HORNO CONTINUO

Above the troposphere is the stratosphere, where air flow is mostly sideways. There is a gradual change from the troposphere to the stratosphere, which starts at around 14 km in altitude. The stratosphere extends from 14 km to around 50 km. Most commercial aircraft travel takes place in the lower part of the strato- sphere. Military aircraft travel at much higher altitudes: Some classified stealth aircraft are thought to graze the boundary of the mesosphere and beyond. NASA’s Space Shuttle generally travels to altitudes between 160 and 500 km.

Although the temperature in the lower stratosphere is cold and constant, hov- ering around at –57°C, there are strong winds in this layer that are part of spe- cific circulation patterns. Extremely high and wispy clouds can form in the lower stratosphere. In general, there are no major weather formations that take place regularly in the stratosphere.

The stratosphere has an interesting feature from midlevel on up. Its tempera- ture jumps up suddenly with an increase in altitude. Instead of a frosty –57°C, the temperature jumps up to a warm 18°C around 40 km in altitude in the upper

The troposphereis where all the local temperature, pressure, wind, and precipitation changes take place.

stratosphere. This temperature change is due to increasing ozone concentrations, which absorb ultraviolet radiation.

The melding of the stratosphere upward into the mesosphere is called the stratopause.

Ozone

Ozone is one of our atmospheric bodyguards. Even small amounts have an important role in protecting planetary life. Concentrated in a thin layer in the upper stratosphere, atmospheric ozone is an exceptionally reactive form of oxygen. It is found in the stratospheric layer, around 15 to 30 km above the Earth’s surface. The ozone layer is largely responsible for absorbing most of the sun’s ultraviolet (UV) radiation. Most importantly, it absorbs the fraction of ultraviolet light called UVB.

Ultraviolet radiation with a wavelength between 200 and 400 nanometers (nm) is usually divided into three main ranges of the spectrum. Table 3-1 shows these different ultraviolet categories and their characteristics.

Ultraviolet radiation is a bad, bad thing! It causes breaks in the body’s nuclear proteins, leaving the door open for cancers and other health issues to get a foothold. UVB has been connected with many serious health problems, like different kinds of skin cancer and cataracts. It is also harmful to certain crops, materials, and marine organisms.

Ozone is much less widespread than normal oxygen. The formation of the ozone layer is a tricky matter. Out of every 10 million air molecules, about 2 mil- lion are normal oxygen and only three are ozone molecules. Instead of two atoms of oxygen like normal oxygen molecules (O₂), ozone (O₃) contains three oxygen atoms. Ozone has a distinctive odor and is blue in color. Regular oxygen has no odor or color.

Only through the production of atmospheric oxygen can ozone form to block ultraviolet radiation from reaching the earth’s surface and the plants and animals

UV radiation type Wavelengths (nm) Effects on life

UVA 400–320 Fairly safe: tanning but not burning

UVB 320–290 Harmful: sunburn; skin cancer; other problems UVC 290–200 Very harmful, but mostly absorbed by ozone

living there. In the past 30 years, there has been intense concern over decreased ozone levels. This big problem must be solved if we want to go on enjoying the outdoors and growing food in the centuries to come!

Ozone molecules are constantly created and destroyed in the stratosphere. The amount is relatively constant and only affected naturally by sunspots, seasons, and latitude. Atmospheric scientists have studied and recorded these annual and geographical fluctuations for years. There are usually yearly cyclic downturns in ozone levels, followed by a recovery. However, as our population increases along with industrialization, global atmospheric changes are taking place as well.

There are other bit players in the stratosphere. These are collections of droplets called polar stratospheric clouds. These unique cloud-like condensa- tions of trace chemicals condense in the cold (–80° C or below) Southern Hemisphere’s winter. During this time, the atmospheric mass above Antarctica is kept cut off from exchanges with midlatitude air by prevailing winds known as the polar vortex. This leads to very low temperatures, and in the cold and continuous darkness of the season, polar stratospheric clouds are formed that contain chlorine. These clouds are often a combination of water and nitric acid. Although very weak, they affect the chemistry of the lower stratosphere by inter- acting with nitrogen and chlorine and providing surface area for other reactions to take place.

Chlorine and nitrogen interact in the atmosphere in ways that form chlorine nitrate. This compound does not interact with ozone or atomic oxygen, so it serves as a storage tank for chlorine in the environment. When polar stratos- pheric clouds form, they tie up stratospheric nitrogen so that it is not available to bind extra chlorine. This interference allows atomic chlorine to interact with ozone and destroy it.

The Antarctic winter (May to September) and the many months of very cold temperatures maintain this interference long enough for ozone levels to drop steeply. As warmer spring temperatures arrive, the combination of returning sun- light and the presence of polar stratospheric clouds leads to the splitting of chlo- rine into highly ozone-reactive radicals that break ozone apart into individual oxygen molecules. A single molecule of chlorine can break down thousands of molecules of ozone.

Ozone Depletion

For the past 50 years, chlorofluorocarbons (CFCs) held the answer to lots of material problems. They were stable, nonflammable, not too toxic, and cheap to produce. They had a variety of uses including applications as refrigerants, solvents, and foam-blowing agents.

Chlorine has been used for everything from disinfecting water to serving as solvents (methyl chloroform and carbon tetrachloride) in chemistry labs.

Unfortunately, these compounds are not so good for the atmosphere. They don’t just break down and disappear. They hang around. This lingering characteristic allows them to be carried by winds into the stratosphere. The net effect is to destroy ozone faster than it is naturally created. Roughly 84% of stratospheric chlorine comes from manmade sources, while only 16% comes from natural sources.

Unfortunately, CFCs break down only by exposure to strong UV radiation. When that happens, CFCs release chlorine. Scientists have found that one atom of chlorine can destroy over 100,000 ozone molecules. As CFCs decay, they release chlorine and damage the ozone layer.

Thirty years ago, researchers started looking at the effects of different chem- icals on the ozone layer. They looked at chlorine and its surface origins. Chlorine from swimming pools, industrial use, sea salt, and volcanic eruptions were found to be minor factors in ozone depletion. Because they mixed with atmos- pheric water first and quickly precipitated out of the troposphere, they didn’t reached the stratosphere.

Stable CFCs, however, act differently. There wasn’t anything in the lower atmosphere to cause them to break down. The outcry from scientists and envi- ronmentalists over ozone depletion led to a 1978 ban on the use of aerosol CFCs in several countries, including the United States.

In 1985, since other types of chlorine compounds were still being used, the policy of the Vienna Convention was adopted to gather international cooperation and reduce the number of all CFCs by half. It’s important to remember that just because CFCs were banned doesn’t mean that long-lived chemicals will disap- pear immediately from the atmosphere. Until CFCs degrade to negligible levels, the annual South Polar ozone hole will keep appearing for many years to come. The annual “hole” or thinning of the ozone layer over Antarctica was first noticed in 1985. This area of extremely low ozone levels showed drops of over 60% during bad years. No corresponding hole was found over the Arctic.

The European Space Agency’s Envisat Earth observation satellite records the arrival of the annual opening of the hole in the Earth’s ozone layer. Since it first appeared, satellites have been tracking its arrival and shape for years, and sci- entists have gotten good at predicting the conditions that create the opening.

The ozone hole usually shows up around the first or second week of September and then closes up again in November or December. When higher temperatures around the South Pole mix ozone-rich air into the region—causing the winds surrounding the South Pole to weaken—ozone-poor air inside the vortex is mixed with ozone-rich air outside it.

Envisat contains an instrument called the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography that provides new atmospheric data on the ozone layer every day.

This information presents a good way of eventually identifying long-term ozone trends.

Subsequent research found that some ozone thinning (though not as severe) also took place over the latitudes of North America, Europe, Asia, Australia, South America, and much of Africa. It became obvious that ozone decreases were a global concern.

In 1992, with new information on the ever-shrinking ozone layer, developed countries decided to totally stop production of halons by 1994 and CFCs by 1996. Halons are compounds in which hydrogen atoms of a hydrocarbon are replaced by bromine or fluorine. The halons are used as fire-extinguishing agents, both in built-in systems and in handheld fire extinguishers.

Halons cause ozone depletion because they contain bromine, which is a lot stronger than CFCs in destroying ozone. Halons are also very stable and break down slowly once formed. The United States’ halon production was stopped by December 31, 1993, because of its contribution to ozone depletion.

This course of action turned out to be what turned the tide in falling ozone levels. Levels of inorganic chlorine in the atmosphere stopped increasing in 1997–1998, and stratospheric chlorine levels peaked and are no longer rising. If nothing happens to change this trend, natural ozone recovery should mend the ozone layer in about 50 years.

However, we need to hold off on celebrations until we see whether this actu- ally takes place. Newly developed industrial chemicals must be watched as well. Whether the ozone layer has begun to recover is a hotly debated subject; scien- tists will know for sure only with time and continued monitoring.