21-9 Ozone Depletion in the Stratosphere

What Is the Threat from Ozone Depletion? A Clear Danger

The ozone layer in the lower stratosphere keeps about 95% of the sun’s harmful ultraviolet (UV) radiation from reaching the Earth’s surface. Measuring instruments on balloons, aircraft, and satellites show considerable seasonal depletion (thinning) of ozone concentrations in the stratosphere above Antarctica and the Arctic. Similar measurements reveal a lower overall loss of stratospheric ozone everywhere except over the tropics. Based on these measurements and on mathematical and chemical models, the overwhelming consensus of researchers in this field is that ozone depletion (thinning) in the stratosphere is a serious threat to humans, other animals, and some of the sun- light-driven primary producers (mostly plants) that support the Earth’s food.

What Causes Ozone Depletion? From Dream Chemicals to Nightmare Chemicals

Rowland and Molina’s research led them to four major conclusions. First, CFCs remain in the troposphere because they are insoluble in water and chemically unreactive. Second, over 11–20 years these heavier-than-air chemicals are lifted into the stratosphere mostly through convection, random drift, and the turbulent mixing of air in the troposphere. Third, once they reach the stratosphere, the CFC molecules break down under the influence of high- energy UV radiation. This releases highly reactive chlorine atoms (Cl), as well as atoms of fluorine (F), bromine (Br), and iodine (I), which accelerate the breakdown of ozone (O3) into O2 and O in a cyclic chain of chemical reactions, one of which is shown in Figure 21-23. This causes ozone in various parts of the stratosphere to be destroyed faster than it is formed. Finally, each CFC molecule can last in the stratosphere for 65–385 years, depending on its type. During that time, each chlorine atom released from these molecules can convert hundreds of molecules of O3 to O2. Overall, according to Rowland and Molina’s calculations and later models and atmospheric measurements of CFCs in the stratosphere, these dream molecules had turned into global ozone destroyers.

What Other Chemicals Deplete Stratospheric Ozone? More Culprits

CFCs are not the only ozone-depleting compounds (ODCs). Others are halons and hydrobromofluorocarbons (HBFCs) (used in fire extinguishers); methyl bromide (a widely used fumigant); hydrogen chloride (emitted into the stratosphere by space shuttles); and cleaning solvents such as carbon tetrachloride, methyl chloroform, n-propyl bromide, and hexachlorobutadiene. The oceans and occasional volcanic eruptions also release chlorine compounds into the troposphere. But most of these do not make it to the stratosphere because they dissolve easily in water and wash out of the troposphere in rain. Bromine compounds may be less likely to wash out of the troposphere, but further study is needed to confirm this possibility.

What Happens to Ozone Levels Over the Earth’s Poles Each Year? Levels Drop Each Winter and Spring

In 1984, researchers analyzing satellite data discovered that 40–50% of the ozone in the upper stratosphere over Antarctica disappeared during the Antarctic late winter and spring (August–November), especially since 1976. The observed loss of ozone above Antarctica often is called an ozone hole. A more accurate term is ozone thinning because the ozone depletion varies with altitude and location.

Measurements indicate that CFCs and other ODCs are the primary culprits. Each winter, steady winds blow in a circular pattern over the Earth’s poles. This creates a polar vortex: a huge swirling mass of very cold air that is isolated from the rest of the atmosphere until the sun returns a few months later.

As summer approaches and temperatures warm, the polar vortex begins to break up and mix again with the rest of the atmosphere. Then new ozone forms over Antarctica until the next dark winter. When the vortex breaks up, huge masses of ozone- depleted air above Antarctica flow northward and linger for a few weeks over parts of Australia, New Zealand, South America, and South Africa. This raises biologically damaging UV-B levels in these areas by 3–10%, and in some years as much as 20%.

Why Should We Be Worried About Ozone Depletion? Life in the Ultraviolet Zone

Humans can make cultural adaptations to increased UV radiation by staying out of the sun, protecting their skin with clothing, and applying sunscreens. However, plants and animals that help support us and other forms of life cannot make such changes except through biological evolution, a process that can take a long time.