7(a) Atmospheric Composition
CHAPTER 7: Introduction to the Atmosphere
 
(a). Atmospheric Composition
Table 7a-1 lists the eleven most                         abundant gases found in the Earth's lower atmosphere                         by volume. Of the gases listed, nitrogen, oxygen, water                         vapor, carbon dioxide, methane, nitrous oxide, and ozone                         are extremely important to the health of the Earth's                         biosphere.
The table indicates that nitrogen and oxygen are                         the main components of the atmosphere by volume. Together                         these two gases make up approximately 99% of the dry                         atmosphere. Both of these gases have very important associations                         with life. Nitrogen is removed from the atmosphere and                         deposited at the Earth's surface mainly by specialized nitrogen                         fixing bacteria, and by way of lightning through                         precipitation. The addition of this nitrogen to the Earth's                         surface soils and various water bodies supplies much                         needed nutrition for plant growth. Nitrogen returns to                         the atmosphere primarily through biomass combustion and denitrification .
Oxygen is exchanged between the atmosphere                         and life through the processes of photosynthesis and respiration .                         Photosynthesis produces oxygen when carbon dioxide and                         water are chemically converted into glucose with the                         help of sunlight. Respiration is a the opposite process                         of photosynthesis. In respiration, oxygen is combined                         with glucose to chemically release energy for metabolism.                         The products of this reaction are water and carbon dioxide.
The next most abundant gas on the table is water                             vapor. Water vapor varies in concentration                             in the atmosphere both spatially and temporally.                             The highest concentrations of water vapor are found                             near the equator over the oceans and tropical rain                             forests. Cold polar areas and subtropical continental                             deserts are locations where the volume of water vapor                             can approach zero percent. Water vapor has several                             very important functional roles on our planet:
It redistributes heat energy on the Earth through latent                                 heat energy exchange.
The condensation of water vapor creates precipitaion                             that falls to the Earth's surface providing needed                             fresh water for plants and animals.
It helps warm the Earth's atmosphere through the greenhouse                                 effect .
The fifth most abundant                           gas in the atmosphere is carbon                         dioxide . The volume of this gas has increased                         by over 35% in the last three hundred years (see Figure                         7a-1 ). This increase is primarily due to human                         induced burning from fossil fuels, deforestation, and                         other forms of land-use change. Carbon dioxide is an                         important greenhouse gas. The human-caused increase                         in its concentration in the atmosphere has strengthened                         the greenhouse                         effect and has definitely contributed                         to global                         warming over the last 100 years. Carbon dioxide                         is also naturally exchanged between the atmosphere and life through the processes of photosynthesis and respiration .
Methane is                         a very strong greenhouse gas. Since 1750, methane concentrations                         in the atmosphere have increased by more than 150%.                         The primary sources for the additional methane added                         to the atmosphere (in order of importance) are: rice                         cultivation; domestic grazing animals; termites; landfills;                         coal mining; and, oil and gas extraction. Anaerobic conditions                         associated with rice paddy flooding results in the formation                         of methane gas. However, an accurate estimate of how                         much methane is being produced from rice paddies has                         been difficult to ascertain. More than 60% of all rice                         paddies are found in India and China where scientific                         data concerning emission rates are unavailable. Nevertheless,                         scientists believe that the contribution of rice paddies                         is large because this form of crop production has more                         than doubled since 1950. Grazing animals release methane                         to the environment as a result of herbaceous digestion.                         Some researchers believe the addition of methane from                         this source has more than quadrupled over the last century.                         Termites also release methane through similar processes.                         Land-use change in the tropics, due to deforestation,                         ranching, and farming, may be causing termite numbers                         to expand. If this assumption is correct, the contribution                         from these insects may be important. Methane is also                         released from landfills, coal mines, and gas and oil                         drilling. Landfills produce methane as organic wastes                         decompose over time. Coal, oil, and natural gas deposits                         release methane to the atmosphere when these deposits                         are excavated or drilled.
The average concentration of the greenhouse gas nitrous   oxide is now increasing at a rate of 0.2 to 0.3% per year. Its part   in the enhancement of the greenhouse effect is minor relative to the other   greenhouse gases already mentioned. However, it does have an important role   in the artificial fertilization of ecosystems. In extreme cases, this fertilization   can lead to the death of forests, eutrophication of aquatic habitats, and species   exclusion. Sources for the increase of nitrous oxide in the atmosphere include:   land-use conversion; fossil fuel combustion; biomass burning; and soil fertilization.   Most of the nitrous oxide added to the atmosphere each year comes from deforestation   and the conversion of forest, savanna and grassland ecosystems into agricultural   fields and rangeland. Both of these processes reduce the amount of nitrogen   stored in living vegetation and soil through the decomposition of organic matter.   Nitrous oxide is also released into the atmosphere when fossil fuels and biomass   are burned. However, the combined contribution to the increase of this gas   in the atmosphere is thought to be minor. The use of nitrate and ammonium fertilizers   to enhance plant growth is another source of nitrous oxide. How much is released   from this process has been difficult to quantify. Estimates suggest that the   contribution from this source represents from 50% to 0.2% of nitrous oxide   added to the atmosphere annually.
Ozone's role                 in the enhancement of the greenhouse effect has been difficult                 to determine. Accurate measurements of past long-term (more than                 25 years in the past) levels of this gas in the atmosphere are                 currently unavailable. Moreover, concentrations of ozone gas                 are found in two different regions of the Earth's atmosphere.                 The majority of the ozone (about 97%) found in the atmosphere                 is concentrated in the stratosphere at                 an altitude of 15 to 55 kilometers above the Earth's surface.                 This stratospheric ozone provides an important service to life                 on the Earth as it absorbs harmful ultraviolet radiation. In                 recent years, levels of stratospheric                 ozone have been decreasing due to the buildup of human                 created chlorofluorocarbons in                 the atmosphere. Since the late 1970s, scientists have noticed                 the development of severe holes in the ozone layer over Antarctica.                 Satellite measurements have indicated that the zone from 65° North                 to 65° South latitude has had a 3% decrease in stratospheric                 ozone since 1978.
Ozone is also highly concentrated at the                         Earth's surface in and around cities. Most of this ozone                         is created as a by product of human created photochemical                         smog . This buildup of ozone is toxic to organisms living at the Earth's surface.