of ozone in the

stratosphere. It is the chlorine that makes a substance ozone depleting; CFCs and HCFCs are a

threat to the ozone layer but HFCs are not.

If the ozone depletion continues, it is likely to have effects on the following:

• human skin, with the development of skin tumors and more rapid aging of the skin,

• human eyes, with an increase in cataracts,

• human immunological system, and

• land and sea biomass, with a reduction in crop yields and in the quantity of phytoplankton.

2.6.2 Ozone Depletion Potential

The ODP is a number that refers to the amount of stratospheric ozone depletion caused by a sub-

stance. The ODP is the ratio of the impact on ozone of a chemical compared to the impact of

a similar mass of R-11. Thus, the ODP of R-11 is deﬁned to be 1.0. Other CFCs and HCFCs

have ODPs that range from 0.01 to 1.0. The halons have ODPs ranging up to 10. Carbon tetra-

chloride has an ODP of 1.2 and methyl chloroform’s ODP is 0.11. HFCs have zero ODP because

they do not contain chlorine. The ODP data of all ozone-depleting substances are tabulated in

Table 2.2.

As an example, a compound with an ODP of 0.2 is roughly about one-ﬁfth as harmful as R-11.

The ODP of any refrigerant (i.e., R-X) is deﬁned as the ratio of the total amount of ozone destroyed

by a ﬁxed amount of R-X to the amount of ozone destroyed by the same mass of R-11, as follows:

ODP (R-X) = (Ozone loss because of R-X)/(Ozone loss because of R-11)

CFCs are considered to be fully halogenated. This means that there are no hydrogen atoms, only

halogens (chlorine, ﬂuorine, bromine, etc.). As mentioned earlier, the refrigerants with hydrogen

atoms are known as HCFCs (e.g., R-22, R-123, R-124, R-141b, and R-142b); they are not fully

halogenated and are less stable than CFCs. The computed ODP values for HCFC refrigerants are

very low (on the order of 0.01 to 0.08) compared to the values estimated for CFCs (on the order

of 0.7 to 1, for R-11, R-12, R-113, and R-114 and about 0.4 for R-115). It is for this reason that

the Montreal Protocol had a main goal of phasing out CFC-type refrigerants. There is a family of

refrigerants with an estimated ODP value of zero and without any chlorine, called HFCs.Some

76 Refrigeration Systems and Applications

Table 2. 2 ODPs, GWPs, and CAS numbers of Class I and II ODSs.

Chemical Name ODP GWP CAS Number

Class I

Group I

CFC-11 Trichloroﬂuoromethane 1.0 4,000 75-69-4

CFC-12 Dichlorodiﬂuoromethane 1.0 8,500 75-71-8

CFC-113 1,1,2-Trichlorotriﬂuoroethane 0.8 5,000 76-13-1

CFC-114 Dichlorotetraﬂuoroethane 1.0 9,300 76-14-2

CFC-115 Monochloropentaﬂuoroethane 0.6 9,300 76-15-3

Group II

Halon 1211 Bromochlorodiﬂuoromethane 3.0 1,300 353-59-3

Halon 1301 Bromotriﬂuoromethane 10.0 5,600 75-63-8

Halon 2402 Dibromotetraﬂuoroethane 6.0 – 124-73-2

Group III

CFC-13 Chlorotriﬂuoromethane 1.0 11,700 75-72-9

CFC-111 Pentachloroﬂuoroethane 1.0 – 354-56-3

CFC-112 Tetrachlorodiﬂuoroethane 1.0 – 76-12-0

CFC-211 Heptachloroﬂuoropropane 1.0 – 422-78-6

CFC-212 Hexachlorodiﬂuoropropane 1.0 – 3182-26-1

CFC-213 Pentachlorotriﬂuoropropane 1.0 – 2354-06-5

CFC-214 Tetrachlorotetraﬂuoropropane 1.0 – 29255-31-0

CFC-215 Trichloropentaﬂuoropropane 1.0 – 1599-41-3

CFC-216 Dichlorohexaﬂuoropropane 1.0 – 661-97-2

CFC-217 Chloroheptaﬂuoropropane 1.0 – 422-86-6

Group IV

CC1

Carbon tetrachloride 1.1 1,400 56-23-5

Group V

Methyl chloroform 1,1,1-trichloroethane 0.1 110 71-55-6

Group VI

Br Methyl bromide 0.7 5 7-55-6

Group VII ––

CHFBr

1.0 – –

CHF

Br (HBFC-12B1) 0.74 – –

Refrigerants 77

Table 2. 2 (continued)

Chemical Name ODP GWP CAS Number

FBr 0.73 – –

HFBr

0.3–0.8 – –

0.5–1.8 – –

0.4–1.6 – –

Br 0.7–1.2 – –

FBr

0.1–1.1 – –

0.2–1.5 – –

Br 0.7–1.6 – –

FBr

0.1–1.7 – –

Br 0.2–1.1 – –

C2H

FBr 0.07–0.1 – –

HFBr

0.3–1.5 – –

0.2–1.9 – –

0.3–1.8 – –

0.5–2.2 – –

0.9–2.0 – –

Br 0.7–3.3 – –

FBr

0.1–1.9 – –

0.2–2.1 – –

0.2–5.6 – –

0.3–7.5 – –

Br 0.9–1.4 – –

FBr

0.08–1.9 – –

0.1–3.1 – –

0.1–2.5 – –

Br 0.3–4.4 – –

FBr

0.03–0.3 – –

0.1–1.0 – –

Br 0.07–0.8 – –

FBr

0.04–0.4 – –

Br 0.07–0.8 – –

FBr 0.02–0.7 – –

(continued overleaf )

78 Refrigeration Systems and Applications

Table 2. 2 (continued)

Chemical Name ODP GWP CAS Number

Class II

HCFC-21 dichloroﬂuoromethane 0.04 210 75-43-4

HCFC-22 monochlorodiﬂuoromethane 0.055 1,700 75-45-6

HCFC-31 monochloroﬂuoromethane 0.02 – 593-70-4

HCFC-121 tetrachloroﬂuoroethane 0.01–0.04 – 354-14-3

HCFC-122 trichlorodiﬂuoroethane 0.02–0.08 – 354-21-2

HCFC-123 dichlorotriﬂuoroethane 0.02 93 306-83-2

HCFC-124 monochlorotetraﬂuoroethane 0.022 480 2837-89-0

HCFC-131 trichloroﬂuoroethane 0.01–0.05 – 359-28-4

HCFC-132b dichlorodiﬂuoroethane 0.01–0.05 – 1649-08-7

HCFC-133a monochlorotriﬂuoroethane 0.02–0.06 – 75-88-7

HCFC-141b dichloroﬂuoroethane 0.11 630 1717-00-6

HCFC-142b monochlorodiﬂuoroethane 0.065 2,000 75-68-3

HCFC-221 hexachloroﬂuoropropane 0.01–0.07 – 422-26-4

HCFC-222 pentachlorodiﬂuoropropane 0.01–0.09 – 422-49-1

HCFC-223 tetrachlorotriﬂuoropropane 0.01–0.08 – 422-52-6

HCFC-224 trichlorotetraﬂuoropropane 0.01–0.09 – 422-54-8

HCFC-225ca dichloropentaﬂuoropropane 0.025 180 422-56-0

HCFC-225cb dichloropentaﬂuoropropane 0.033 620 507-55-1

HCFC-226 monochlorohexaﬂuoropropane 0.02–0.1 – 431-87-8

HCFC-231 pentachloroﬂuoropropane 0.05–0.09 – 421-94-3

HCFC-232 tetrachlorodiﬂuoropropane 0.008–0.1 – 460-89-9

HCFC-233 trichlorotriﬂuoropropane 0.007–0.2 – 7125-84-0

HCFC-234 dichlorotetraﬂuoropropane 0.01–0.28 – 425-94-5

HCFC-235 monochloropentaﬂuoropropane 0.03–0.52 – 460-92-4

HCFC-241 tetrachloroﬂuoropropane 0.004–0.09 – 666-27-3

HCFC-242 trichlorodiﬂuoropropane 0.005–0.13 – 460-63-9

HCFC-243 dichlorotriﬂuoropropane 0.007–0.12 – 460-69-5

HCFC-244 monochlorotetraﬂuoropropane 0.009–0.14 – –

HCFC-251 trichloroﬂuoropropane 0.001–0.01 – 421-41-0

HCFC-252 dichlorodiﬂuoropropane 0.005–0.04 – 819-00-1

HCFC-253 monochlorotriﬂuoropropane 0.003–0.03 – 460-35-5

HCFC-261 dichloroﬂuoropropane 0.002–0.02 – 420-97-3

HCFC-262 monochlorodiﬂuoropropane 0.002–0.02 – 421-02-03

HCFC-271 monochloroﬂuoropropane 0.001–0.03 – 430-55-7

Source: U.S. Environmental Protection Agency, EPA (2009).

Refrigerants 79

examples of HFCs mentioned above are R-125, R-134a, R-143a, and R-152a. Research and devel-

opment activities have focused on the use of these ozone- and environment-friendly refrigerants.

2.6.3 Montreal Protocol

The world’s leading ecologists and their counterparts in industry and commerce all agree that

CFCs are the primary cause of ozone-layer depletion in the atmosphere. Ozone layer depletion

and the greenhouse effect (direct or indirect) are the ﬁrst environmental problems that have arisen

from the use of CFCs. In 1974, Molina and Rowland observed a hole in the ozone layer over

Antarctica, which they thought to be abnormal. There seemed to be a direct connection with CFCs.

In 1977, 3 years after Molina and Rowland presented their hypothesis of ozone destruction by

CFCs, the United Nations Environment Program organized a crucial conference to initiate action.

Since then, this situation has been discussed at several meetings and symposia. On September 19,

1987, 24 countries meeting in Montreal signed the Protocol on Substances Depleting the Ozone

Layer. The Montreal Protocol was the international treaty governing the protection of stratospheric

ozone. The Montreal Protocol and its amendments control the phaseout of ODS production and

use. Under the Protocol, several international organizations report on the science of ozone depletion,

implement projects to help move away from ODS, and provide a forum for policy discussions. In

addition, the Multilateral Fund provides resources to developing nations to promote the transition

to ozone-safe technologies.

This protocol provided for a reduction in consumption (of 20% of the 1986 consumption by

July 1, 1993 and of 50% by July 1, 1998), with later deadlines for developing countries. In addition,

many countries (over 70) signed the Protocol and accepted the regulations in the following Helsinki

Conference (May 1989) and London Conference (June 1990) and so on. Later, many countries have

adopted regulations stricter than those of the Montreal Protocol.

After the Montreal Protocol, there was a tremendous effort within the refrigeration and air-

conditioning industry to ﬁnd proper replacements for CFCs now under phaseout. In this respect,

the thermodynamic aspects of replacement refrigerants, in particular, the consequences for system

operating efﬁciencies and the desired operating temperatures and pressures for conventional refrig-

eration equipment, are being investigated. Recently, there has been increasing interest in research

and development in many areas, for example, ecological phenomena, ﬂuid toxicology, thermody-

namic and technological properties of the alternative refrigerants and equipment, and use of the

new cycles and systems.

2.7 Greenhouse Effect (Global Warming)

Although, the term greenhouse effect has generally been used for the role of the whole atmosphere

(mainly water vapor and clouds) in keeping the surface of the earth warm, it has been increasingly

associated with the contribution of CO

(currently, it is estimated that CO

contributes about 50%

to the anthropogenic greenhouse effect). However, several other gases such as CH

, CFCs, halons,

O, ozone, and peroxyacetylnitrate (so-called greenhouse gases) produced by the industrial and

domestic activities can also contribute to this effect, resulting in a rise in the earth’s temperature.

A schematic representation of this global problem is illustrated in Figure 2.2.

In the greenhouse effect phenomenon, the rays of the sun reach the earth and maintain an average

temperature level of around +15

◦

C. A large part of the infrared rays reﬂected off the earth are

caught by CO

O, and other substances (including CFCs) present in the atmosphere and kept

from going back into space. The increase in the greenhouse effect would result in a sudden rise

in temperature and it is very likely linked with human activity, in particular, the emissions from

fossil fuel consumption.

80 Refrigeration Systems and Applications

Earth‘s surface

Trap heat and raise the earth's surface temperature

Greenhouse effect

Atmosphere

Figure 2.2 A schematic representation of the greenhouse effect.

Also, increasing the greenhouse effect may result in the following (Dincer, 2003):

• an intermediate warming of the atmosphere (estimated as 3 to 5

◦

C by 2050),

• a rise in the level of the oceans (estimated as 20 cm by 2050), and

• climatic effects (increases in drought, rain, snow, warming, and cooling).

Increasing atmospheric concentrations of CFCs have accounted for about 24% of the direct

increase in the radiative heating from greenhouse gases over the last decade. However, an observed

decrease in stratospheric ozone, thought to be connected to increasing stratospheric chlorine from

CFCs, suggests a negative radiative heating or cooling tendency over the last decade.

The release of CFCs into the atmosphere affects climate in two different ways (Badr et al., 1990):

• CFCs are highly harmful greenhouse gases (relative to CO

) because of their stronger IR band

intensities, stronger absorption features, and longer atmospheric lifetimes.

• CFCs deplete the stratospheric ozone layer that affects earth’s surface temperature in two

ways: more solar radiation reaching the surface–lower troposphere system, resulting in a

warmer climate and leading to lower stratospheric temperatures and, therefore, less IR radiation

being passed to the earth’s surface–lower troposphere system, resulting in lower ground-level

temperatures.

Therefore, the net effect is dependent on the altitudes where the ozone change takes place.

2.7.1 Global Warming Potential

GWP is a number that refers to the amount of global warming caused by a substance. GWP

is the ratio of the warming caused by a substance to the warming caused by a similar mass of