Composition of the atmosphere

  1. Effect of the atmosphere
  2. Energy transfer within the earth-atmosphere system
  3. The layering of the atmosphere

Air is a mechanical mixture of gases, not a chemical compound. Nitrogen, oxygen and argon account for 99.9 per cent of the air by volume. Moreover, rocket observations show that these gases are mixed in remarkably constant proportions up to about 80 km (50 miles).

Of especial significance, despite their relative scarcity, are the so-called greenhouse gases, which play an important role in the thermodynamics of the atmosphere by trapping long-wave terrestrial reradiation, producing the greenhouse effect. The concentrations of these gases are particularly susceptible to human (i.e. anthropogenic) activities.

1. Carbon dioxide (CO2) Is involved in a complex global cycle. It is released from the interior of the earth and produced by respiration of biota, soil processes, combustion and oceanic evaporation. Conversely, it is dissolved in the oceans and consumed by the process of plant photosynthesis.

2. Methane (CH4) Is produced primarily through anaerobic (ie oxygen-deficient) processes by natural wetlands and rice paddies (together about 40 per cent of the total), as well as by enteric fermentation in animals, by termites, through coal and oil extraction, biomass burning , and from landfills.

CO2 + 4H2> CH4+ 2H20

Almost two-thirds of the total is related to anthropogenic activity. Methane is oxidized to CO2and H2O by a complex photochemical reaction system.

CH4+ O2+ 2x > CO2+ 2x H2

where x denotes any specific methane destroying species (such as H, OH, NO, Cl or Br).

3 Nitrous oxide (N2O) is produced by biological mechanisms in the oceans and soils, by industrial combustion, automobiles, aircraft, biomass burning, and as a result of the use of chemical fertilizers. It is destroyed by photochemical reactions in the stratosphere involving the production of nitrogen oxides (NOx).

4 Ozone (O3) Is produced by the high-level breakup of oxygen molecules by solar ultraviolet radiation and destroyed by reactions involving nitrogen oxides and chlorine (Cl) (the latter generated by CFCs, volcanic eruptions and vegetation burning) in the middle and upper stratosphere.

5 Chlorofluorocarbons (CFCs: chiefly CFC13(F-11) and CF2C13(F-12)) are entirely anthropogenically produced by aerosol propellants, refrigerator coolants (e.g. 'freon'), cleansers and air conditioners, and were not present in the atmosphere until the 1930s. CFC molecules rise slowly into the stratosphere and then move poleward, being decomposed by photochemical processes into chlorine after an estimated average lifetime of some 65-130 years.

6 Hydrogenated halocarbons (HFCs and HCFCs) are also entirely anthropogenic gases. They have increased sharply in the atmosphere over the last few decades, following their use as substitutes for CFCs. Trichloroethane (C2H3C13), For example, which is used in dry-cleaning and degreasing agents, increased fourfold in the 1980s and has a seven-year residence time in the atmosphere. They generally have lifetimes of a few years, but still have substantial greenhouse effects.

Water vapour (H2O), the primary greenhouse gas, is a vital atmospheric constituent. It averages about 1 per cent by volume but is very variable both in space and time, being involved in a complex global hydrological cycle.

In addition to the greenhouse gases, important reactive gas species are produced by the cycles of sulphur, nitrogen and chlorine. These play key roles in acid precipitation and in ozone destruction. Sources of these species are as follows:

Nitrogen species. The reactive species of nitrogen are nitric oxide (NO) and nitrogen dioxide (NO2).

NOxrefers to these and other odd nitrogen species with oxygen. Fossil fuel combustion (approximately two-thirds for heating, one-third for cars and other transport) is the primary source of NOx(Mainly NO) accounting for 15-25 * 109kg N / year. Biomass burning and lightning activity are other important sources. NOxemissions increased by some 200 per cent between 1940 and 1980. The total source of NOxis about 40 * 109kg N / year. About 25 per cent of this goes into the stratosphere, where it undergoes photochemical dissociation. It is also removed as nitric acid (HNO3) In snowfall. Odd nitrogen is also released as NHxby ammonia oxidation in fertilizers and by domestic animals (6-10 * 109kg N / year).

Sulphur species. Reactive species are sulphur dioxide (SO2) And reduced sulphur (H2S, DMS). Atmospheric sulphur is almost entirely anthropogenic in origin: 90 per cent from coal and oil combustion, and much of the remainder from copper smelting. The major sources are sulphur dioxide (80-100 * 109 kg S / year), hydrogen sulphide (H2S) (20-40 * 109g S / year) and dimethyl sulphide (DMS) (35-55 * 109 kg S / year). DMS is primarily produced by biological productivity near the ocean surface. SO2emissions increased by about 50 per cent between 1940 and 1980. Volcanic activity releases approximately 109kg S / year as sulphur dioxide. Because the lifetime of SO2and H2S in the atmosphere is only about one day, atmospheric sulphur occurs largely as carbonyl sulphur (COS), which has a lifetime of about one year. The conversion of H2S gas to sulphur particles is an important source of atmospheric aerosols.

Despite its short lifetime, sulphur dioxide is readily transported over long distances. It is removed from the atmosphere when condensation nuclei of SO2are precipitated as acid rain containing sulphuric acid (H2SO4). The acidity of fog deposition can be more serious because up to 90 per cent of the fog droplets may be deposited. In Californian coastal fogs, pH values ??of only 2.0 - 2.5 are not uncommon. Peak pH readings in the eastern United States and Europe are <4.3 (pH = 7 is neutral). In these areas and central southern China, rainfall deposits> 1 g m-2of SO2annually.

There are also significant quantities of aerosols in the atmosphere. These are suspended particles of sea salt, mineral dust (particularly silicates), organic matter and smoke. Aerosols enter the atmosphere from a variety of natural and anthropogenic sources. Some originate as particles - soil grains and mineral dust from dry surfaces, carbon soot from coal fires and biomass burning, and volcanic dust. Others are converted into particles from inorganic gases (sulphur from anthropogenic SO2and natural H2S; ammonium salts from NH3; nitrogen from NOx). Sulphate aerosols, two-thirds of which come from coal-fired power station emissions, are now playing an important role in countering global warming effects by reflecting incoming solar radiation. Other aerosol sources are sea salts and organic matter (plant hydrocarbons and anthropogenically derived). Natural sources are about eight times larger than anthropogenic ones on a global scale, but the estimates are wide-ranging. Moreover, there is considerable spatial variability. For example, some 1,500 Tg (1012g) of crustal material is picked up by the air annually, about one-half from the Sahara and the Arabian Peninsula. Most of this is deposited downwind over the Atlantic. Large particles originate from mineral dust, sea salt spray, fires and plant spores; these sink rapidly back to the surface or are washed out (scavenged) by rain after a few days. Small (Aitken) particles form by the condensation of gas-phase reaction products and from organic molecules and polymers, including natural and synthetic fibres, plastics, rubber, and vinyl. Fine particles from volcanic eruptions may reside in the stratosphere above the level of weather processes for 1-3 years. Intermediate-sized particles originate from natural sources, such as soil surfaces, from combustion, or they accumulate by random coagulation and by repeated cycles of condensation and evaporation. Particles with diameters of 0.1- 1.0 ?m are highly effective in scattering solar radiation, and those of about 0.1 m diameter are important in cloud condensation.

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ISBN 978-5 -86813-306-0 | ISBN 978-5-86813-306-0 | Solar radiation | Altitude of the sun | Distance from the sun | Length of day | Energy transfer within the earth-atmosphere system | Variations with latitude and season | The layering of the atmosphere | Ozone layer reduction |

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