Ozone is found both in the Earth’s troposphere, which extends from ground level to 7 miles up, and in the stratosphere, which is 7 to 30 miles above the Earth’s surface. Ozone is chemically identified as 0 3, which is an oxygen molecule ( 0 2) with an additional oxygen atom attached.
Tropospheric ozone is undesirable because it contributes to smog and has a strong odor.
Ozone within the Earth’s stratosphere is beneficial because it protects the Earth and its inhabitants from exposure to excessive amounts of ultraviolet radiation. In sufficient quantities, ultraviolet radiation can cause an increase in skin cancer and other health problems for humans and animals. It can also have an adverse effect on crops and other plant growth.
For millions of years, nature has created and destroyed ozone in the stratosphere in an environmentally-balanced process. With the introduction of man-made chemicals containing chlorine into the stratosphere, the process has become unbalanced. The result is that the ozone is being depleted faster than it is being generated. This allows too much harmful ultraviolet radiation to reach the Earth.
Chlorofluorocarbon (C FC ) refrigerants are considered the most damaging to the Earth’s ozone layer because they are chemically stable and can survive the long trip to the stratosphere. CFC refrigerants contain chlorine, fluorine, and carbon atoms. It is the chlorine that reacts with the sun’s ultraviolet radiation to alter the ozone molecules present in the stratosphere.
Figure 3-1 shows an example o f the ozone depletion process. Simply speaking, it involves the transfer of an oxygen atom from ozone to another molecule.
The process starts when ultraviolet radiation from the sun strikes a C FC -12 molecule, causing one of its chlorine (Cl) atoms to break off. This free chlorine atom then bonds with one of the three oxygen atoms in an ozone molecule and forms a molecule of harmful chlorine monoxide (CIO). Also, because the third oxygen atom is removed, the ozone molecule ( 0 3) is converted into an oxygen molecule (Oz). Thus an ozone mob ecule has been removed or depleted. The process continues when the oxygen atom in the chlorine monoxide molecule reacts with a free oxygen atom. The two oxygen atoms bond to form an oxygen molecule. This sets the remaining chlorine atom free again, allowing it to repeat the cycle by attacking and breaking down another ozone molecule.
HCFC refrigerants are not as harmful to the ozone layer as C FC refrigerants, but they still contribute to ozone depletion. HCFC refrigerants contain chlorine as well as hydrogen, fluorine, and carbon. It is the hydrogen in HCFCs that causes them to be less stable. This instability allows HCFC refrigerants to decompose in the troposphere. The result is that very few HCFC molecules reach the stratosphere intact to react with the ozone. HFC refrigerant molecules do not contain chlorine atoms; therefore, they do not contribute to ozone depletion.
A measure of the ability of a refrigerant to attack and deplete the ozone layer is called its Ozone Depletion Potential (ODP). The ODP values of all refrigerants are compared to C FC -11, which is the most harmful and thus is given an ODP of 1. The lower the ODP number, the less harmful the refrigerant is to the environment. Some common ODP values for refrigerants are:
Categories: Consideration | Leave a comment