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Atmospheric Chemistry

Atmospheric Chemistry


With each breath, the lungs inhale air filled with nitrogen and oxygen, the most abundant natural gases in the atmosphere. Also inhaled, however, are small quantities of gases and particles that are pollutants. Understanding the effects of these pollutants and how to control their emissions has been a continuing challenge for many centuries.

History

Humans have made a large impact on the state of the atmosphere ever since they first began using fire for light, cooking, and heat. When early populations traded their nomadic lifestyle for one based on agriculture, concentrations of air pollutants began to accumulate around permanent communities. With population growth came an increase in the use of fire, along with an increasing demand for wood.

In the twelfth and thirteenth centuries as wood became more difficult to find, growing cities began looking for alternate sources of energy. The solution was coalplentiful, cheap, long-lasting, and hailed as the perfect new source of energy. The only "inconvenience" was the heavy black smoke that resulted from its burning. It was not long, however, before this inconvenience became a serious health concern.

As industrialization increased over the next few hundred years, the burning of coal and wood produced some of the unhealthiest air ever recorded. In some European cities, hundreds of deaths were blamed on episodes of excessive smoke and soot. London's air was especially filthy: The mixture of smoke and fog (later termed "smog") was at times so thick that it affected visibility. As little was done to control the burning of coal, many cities in Europe, Asia, and the Americas suffered from poor air quality.

Although the burning of coal remains one of the largest global sources of air pollution, the rise of automobiles in the mid-twentieth century was another major source. This was especially true on the west coast of the United States, where the ever-rising automobile population, coupled with a large petroleum industry, generated a different type of pollutant; cities became known for "brown smog," a layer of pollution that forms in sunny skies and causes irritation to the lungs and eyes.

Acid Rain

Although Earth has its own sources of naturally produced air pollutants, humans have had a far larger effect on Earth's atmosphere. One of the most striking examples of this pollution is acidic deposition, or "acid rain." Acid rain occurs when emissions of sulfur dioxide and nitrogen oxides, which typically come from coal-burning power plants and automobile emissions, react with water and oxygen to form acidic compounds such as nitric acid and sulfur-containing acids, according to the following reactions:

S + O2 SO2

SO2 + O3 SO3 + O2

SO3 + H2O H2SO4

SO2 + H2O H2SO3

2NO + O2 2NO2

2NO2 + H2O 2HNO3 + NO

NO + O2 NO2

Once airborne, these pollutants can travel long distances before returning to Earth's surface as rain or snow, or in a dry form.

Not all atmospheric acidity is due to atmospheric pollution, as Earth also has natural sources of sulfur and nitrogen. Natural sources of sulfur dioxide include volcanoes and forest fires. Air is a natural source of nitrogen oxides, as is lightning, according to the following reactions:

O2 + N2 2NO

2NO + O2 2NO2

Even "normal" rain (rain that has not formed in a polluted atmosphere) is acidic because of the presence of carbon dioxide in the atmosphere:

CO2 + H2O H2CO3

Over the millennia, normal rain has created limestone caves because calcium carbonate is slightly soluble in solutions of H2CO3:

CaCO3 (s) + H2CO3 (aq) Ca(HCO3)2 (aq)

The additional burden of human air pollution, however, has made acid rain an important environmental concern. Acid rain may acidify lakes and streams, making the water unsuitable for some fish and other wildlife. Further damage has been reported in soil and tree vegetation, which are sensitive to the acid level of rainwater.

Acid rain also speeds the decay of buildings, statues, and other man-made structures. Natural treasures such as the Taj Mahal in India, the Acropolis in Greece, and cathedrals in Germany and Britain have suffered significant damage due to acid rain. Although large sums of money are being used to help repair these structures, some damage is beyond repair.

Many governments have enacted controls on the chemicals responsible for acid rain. Although there have been some improvements in acid levels in lakes and streams, there are still many scientists who believe that stricter controls are necessary to reduce the risk to Earth's land and water.

Fluorocarbons and Ozone Depletion

Ozone (O3) is a gas consisting of three oxygen atoms. The ozone layer of the atmosphere acts as a shield protecting Earth's surface from the Sun's harmful ultraviolet (UV) radiation. When ozone absorbs UV radiation, it decomposes into an oxygen molecule (O2) and an oxygen atom (O) as:

2O3 + UV radiation 3O2 + O

Even a relatively small decrease in the ozone layer could produce significant risks to human, animal, and plant life. For example, scientists estimated that even a 1 percent decrease in global ozone levels would produce 10,000 more cases of skin cancer each year.

Fluorocarbons are a class of chemicals widely used in various technologies, including air conditioning, aerosol cans, and fire extinguishers. While the chemicals have proved extremely useful, it was not until the 1970s (when growing concentrations of chlorine were detected in the upper atmosphere) that scientists first realized that chlorofluorocarbons (CFCs) , a type of fluorocarbon, could potentially destroy ozone.

The widespread growth of CFCs produced an unsuspecting increase in upper atmosphere chlorine. When a CFC molecule is released into the atmosphere, it can remain for many years without reacting with other chemicals. Once the CFC molecule reaches the upper atmosphere, however, it can be broken apart by UV radiation, thus releasing a chlorine atom. It is this release of chlorine that poses the serious risk to Earth's ozone layer, because it is involved in a series of ozone depleting reactions in which a chemical family or a particular species is depleted, leaving the catalyst unaffected. Ozone can be affected by such a cycle. In the presence of a chlorine atom (Cl), atomic oxygen and an ozone molecule are converted into molecular oxygen via the following two-step process:

O3 + Cl ClO + O2

ClO + O Cl + O2

A single chlorine atom can potentially destroy many thousands of ozone molecules. Notice that a chlorine atom is consumed in the first reaction and preserved in the second reaction. Chlorine atoms thus act as catalysts in the depletion of ozone.

It was not until the Antarctic ozone hole was discovered in 1985 that scientists first realized how fragile the ozone layer can be to specific chemicals. In 1987 thirty-one countries agreed to protect the ozone layer through a reduction and elimination of the chemicals that destroy ozone. This international agreement, known as the Montreal Protocol, has successfully reduced the use and production of CFCs, with the long-term goal of restoring the ozone layer to its original state.

Summary

The struggle to improve air quality has persisted for many years. Although many cities still have air pollution episodes that are classified as unhealthy, stricter emission controls mean that the air over most major cities is cleaner today than it was in the mid-twentieth century. Even so, issues such as acid rain and ozone depletion continue to pose serious environmental challenges that require cooperation between science and policy.

see also Air Pollution.

Eugene C. Cordero

Bibliography

Jacobson, Mark Z. (2002). Atmospheric Pollution: History, Science, and Regulation. New York: Cambridge University Press.

Scorer, Richard S. (2002). Air Pollution Meteorology. Chichester, U.K.: Horwood.

World Meteorological Organization (1998). Scientific Assessment of Ozone Depletion: 1998. Report No. 44. Geneva: WMO Global Ozone Research and Monitoring Project.

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Atmospheric Chemistry

Atmospheric chemistry

Man lives at the bottom of an ocean of air. We may ordinarily take the atmosphere for granted and focus much more concern on the weather . This ocean of air, however, has profound consequences for life on Earth.

The surface density of air is about 0.074 lb/ft3 (1.184 g/l) and surface pressure is about 14 lb/ft2 (1 atm). This mass of air presses downward at all times. At a higher altitude, however, both the pressure and the density of air decrease. This explains why passenger jets, which often fly near 40,000 ft (12,192 m) to take advantage of the thin or low-density air, require pressurized cabins. Without them, passengers would not be able to take in enough oxygen with each breath.

The atmosphere is generally divided into four zones or layers. Starting at sea level and increasing in altitude, they are the troposphere (010 mi [016.1 km]), the stratosphere (1030 mi [16.148.3 km]), the mesosphere (3060 mi[48.396.6 km]), and the thermosphere (beyond 60 mi [96.6 km]). These altitudes are approximate and depend upon a variety of conditions, and are clearly distinct in both their physical properties (e.g., temperature ) and their chemistry .

The troposphere is the region of air closest to the ground. It is where the clouds and storm systems are to be found, and where our weather occurs. The troposphere is in direct contact with effluent chemicals generated by living things. These can range from the carbon dioxide and water vapor we exhale to industrial or automotive pollutants. In the absence of such compounds, atmospheric chemistry is very simple. Since the splitting of both the nitrogen and oxygen molecules requires a great deal of energy, the atmospheric composition is fairly constant at sea level, and without interfering compounds.

Smog is the term applied to the mixture of nitrous oxides, spent hydrocarbons , carbon monoxide, and ozone that is generated by automobiles and industrial combustion. Smog is the thick brown haze that hovers over large populated areas. This combination of gases is reactive. The addition of water vapor or raindrops, for example, can result in the scrubbing of these compounds from the air but also the generation of nitrous, nitric, and carbonic acid. Ozone is a powerful oxidizing agent and results in the degradation of plastics and other materials. However, it is also capable of reacting with spent hydrocarbons to generate noxious chemicals.

Industrial pollutants, such as sulfur dioxide generated by coal-burning power plants, can generate acid rain as the sulfur dioxide is converted to sulfurous and sulfuric acid. Even forest fires contribute a large variety of chemical compounds into the atmosphere and induce chemical reactions. And, the largest of all natural disasters, a volcanic eruption, spews tons of chemical compounds into the troposphere where they react to produce acids and other compounds.

The stratosphere is the home of the ozone layer, which is misleading as it implies a distinct region in the atmosphere that has ozone as the major constituent. Ozone is never more than a minor constituent of the atmosphere, although it is a significant minor constituent. The concentration of ozone achieves its maximum in the stratosphere. It is here that the chemistry occurs that blocks incoming ultraviolet radiation.

The complete spectrum of radiation from the sun contains a significant amount of high energy ultraviolet light and the energy of these photons is sufficient to ionize atoms or molecules. If this light penetrated to Earth's surface, life as we know it could not exist as the ionizing radiation would continually break down complex molecules.

Within the ozone layer, this ultraviolet energy is absorbed by a delicate balance of two chemical reactions. The first is the photolytic reaction of molecular oxygen to give atomic oxygen, which subsequently combines with another oxygen molecule to give ozone. The second reaction is the absorption of another photon of ultraviolet light by an ozone molecule to give molecular oxygen and a free oxygen atom .

hν
O2O+O
O+O2O3
hν
O3O2+O

It is the combination of these two reactions that allows the ozone layer to protect the planet. These two reactions actually form an equilibrium with the forward reaction being the formation of ozone and the backwards reaction being the depletion.

hν
3O22O3

The ozone concentration is thus at a constant and relatively low level. It occurs in the stratosphere because this is where the concentration of gases is not so high that the excited molecules are deactivated by collision, but not so low that the atomic oxygen generated can not find a molecular oxygen with which to react.

In the last half of the twentieth century, the manufacture of chlorofluorocarbons (CFCs) for use as propellants in aerosol sprays and refrigerants has resulted in a slow mixing of these compounds with the stratosphere. Upon exposure to high-energy ultraviolet light, the CFCs break down to atomic chlorine, which interferes with the natural balance between molecular oxygen and ozone. The result is a shift in the equilibrium and a depletion of the ozone level. The occurrence of ozone depletion was first noted over Antarctica . Subsequent investigations have demonstrated that the depletion of ozone also occurs over the Arctic, resulting in higher than normal levels of ultraviolet radiation reaching many heavily populated regions of North America . This is, perhaps, one of the most important discoveries in atmospheric chemistry and has lead to major changes in legislation in all countries in an attempt to stop ozone depletion.

Beyond the stratosphere, the energy levels increase dramatically and the available radiation is capable of initiating a wide variety of poorly characterized chemical reactions. Understanding all of the complexities of atmospheric chemistry is subject for much ongoing research.

See also Atmospheric composition and structure; Atmospheric pollution; Global warming

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