Self-study
By 1970 it was well established that ultraviolet light from the sun contributes to skin cancer. Fortunately, much of the sun's most damaging ultraviolet radiation is screened out by a thin, diffuse layer of ozone—a toxic form of oxygen—in the stratosphere, 10 to 25 miles above the earth's surface.
During the 1970s, however, public policy makers worldwide were alerted to the fragility of the ozone layer through the pioneering research and advocacy of two Nobel Prize-winning scientists, Mario Molina and F. Sherwood Rowland. In the absence of pollutants, stratospheric ozone concentrations should remain stable over time, with natural production and destruction of the gas in rough equilibrium. Molina and Rowland showed how manufactured chlorofluorocarbons (CFCs)—highly volatile chemicals, millions of tons of which had been used each year in products such as aerosol sprays and refrigerants—chemically attack and deplete the ozone layer, diminishing its effectiveness as a shield against ultraviolet radiation. Studying two freon gases—types of CFCs—they observed that, when released into the lower atmosphere (troposphere), these gases slowly diffuse upward into the stratosphere. There, subjected to massive ultraviolet radiation, they break down into their constituent elements, including chlorine. The resulting increase in the concentration of chlorine in the stratosphere is devastating to the ozone layer. Chlorine and ozone chemically react in a way that both destroys the ozone and regenerates the chlorine atoms. As a result of this chemical reaction, each chlorine atom could destroy as many as 100,000 ozone molecules before becoming inactive.
In 1974 the two scientists estimated that the atmosphere contained the accumulation of five years of global CFC production. This meant that, given the rate of diffusion and breakdown of CFCs in the atmosphere, the depletion of the ozone layer would continue for years, if not decades, even if the production and use of CFCs were to cease immediately. Recognizing this as a pressing environmental threat, Molina and Rowland became public advocates for a prompt and proportionate public policy response. As a result, Molina was invited to testify before the U.S. Congress and was later appointed to the U.S. National Science Foundation Committee on Fluorocarbon Technology Assessment.
Predictably, the work of Molina and Rowland and their advocacy of dramatic policy changes were subjected to attacks by critics, especially scientists with ties to the CFC industry. However, over time their views were corroborated, especially by the discovery of a hole in the ozone layer over Antarctica, and this led to the development of an international agreement (the Montreal Protocol of 1987) to ban the production of ozone-depleting gases. In North America, CFCs were banned in the late 1970s, leading to a transformation in packaging for consumer spray products and the development of more environmentally friendly refrigerant chemicals.
26.
Based on the passage, the information yielded by which one of the following experiments would be most useful in determining whether a particular chemical could replace CFCs without damaging the ozone layer?
a
testing to see whether the chemical is capable of reacting with forms of oxygen other than ozone
It’s not clear that forms of oxygen other than ozone are relevant to the process by which CFCs can damage the ozone. So we have no reason to think (A) would be useful.
b
testing to see whether the chemical, when released into the lower atmosphere, would react with other chemicals commonly found there
The reaction between chlorine and ozone occurs in
c
testing the chemical to determine whether it would chemically react with chlorine
Damage to the ozone layer results when chlorine reacts with ozone. What matters is whether the replacement chemical would react with ozone, not with whether it would react with chlorine.
d
testing to see what chemical properties the chemical or its constituent elements share with chlorine
This would be useful because the damaging element in CFCs is chlorine. Chlorine reacts with ozone in a way that destroys ozone. If the replacement chemical for chlorine has very similar chemical properties as those in chlorine, that raises the chance the replacement chemical might also destroy ozone. But if the replacement chemical has very few chemical properties shared with chlorine, that provides evidence it is less likely to destroy ozone.
e
testing the chemical to see if it would break down into its components when subjected to ultraviolet radiation
What matters is whether the replacement chemical destroys ozone. The fact CFCs broke down into constituent chemicals mattered only because one of those chemicals was chlorine, which destroys ozone. But the breaking down into constituent chemicals wouldn’t matter if none of the chemicals in CFC could damage the ozone layer. So when we’re considering a replacement chemical for CFCs, we don’t care whether the chemical would break down. We care about whether the chemical can damage ozone (in the same way that chlorine can damage ozone).