The Ozone Shield

A gaseous shield,

Essential,

Dynamic,

Destructible.

 

         Stratospheric ozone (O₃) depletion is an environmental problem of global proportions. A 90% increase in skin cancer in Americans over the past 30 years has been largely attributed to ozone depletion. Moreover, O₃ depletion increases the incidence of cataracts and may weaken the human immune system. In Australia, which is close to the Antarctic ozone hole, children are required by law to wear hats and long-sleeved shirts when outdoors as protection against UV irradiation that is normally absorbed by stratospheric O₃.

         The causes of ozone destruction are fairly well understood, and a Nobel Prize has been given for studies that led to this understanding. The anthropogenic emission of certain chemicals is largely responsible. Based on extensive scientific evidence, international agreements have restricted the use of these ozone-destroying chemicals, particularly the chlorofluorocarbons. As a result of these agreements, concentrations of these compounds in the atmosphere have already stabilized, giving hope for future closure of the ozone hole.

         The composition of dry air at ground level is roughly 80% nitrogen (N₂) and 20% oxygen (O₂). Other gases are present in trace amounts, but many of these trace gases are important for normal temperature maintenance and for ozone retention. Present in excess, these gases cause global warming, changing our environment and causing extensive species extinction. Thus, in addition to water vapor, tropospheric carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃) are all greenhouse gases that are produced as a result of various man-related activities. Tropospheric ozone (at the surface of the Earth and up about 10 kilometers) is harmful, both because of its greenhouse effect and because of its oxidizing properties. However, stratospheric ozone (roughly from 10 to 50 kilometers above the Earth's surface) is not just beneficial, it is essential for terrestrial plant and animal life, most of the life on Earth as we know it. But man-produced trace atmospheric chemicals called chlorofluorocarbons, particularly CFCl₃ or CFC-11 and CF₂Cl₂ or CFC-12, are largely responsible for the stratospheric ozone depletion that has occurred in recent decades.

         How does the stratospheric ozone shield protect life on Earth? It absorbs over 99% of the sun's hazardous ultraviolet (UV) radiation. In fact, because of the ozone shield, the most dangerous form of UV radiation, UV-C, never reaches the Earth's surface. It is completely absorbed in the stratosphere where it functions beneficially to convert O₂ into O₃ via the two reactions: (1) O₂ + UV ® 2O, and (2) O + O₂ ® O₃. UV radiation of intermediate energy, UV-B is the form of ultraviolet radiation that causes cancer and damages life at the Earth's surface. This is the type of radiation that stratospheric O₃ protects against but does not completely eliminate. The least energetic form of UV, UV‑A, is not appreciably absorbed by O₃, and fortunately, it causes minimal damage to living organisms.

         The "ozone hole" over Antarctica is the region of the Earth's stratosphere where the O₃ concentration is lowest. Human activities since 1970 have reduced this concentration to less than one-third of its previous value. Which man-made compounds are destroying O₃, and why do we produce it? In 1974, two scientists, Mario Molina and F. Sherwood Rowland, who 21 years later shared the Nobel Prize in Chemistry with Paul J. Crutzen, proposed that chlorofluorocarbons (CFCs or "freons") are primarily responsible. These and other related compounds are used as aerosols in spray cans of insecticides, paints, deodorants, shaving creams and hair sprays. They are also used as refrigerants, in air conditioners, as cleaning solvents, and in fire extinguishers. Worldwide, we produce over a million metric tons of CFCs every year. While the production of some of these is now under control, due to international agreements, others are still being made in alarming quantities.

         Molina and Rowland's proposal included three features, all of which were based on firm experimental evidence and principles generated in part by Paul Crutzen. First, they noted that CFCs are extremely stable, lasting about 100 years in the troposphere. Second, they suggested that through atmospheric mixing, CFCs can enter the stratosphere where solar UV breaks them down into reactive chlorine (Cl). Finally, they showed that chlorine reacts with O₃ to break it down, depleting the supply. Simplified reactions, first demonstrated by Ralph Cicerone, that largely account for O₃ destruction include:

(1)           Cl + O₃ Õ ClO + O₂

(2)           O₃ + UV Õ O₂ + O

(3)     ClO + O Õ Cl + O₂

         ________________________

         Net:   2O₃ Õ 3O₂

Because the chlorine free radical, Cl, is regenerated, it creates a catalytic chain reaction that can cause the destruction of up to 100,000 O₃ molecules per Cl generated. Consequently, you can easily imagine how much damage a million metric tons of CFCs can do! And this is the amount we produce in just one year!

These reactions involving Cl are similar to but much more potent than reactions that had been demonstrated previously by Paul Crutzen involving nitrogen oxides, produced, for example, by supersonic aircraft. These analogous reactions are:

         (1)     NO + O₃ Õ NO₂ + O₂

         (2)     O₃ + UV Õ O₂ + O

         (3)     NO₂ + O Õ NO + O₂

         _________________________

         Net:   2O₃ Õ 3O₂

          How big is the Antarctic ozone hole? In 1980, it encompassed about one million square kilometers, but by 1990 it covered over 20 million square kilometers. Today it is only a little larger, roughly 24 million square kilometers, a surface area that however exceeds the entire area of North America! Happily, the rate of expansion of this hole has slowed due to international agreements restricting CFC production in developed countries. This success leads to the hope that similar agreements can eventually protect us against other types of environmental damage.

         Why is O₃ depletion greatest over Antarctica? The answer is related to the low temperatures that promote formation of polar stratospheric clouds, present in the lower stratosphere, between 14 and 24 km high. These clouds can be huge: 10-100 km wide and several kilometers thick. They create an eerie beauty with an iridescent glow, the color of mother-of-pearl. But unfortunately for us, they accelerate O₃ destruction by chlorine and other free radical compounds.

         Polar stratospheric clouds form during the polar winter when there is little sunlight. During these periods, the Antarctic air mass circulates only above the pole, and there it cools, condenses and descends. This isolated, circulating Antarctic air mass is known as the Antarctic polar vortex. Its temperature drops to about -80ºC (‑110ºF). At these low temperatures, small sulfuric acid (H₂SO₄) droplets, about 0.1 mm in diameter, freeze and serve as seed particles for nitric acid (HNO₃). Water vapor can then condense around them increasing their size. These particles give the clouds their beautiful mother-of-pearl color, but because they are large, the particles yield to gravity and fall out of the stratosphere into the troposphere taking most of the beneficial nitrogen oxides with them. When the sunlight returns in the early spring, there are practically no nitrogen oxides left in the stratosphere to quench or inhibit the chlorine-induced destruction of ozone. Under these conditions, as much as 2% of the local O₃ may be destroyed in a single day!

          There is an arctic polar vortex over the North Pole that similarly stimulates O₃ destruction, but it is generally weaker and shorter lived than the Antarctic polar vortex. When on occasion it persists, local O₃ loss can be substantial. Subsequently, when these arctic polar vortices warm and break up, O₃ deficient air masses move southward where they drift over populated areas of North America and Europe. This happened in the Spring of 1995 after a particularly severe Arctic winter. A tremendously increased risk to human health resulted.

         There are many deleterious consequences of ozone depletion in addition to those on human and animal health. Damage to food chains on land and in the oceans can radically upset these systems, causing extensive death and, in severe cases, species extinction. For example, a 10% loss of marine surface photosynthetic algae and bacteria (phytoplankton) in the Antarctic oceans, due to O₃ loss, has been documented, and these microscopic organisms are at the base of the food chain! As these organisms also remove CO₂ from the atmosphere, O₃ depletion can exacerbate global warming. Moreover, ozone depletion negatively impacts agricultural productivity. Diminishing food availability, even to the extent of 10%, can have tremendous social and political consequences, particularly in poorer countries around the world. It is amazing how detrimental our well-intentioned technological activities can be.

In 1987, a diplomatic achievement of monumental proportions was the signing of the Montreal Protocol by 146 nations. This protocol outlined a plan to reduce global emissions of CFCs to 50% of the 1986 levels. Most industrialized countries, including the U.S., had largely stopped CFC production by 1996. An eventual total phase-out is planned. If this and subsequent protocols are strictly followed, it will nevertheless take until 2050 to return to the 1980 CFC emissions levels. Since these compounds are long lived, this means that ozone depletion may continue for many decades. However, stratospheric CFC levels have at least begun to stabilize, giving hope that their effects will eventually be reversed.

         There is tremendous need for the development of cheap, safe and effective substitutes for CFCs. Hydrofluorocarbons (HFCs) which lack chlorine offer possibilities and are under study. But to a lesser extent, these can also cause O₃ depletion. Other possible alternatives are being developed. However, scientific research is a slow and expensive process. We have no guarantees that suitable safe replacements will be found. If they are, what unexpected side effects will subsequently be discovered? There are many unknowns that emphasize the precarious nature of our existence, an existence that depends on the maintenance of the fragile environmental conditions that support life.

 

 

See http://nobelprize.org/chemistry/laureates/1995/press.html

 

Graedel, T.E. and Crutzen, P.J. (1995). Atmosphere, Climate and Change. Scientific American Library.

Rowland, F.S. and Molina, M.J. (1994). Ozone depletion: 20 years after the alarm. Chemical & Engineering News 72, 8-13.

Toon, O.B. and Turco, R.P. (1991). Polar stratospheric clouds and ozone depletion. Scientific American 264, 68-74.