Atmospheric Chemistry

Introduction

Since the beginning of industrial development and its associated increase in the burning of fossil fuels, humans have been altering the composition and chemistry of the atmosphere. More recently, the introduction of human-made halocarbons has also had an impact on atmospheric composition. The atmosphere is a mixture of gases that includes about 78% nitrogen and 21% oxygen at Earth's surface. The rest of the atmosphere near Earth's surface is made up of carbon dioxide and trace amounts of a number of other gases. It is the increase in carbon dioxide and trace gases that has altered atmospheric chemistry.

Historical Background and Scientific Foundations

The carbon dioxide content of the atmosphere has grown significantly in the last 50 years as a byproduct of the burning of fossil fuels. There has been a 36% increase in atmospheric carbon dioxide since the mid-1800s with almost all of the increase due to human activities.

Methane, a trace gas, is naturally found in the atmosphere as the product of decaying organic matter. The concentration of methane in the atmosphere has risen to 148% above its pre-industrial age level. Anthropogenic (human-caused) sources of methane include

landfills, the management of livestock, and natural gas and petroleum processing.

Nitrous oxide is another gas that is naturally found in trace amounts in the atmosphere. The concentration of nitrous oxide is largely derived from agricultural sources, with vehicle emissions also contributing nitrous oxide to the atmosphere. The concentration of nitrous oxide in the atmosphere has risen approximately 18% in the last 200 years.

Carbon dioxide, methane, and nitrous oxide are greenhouse gases. Their increased concentration in the atmosphere is contributing to global warming. Greenhouse gases cause global warming by absorbing reflected heat from Earth's surface thereby warming the atmosphere.

In addition to carbon dioxide, methane, and nitrous oxide, there are entirely human-made trace gases in the atmosphere that have added significantly to global warming. Halocarbons, particularly chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are found in a variety of products and substances, including aerosol propellants, solvents, and refrigerants, and are released into the atmosphere from these sources. Other fluorinated gases—such as sulfur hexafluoride that has escaped from applications in electric power equipment and perfluorocarbons that have escaped from aluminum processing and the manufacture of semiconductors—are also found in trace amounts in the atmosphere.

Earth's atmosphere is actually comprised of four layers. The troposphere starts at ground level and extends up approximately 10 mi (16 km). The next layer is the stratosphere, which is roughly 10–30 mi (16–48 km) above Earth's surface. Above the stratosphere is the mesosphere (30–60 mi/48–97 km above Earth's surface); and beyond the mesosphere is the thermosphere. Although each layer has a distinct composition, the troposphere and stratosphere are the layers that are critical to climate change issues.

WORDS TO KNOW

ACID RAIN: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.

HALOCARBON: Compound that contains carbon and one or more of the elements known as halons (chlorine, fluorine, or bromine). Halocarbons do not exist in nature; all are manufactured. When released into the atmosphere, many halocarbons deplete the ozone layer and have high global warming potential.

OZONE LAYER: The layer of ozone that begins approximately 9.3 mi (15 km) above Earth and thins to an almost negligible amount at about 31 mi (50 km) and shields Earth from harmful ultraviolet radiation from the sun. The highest natural concentration of ozone (approximately 10 parts per million by volume) occurs in the stratosphere at approximately 15.5 mi (25 km) above Earth. The stratospheric ozone concentration changes throughout the year as stratospheric circulation changes with the seasons. Natural events such as volcanoes and solar flares can produce changes in ozone concentration, but man-made changes are of the greatest concern.

PHOTOCHEMICAL SMOG: A type of smog that forms in large cities when chemical reactions take place in the presence of sunlight; its principal component is ozone. Ozone and other oxidants are not emitted into the air directly but form from reactions involving nitrogen oxides and hydrocarbons. Because of its smog-making ability, ozone in the lower atmosphere (troposphere) is often referred to as “bad” ozone.

TRACE GASES: Gases present in Earth's atmosphere in trace (relatively very small) amounts. All greenhouse gases happen to be trace gases, though some are more abundant than others; the most abundant greenhouse gases are CO₂(0.037% of the atmosphere) and water vapor (0.25% of the atmosphere, on average).

ULTRAVIOLET RADIATION: The energy range just beyond the violet end of the visible spectrum. Although ultraviolet radiation constitutes only about 5 percent of the total energy emitted from the sun, it is the major energy source for the stratosphere and mesosphere, playing a dominant role in both energy balance and chemical composition.

In the troposphere, the concentration of nitrogen is around 78% and oxygen is about 21%. This is the atmospheric layer where carbon dioxide, methane, nitrous oxide, sulfur hexafluoride, and the halocarbons collect. In the stratosphere, a naturally occurring layer of ozone is formed from the reaction of sunlight with oxygen. Ozone is a form of oxygen that has three atoms in each molecule instead of the usual two atoms, and it plays an important protective role by diminishing the amount of potentially damaging ultraviolet radiation reaching Earth. CFCs and HCFCs are drifting from the tropo-sphere into the stratosphere and destroying some of this natural ozone layer. The chlorine in the halocarbons

reacts with ozone to convert molecules of ozone to oxygen, thereby depleting the ozone layer.

Also in the troposphere, carbon dioxide reacts with water vapor to make a very weak acid (carbonic acid), a reaction that depends on the temperature. Carbon dioxide gas is more soluble at colder temperatures, but saturation of the gas in water vapor is quickly reached with normal amounts of the gas in the atmosphere.

Increasing the concentration of the carbon dioxide in the atmosphere does not make the water vapor any more acidic. However, the introduction into the atmosphere of sulfur and nitrogen oxides, which react with water to form strong acids, does make the water vapor and the precipitation derived from it more acidic. This acid rain was the first recognized negative climate change that is caused by human activities.

Satellite observations have provided data on the chemical composition of the stratosphere and changes in the ozone layer. In the troposphere, no satellites are needed to observe the damaging chemical changes that have periodically occurred as a result of human activities. Photochemical smog is a serious air pollution problem that occurs when sunlight acts on nitrogen oxides, hydrocarbons, and other pollutants that are created from fossil fuel combustion. This type of smog is seen most often in highly populated areas with large numbers of vehicles during periods of high temperature and intense sunlight. In such situations, concentrations of the offending gases near ground level increase.

Impacts and Issues

Observing atmospheric chemistry requires international efforts. The World Meteorological Organization (WMO), an agency of the United Nations, has 188 participating member states and territories cooperating in a continuous study of all aspects of the atmosphere. The WMO working with the United Nations Environmental Programme established the Intergovernmental Panel on Climate Change (IPCC) in 1988 to assess information being gathered about atmospheric composition and chemistry. The IPCC is not a research organization. It issues reports on the data collected from the WMO and other sources.

In 1987, 145 countries signed the Montreal Protocol to ban the production of ozone-depleting CFCs by the year 2000. Although the use of CFCs was banned by the Montreal Protocol in 1987, the existing CFCs will remain in the atmosphere for hundreds of years. HCFCs are less damaging to the ozone layer, but their use is also being phased out because they are greenhouse gases.

IN CONTEXT: HUMAN ACTIVITY IMPACTS NATURAL BALANCE

“The current concentration of a greenhouse gas in the atmosphere is the net result of the history of its past emissions and removals from the atmosphere. The gases and aerosols considered [by the Intergovernmental Panel on Climate Change] are emitted to the atmosphere by human activities or are formed from precursor species emitted to the atmosphere. These emissions are offset by chemical and physical removal processes. With the important exception of carbon dioxide (CO₂), it is generally the case that these processes remove a specific fraction of the amount of a gas in the atmosphere each year and the inverse of this removal rate gives the mean lifetime for that gas. In some cases, the removal rate may vary with gas concentration or other atmospheric properties (e.g., temperature or background chemical conditions).”

“Long-lived greenhouse gases (LLGHGs), for example, CO₂, methane (CH₄) and nitrous oxide (N₂O), are chemically stable and persist in the atmosphere over time scales of a decade to centuries or longer, so that their emission has a long-term influence on climate.”

“Because these gases are long lived, they become well mixed throughout the atmosphere much faster than they are removed and their global concentrations can be accurately estimated from data at a few locations. Carbon dioxide does not have a specific lifetime because it is continuously cycled between the atmosphere, oceans and land biosphere and its net removal from the atmosphere involves a range of processes with different time scales.”

“Short-lived gases (e.g., sulphur dioxide and carbon monoxide) are chemically reactive and generally removed by natural oxidation processes in the atmosphere, by removal at the surface or by washout in precipitation; their concentrations are hence highly variable. Ozone is a significant greenhouse gas that is formed and destroyed by chemical reactions involving other species in the atmosphere. In the troposphere, the human influence on ozone occurs primarily through changes in precursor gases that lead to its formation, whereas in the stratosphere, the human influence has been primarily through changes in ozone removal rates caused by chlorofluorocarbons (CFCs) and other ozone-depleting substances.”

SOURCE:Solomon, S., et al. “Technical Summary.” In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press. 2007.

In 1992, 189 countries joined the United Nations Framework Convention on Climate Change (UNFCCC) to set general goals and rules to address climate change. One result of the activities of UNFCCC was the 1997 Kyoto Protocol, which included an outline of controls for the industrial nations to contain six greenhouse gases—carbon dioxide, methane, nitrous oxide, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride.

See Also Acid Rain; Climate Change; Greenhouse Gases.

BIBLIOGRAPHY

Web Sites

“Atmosphere Changes.” U.S. Environmental Protection Agency. < http://www.epa.gov/climatechange/science/recentac.html> (accessed August 16, 2007).

“International Global Atmospheric Chemistry Science Plan and Implementation Strategy.” IGBP Secretariat. < http://www.igac.noaa.gov/> (accessed August 16, 2007).

“The Nobel Prize in Chemistry 1995.” Nobelprize.org. < http://nobelprize.org/nobel_prizes/chemistry/laureates/1995/index.html> (accessed August 16, 2007).

“Photochemical Smog—What It Means for Us.” U.S. Environmental Protection Agency, March 2004. < http://www.epa.sa.gov.au/pdfs/info_photosmog.pdf> (accessed August 16, 2007).

“WMO in Brief.” World Meteorological Organization. < http://www.wmo.ch/pages/about/index_en.html> (accessed August 16, 2007).

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 coal—plentiful, 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.

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 (0–10 mi [0–16.1 km]), the stratosphere (10–30 mi [16.1–48.3 km]), the mesosphere (30–60 mi[48.3–96.6 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
O₂→O+O
O+O₂→O₃
hν
O₃→O₂+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
3O₂⇆2O₃

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