A Hole in the Sky: Ozone Depletion - Ozone-depleting Chemicals

Most ozone destruction in the atmosphere is believed to be anthropogenic (caused by humans). In 1999 the World Meteorological Organization in Geneva, Switzerland, estimated that only 18 percent of the sources contributing to ozone depletion were natural. The remaining 82 percent of sources contributing to ozone depletion were industrial chemicals. The blame is largely placed on chemicals developed by modern society for use as refrigerants, air conditioning fluids, solvents, cleaning agents, and foam-blowing agents. These chemicals can persist for years in the atmosphere. Thus, there is a significant lag between the time that emissions decline at the Earth's surface and the time at which ozone levels in the stratosphere recover.

Table 3.1 lists the chemicals of particular concern to scientists worried about thinning ozone levels. Each chemical is assigned a value called an ozone depletion potential (ODP) based on its harmfulness to the ozone layer. The most common depleters are the chlorofluorocarbons known as CFC-11, CFC-12, and CFC-13. Each of these chemicals is arbitrarily assigned an ODP of 1. The ODPs for other chemicals are determined by comparing their relative harmfulness to that of CFC-11. Class I chemicals are those with an ODP value greater than or equal to 0.2. Class II chemicals have ODP values less than 0.2.

Class I Chemicals

Although a number of chemicals can destroy stratospheric ozone, CFCs are the main offenders because they are so prevalent. When CFCs were invented in 1930, they were welcomed as chemical wonders. Discovered by Thomas Midgley, Jr., they were everything the refrigeration industry needed at the time—nontoxic, nonflammable, noncorrosive, stable, and inexpensive. Their artificial cooling provided refrigeration for food and brought comfort to warm climates. The compound was originally marketed under the trademark Freon.

Over time, new formulations were discovered and the possibilities for use seemed endless. CFCs could be used as coolants in air conditioners and refrigerators, as propellants in aerosol sprays, in certain plastics such as polystyrene,

TABLE 3.1
Lifetime and ozone depletion potential of various chemicals

	Lifetime in years	Ozone depletion potential
CLASS I
CFC-11	45	1
CFC-12	100	1
CFC-13	640	1
CFC-113	85	0.8–1
CFC-114	300	0.94–1
CFC-115	1700	0.44–0.6
Halon 1211	16	3–6
Halon 1301	65	10–12
Halon 2402	20	6–8.6
Carbon tetrachloride	26	0.73–1.1
Methyl bromide	0.7	0.38–0.6
Methyl chloroform	5	0.1–0.12
CLASS II
HCFC-21	1.7	0.04
HCFC-22	1.2	0.05–0.055
HCFC-123	1.3	0.02–0.06
HCFC-124	5.8	0.02–0.04
HCFC-141b	9.3	0.1–0.12
HCFC-142b	17.9	0.06–0.07
HCFC-225ca	1.9	0.02–0.025
HCFC-225cb	5.8	0.03–0.033
The ODP is the ratio of the impact on ozone of a chemical compared to the impact of a similar mass of CFC–11. Thus, the ODP of CFC–11 is defined to be 1.0. Other CFCs and HCFCs have ODPs that range from 0.01 to 1.0.
SOURCE: Adapted from "Class I Ozone-Depleting Substances," and "Class II Ozone-Depleting Substances," in Ozone Depletion Chemicals, U.S. Environmental Protection Agency, Washington, DC, February 12, 2004 [Online] http://www.epa.gov/ozone/ods.html [accessed April 10, 2004]

in insulation, in fire extinguishers, and as cleaning agents. World production doubled every five years through 1970, and another growth spurt occurred in the 1980s as new uses were discovered—primarily as a solvent to clean circuit boards and computer chips.

CFCs are extremely stable; it is this stability that allows them to float intact through the troposphere (the layer of air nearest the Earth's surface) and into the ozone layer. They reach the stratosphere after six to eight years. Once there, some can survive for hundreds of years. CFCs do not degrade in the lower atmosphere but, on entering the stratosphere, they encounter the sun's UV radiation and eventually break down into chlorine, fluorine, and carbon. Many scientists believe it is the chlorine that damages the ozone layer. (See Figure 3.4.)

While CFCs are primarily blamed for ozone loss, other gases are also at fault. One of those gases is halon, which contains bromine. As shown in Table 3.1, halons have much higher ODP values than do CFCs. The bromine atoms in halons destroy ozone in a manner similar to that shown in Figure 3.4 for chlorine, but they are chemically more powerful. This means that the impact to ozone of a particular mass of halon is more destructive than a similar mass of a CFC. Halons are relatively longlived FIGURE 3.4
Destruction of ozone in the atmosphere, lingering for up to sixty-five years before being broken down. Halon is used primarily for fighting fires. Civilian and military fire-fighting training accounts for much of the halon emission.

Other Class I ozone destroyers include carbon tetrachloride, methyl bromide, and methyl chloroform. These chemicals are commonly used by industry as solvents and cleaning agents.

Class II Chemicals

The most common Class II ozone-depleting chemicals are hydrochlorofluorocarbons (HCFCs). HCFCs contain hydrogen. This makes them more susceptible to atmospheric breakdown than CFCs. As shown in Table 3.1, most HCFCs have a lifetime of less than six years. The most long-lived, HCFC-142b, lasts for only 17.9 years. HCFCs have much lower ODP values than CFCs, halons, and industrial ozone depleters. HCFCs are considered good short-term replacements for CFCs. Although HCFCs are less destructive to ozone than the chemicals they are replacing, scientists believe that HCFC use must also be phased out to allow the ozone layer to fully recover.