4 -- SOURCES OF Cl, Br, ClOx, AND BrOx

The most reactive compounds to stratospheric ozone are chlorine and bromine oxides, members of the chemical family called the halogens. Free radical chlorine or bromine atoms can extract an oxygen atom from ozone producing oxidized forms of chlorine (ClOx) and bromine (BrOx). These halogen oxides participate in catalytic reactions leading to ozone loss.

4.1 Sea Salt Production of Sodium Chloride (NaCl) and Hydrochoric Acid (HCl)

The most common source of chlorine to the atmosphere is sea salt spray. Breaking waves and wind-blown foam spray millions of tiny particles and gases into the atmosphere. The tiny particles contain dissolved sodium chloride (NaCl or ordinary table salt). As the particles evaporate, both NaCl and hydrogen chloride (HCl) are released as a gas. Both of these molecules are highly soluble, and are removed by redissolving in the ocean or by dissolving in rainwater and being redeposited in the ocean or on land. Their atmospheric residence time is of the order of one week and they contribute negligible amounts of chlorine to the stratosphere.

4.2 Ocean Production of Methyl Chloride and Methyl Bromide

Biological activity in the ocean processes some of the dissolved chlorine, bromine, and iodine salts. Seaweed produces methyl iodide (CH3I). Reactions in seawater replace the iodine with bromine to produce methyl bromide (CH3Br). Further reactions replace bromine with chlorine to produce methyl chloride (CH3Cl). A portion of each of these substances escapes to the atmosphere. CH3Br from the atmosphere may also dissolve in the ocean and be converted to CH3Cl. Methyl chloride has an atmospheric residence time of 1.5 years and is the most important natural source of chlorine for the stratosphere.

4.3 Volcanic Emissions as Halogen Sources

Volcanos are often mentioned as a potential source of chlorine to the stratosphere. Gases are emitted from volcanically active vents and from frequently erupting, non explosive volcano s (such as Kilauea crater in Hawaii). The chlorine is predominately in the form of HCl which is soluble in water and rains out like the gases produced from sea salt spray. Only the most explosive volcano s like Mt. Pinatubo are of potential direct importance to the stratosphere (see section 5 below for more details).

4.4 Industrial Production of Halogen Compounds

By far the most important source of chlorine to the stratosphere at present is the suite of industrially produced chlorinated hydrocarbons. These are sometimes known as chlorocarbons, chlorofluorocarbons (CFCs) or just fluorocarbons (somewhat of a misnomer since not all fluorocarbons contain chlorine). The large number of CFCs are discussed in more detail below.

4.5 Chlorofluorocarbons (Including the Halons)

4.5.1 CFC Molecules: Descriptions and Naming Conventions -- The chlorofluorocarbons (CFCs) are a set of compounds based mainly on methane (CH4) and ethane (C2H6) with chlorine, fluorine, and bromine substituted for some or all of the hydrogen atoms. A carbon atom has the potential for making four bonds with various kinds of atoms. In methane, all four bonds are with hydrogen atoms. If one hydrogen atom is replaced with a chlorine atom, the compound is methyl chloride (CH3Cl). If all of the hydrogen atoms are replaced by chlorine and fluorine, a number of different molecules can be produced. One of these is CFC-11 or CFCl3 with one fluorine and three chlorine atoms bonded to the carbon. CFC-12 is CF2Cl2 with two fluorine atoms and two chlorine atoms bonded to carbon.

The numbers attached to these CFCs are part of a naming convention. This convention is in the form CFC-xyz where x, y, and z stand for
x = the number of carbon atoms minus 1
y = the number of hydrogen atoms plus 1
z = the number of fluorine atoms.

Thus, CFC-11 has one carbon atom since x=0, no hydrogen atoms since y=1, and one fluorine atom since z=1. Technically it should be written as CFC-011, but the zero is usually dropped. Given this formulation, the number of chlorine atoms has to be inferred from the knowledge that there are four available bonds for a single carbon atom. Thus CFC-11, with one F atom and no H atoms, has three bonds availble for chlorine atoms.

For a two-carbon molecule, one bond for each carbon is used to bond to the other carbon. Thus, there are six bonds available as in C2H6. CFC-113 would be C2F3Cl3 (work it out!).

4.5.2 Properties of CFC's -- An extremely important property of most CFCs is their chemical inertness. They are tightly bound, nonreactive molecules. They are not soluble in water. They don't absorb visible or near-ultraviolet radiation. These are the properties which give them their long atmospheric residence times and make them candidates for delivery of their constituent atoms to the stratosphere. These are also the properties which make them useful for many applications. Table 1 lists some of the most important CFCs and gives their atmospheric lifetimes.

Table 1: Estimated CFC Lifetimes (WMO, 1994)


Chem Formula

Conc (1992)




0.27 ppbv

50 years



0.50 ppbv

100 years



0.082 ppbv

85 years



0.020 ppbv

300 years


0.13 ppbv

42 years

An important set of properties of the CFCs is their thermodynamic characteristics. When placed under pressure at room temperature, they condense (liquify) so that they can be stored in a much smaller volume such as a can. Their nonreactivity also made them an ideal carrier gas for aerosol sprays. When the pressure is released by a valve, they expand rapidly and exit the can through the valve in a fine spray carrying along any product being also in the can (hair spray or deodorant, for example.). Or the CFCs could simply be in the container without anything else and can be sprayed on the skin. The rapid expansion through the valve causes cooling so that the application area will be rapidly chilled.

CFCs were first used as refrigerants, replacing compounds like ammonia. Their lack of reactivity rendered them non-toxic in direct contrast to ammonia. Servicing a refrigerator became a less dangerous task. Their use in the compression-expansion cycles of refrigeration is possible because they can be compressed and liquified readily at normal temperatures. Mechanical energy (converted from electricity) can be used to compress the refrigerant. When it liquifies it gives up energy to its surroundings. When it reexpands it takes energy from its surroundings. The refrigerator is arranged so that the expansion takes energy from inside the refrigerator and the compression puts the excess energy into the air behind or around the refrigerator.

Another property of some specific CFCs, called halons, is their chemical reactions at high temperatures. Halons are CFCs that also contain bromine. They are inert and long lived at normal atmospheric temperatures. They are used in fire extinguishers because at higher temperatures the bromine participates in chemical reactions which actually scavenge important chain-propagating species from the fire. The rapid oxidation reactions that are sustaining the fire by giving off heat are stopped, and the fire is put out without having to drown it with water or smother it with CO2. The biggest use of halon extinguishers is in computer rooms and in military vehicles such as ships and airplanes.

4.5.3 CFC Usage Statistics -- In 1974 the predominant use of CFCs was as propellants in aerosol cans (hair spray, deodorants, etc.). The propellant usage amounted to 69% of an estimated production of 970 million kilograms (~2 million pounds). Usage as refrigerants made up another 18%, while cleaning agents were 6% and foam blowing 5% (see Figures 10.05a, b, and c). By 1986 the total production was up to 1130 million kilograms, but the use as propellants was only 28% of the total. This is about the time the Montreal Protocol (discussed below) was put into place, and is nearly a decade after the first regulations were passed in the United States outlawing the usage of CFCs in spray cans. By 1991 the total production was down to 68 million kilograms and the aerosol usage was only 18% of this total. Refrigeration was up to 32% of the total. Although the total amount used in refrigeration had decreased, the total production decreased even more, so that the fraction used in refrigeration increased. In 1991 cleaning agents made up 20% of the usage and foam blowing another 28%. All of this information is summarized in Figure 10.05 as pie charts for 1974, 1986, and 1991.

4.5.4 CFC Lifetimes and Trends -- The first chlorofluorocarbons were invented by Midgley at DuPont in the 1930s. Prior to that time the atmospheric concentrations of these compounds were negligible. If we begin from zero concentration and begin to inject a long-lived compound into the atmosphere at a constant rate, its concentration will build up until a steady-state is reached. The concentration will reach about 2/3 of the steady-state value in one atmospheric residence time and 95% in 2 atmospheric residence times. This means that a chlorofluorocarbon with a 50 year residence time will reach 95% of its steady-state concentration in 100 years, assuming that the source rate is kept constant during this entire time.

What actually happened for most CFCs such as CFC-11 and CFC-12 (with lifetimes of 50 and 100 years, respectively) was that their emission rates into the atmosphere increased exponentially during the 1960s and 1970s. This meant that the steady-state value (towards which their atmospheric concentrations were increasing) continually changed to higher values. As Figures 10.06a and b and 10.07a and b show, when production was first slowed in the late 1970s and then decreased rapidly in the late 1980s and 1990s, the atmospheric concentrations continued to increase because they were still well below the steady-state concentrations consistent with the new source rates. Only recently have the source rates for CFC-11 and CFC-12 decreased to the point where the steady-state concentration is less than the atmospheric concentration so that the atmospheric concentrations are beginning to decrease.

Halon usage is still small compared to the usage of CFC-11 and CFC-12. The atmospheric concentrations of the two major halons (1211 and 1301) are still increasing, as shown in Figure 10.08. The chemical formulas and lifetimes of these halons are given in Table 2. The halons are particularly important because they can deliver bromine to the stratosphere. Bromine is a stronger catalyst for ozone loss than chlorine because of the reaction of BrO with itself and because of the reaction of BrO with ClO. (For details see Chapter 5.)

Table 2: Estimated Halon Lifetimes (WMO, 1994)


Chem Formula




20 years



65 years

4.5.5 Hydrochlorofluorcarbons (HCFCs) and Hydrofluorocarbons (HFCs) -- With the restrictions on the usage of the primary fully halogenated CFCs (i.e., those containing no hydrogen atoms), replacements had to be found which could perform the same functions. The most obvious requirements of a replacement are that it have nearly the same physical properties and that it have a shorter atmospheric lifetime. This can be accomplished by replacing one or more of the halogens by a hydrogen atom.

A CFC with one or more halogens replaced by hydrogen is called a hydrochlorofluorocarbon or HCFC. The key chemical property resulting from the presence of a hydrogen atom is the reaction of the HCFC with hydroxyl radicals (OH). This reaction extracts the hydrogen atom and combines it with OH to form water. The resulting fragment is a reactive radical and rapidly reacts to convert the chlorine and fluorine atoms to oxides and then to the acids HCl and HF. The reaction with OH occurs rapidly enough that much of the HCFC is mostly destroyed in the troposphere where the HCl and HF can be dissolved in water and rained out of the atmosphere. Only a small fraction of the HCFC makes it to the stratosphere.

The reaction rates of HCFCs are measured in the laboratory. For molecules with faster reaction rates with OH, the lifetimes are shorter and a smaller portion of the chlorine release occurs in the stratosphere. So the replacement affects chlorine delivered to the stratosphere in two ways; shorter lifetimes mean less buildup of the gas for a given amount of release; and less of the chlorine that is released is in the stratosphere. Table 3 gives a list of several important HCFCs and their lifetimes.

Table 3: Estimated HCFC Lifetimes (WMO, 1994)


Chem Formula




13.3 years



1.4 years



5.9 years



9.4 years



19.5 years



2.5 years



6.6 years

(*) The letters a,b,c refer to different arrangements of the atoms in the molecule (specific chemical isomers)

Some replacement compounds include hydrogen but have all of the chlorine removed. These are hydrofluorocarbons or HFCs. They have zero potential for delivering chlorine to the stratosphere. These frequently have relatively long lifetimes and can accumulate in the atmosphere. While they deliver no chlorine to the stratosphere, they are absorbers of infrared radiation and can contribute to global warming (as can CFCs and HCFCs). A number of these compounds are listed below in Table 4 along with their lifetimes in the atmosphere.


Table 4: Estimated HFC Lifetimes (WMO, 1994)


Chem Formula




250 years



6.0 years



36 years



11.9 years



14 years



3.5 years



55 years



1.5 years



41 years



250 years



7 years

(*) The letters a,c,e,f refer to different arrangements of the atoms in the molecule (isomers)

Most of these replacement compounds have only been recently produced, and released to the atmosphere. An exception is HCFC-22, which has been in use in home and industrial air conditioners for quite sometime now. The measured atmospheric concentration of HCFC-22 is shown in the top panel of Figure 10.09. Note that its concentration is increasing in time and is a little over 100 parts per trillion by volume (pptv). The other two panels of Figure 10.09 show the measured atmospheric concentrations of HCFC-142b and HCFC-141b as a function of time for the last several years. Both of their concentrations are small, but are increasing at a relatively rapid rate as they come into increased use as replacement compounds.

4.6 Chlorine From Solid Rocket Boosters

Another potential source of chlorine to the stratosphere is from solid rocket boosters. The space shuttle uses two strap-on solid rocket boosters that are ejected as the main engine takes over and boosts the system into its final orbit. These solid boosters use a fuel of ammonium perchlorate which is contained in an aluminum structure. The fuel burns to give many products among which are hydrochloric acid (HCl) and alumina (Al2O3). As the shuttle passes through the stratosphere on its way to orbit, a fraction of the exhaust products, including HCl, are deposited there. The estimated amount of chlorine deposited in the stratosphere from 9 shuttle launches per year and 6 Titan IV launches per year is about 0.7 million kilograms.

4.7 Methyl Bromide

Methyl bromide (CH3Br) is widely used as a pesticide or fumigant. It is used on crops like strawberries which grow close to the ground and are subject to a number of pests. A field is covered with a large plastic sheet and CH3Br is injected underneath this cover to kill all of the pests. The field is then uncovered and the crop can be grown. Some of this methyl bromide escapes to the atmosphere.

As mentioned previously, the ocean is also a natural source for CH3Br. Methyl iodide (CH3I) is produced by seaweed. When released into the ocean, CH3I reacts with bromide ions dissolved in seawater to form CH3Br and release iodide ions. The CH3Br thus produced can escape to the atmosphere or it can undergo further reaction with chloride ions in the seawater to form methyl chloride (CH3Cl). The release of this CH3Cl to the atmosphere is the major natural source of stratospheric chlorine. It accounts for about 0.6 ppbv of chlorine.

Once CH3Br is released to the atmosphere, it reacts with OH with a lifetime of a little more than 1.5 years. Thus much of it is destroyed in the troposphere. But the remaining portion is transported to the stratosphere where it is a source for bromine oxides, extremely powerful catalysts for stratospheric ozone loss.

The ocean can also serve as a sink for CH3Br. Atmospheric CH3Br can be dissolved in the ocean where reaction with chloride ions produces CH3Cl. The resulting atmospheric amounts are plotted in Figure 10.10. This process reduces the atmospheric lifetime of CH3Br to about one year. There remain uncertainties as to whether or not there is an additional natural source of CH3Br. The production/loss budget has not yet been balanced. The current atmospheric concentration is a little more than 10 pptv. There does not yet exist a long enough time series to determine by how much the concentration is increasing due to anthropogenic sources.

4.8 Montreal Protocol

In 1987 the governments of the world, through the United Nations Environment Program (UNEP), agreed to a protocol to limit the production and release of a variety of CFCs. This protocol was put forward at a meeting in Montreal, Canada and has become known as the Montreal Protocol. It has been ratified or accepted by 165 countries.

Since the original protocol, its provisions have been amended; in 1990 at a conference in London and in 1992 at a conference in Copenhagen. As of mid-1998, these amendments have been ratified or accepted by 120 and 78 countries respectively. In 1997, a further amendment was adopted at another conference in Montreal. See the homepage of UNEP at http://www.unep.ch/ozone for further information.

The Montreal Protocol put forward schedules for phaseouts of various CFCs and other ozone depleting substances (e.g., halons) based on their calculated "ozone depletion potential (ODP)". These calculations compared the projected ozone loss for a given release of a CFC compared to that for the same release of CFC-11. Substances with an ODP close to that for CFC-11 were scheduled for rapid phaseout. Those with lower ODPs were scheduled for slower phaseout. Replacement substances such as HCFCs were scheduled to buildup over a period of time as they served as replacements, to be phased out later in favor of substances with even lesser (or zero) ODPs.

The effects of the protocol can already be seen in the slowing down and reduction of the atmospheric concentrations of some CFCs such as CFC-11 as seen in Figure 10.11. It can also be seen in the buildup of some of the replacement substances as illustrated in Figure 10.09.

The chlorine amount in the stratosphere can be estimated from the combination of all of the known sources. The amount in the stratosphere is the sum of all of the chlorine amounts in the CFCs, HCFCs, and CH3Cl in the troposphere with the proviso that there is about a 3-5 year delay for changes in the tropospheric amounts to reach the stratosphere.

Figure 10.11 shows an estimate of the past contributions of the various compounds to chlorine and a projection into the future, assuming that the provisions of the Montreal Protocol and subsequent amendments are followed. The chlorine amount increases from less than 2 ppbv in the mid 1970s to a peak of about 3.8 ppbv in 1994. With the 3-5 year time delay to reach the stratosphere, the chlorine amount in the stratosphere should be peaking about now (1998). The projected chlorine amount then declines slowly reaching about 2 ppbv in 2050. Note that the decline is somewhat slowed between now and 2020 because of the continued increase of some of the replacement compounds. These are then phased out and the decline is more rapid because of their relatively short lifetimes. The major remaining CFCs are the long-lived ones CFC-11 and CFC-12, which have very long lifetimes and will remain in the atmosphere long after their phaseout is complete.