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Composition of the atmosphere
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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 (i.e. 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 CO2 and 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, CI 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.
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Water vapour (H20), 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).
NOx refers 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 NOv (mainly NO) accounting for 15-25×109 kg N/year. Biomass burning and lightning activity are other important sources. NOv emissions increased by some 200 per cent between 1940 and 1980. The total source of NOv is about 40×109 kg N/year. About 25 per cent of this goes into the stratosphere, where it undergoes photochemical dissociation. It is also removed as nitric acid (HNO-,) in snowfall. Odd nitrogen is also released as NHV by ammonia oxidation in fertilizers and by domestic animals (6-10×109 kg N/year).
Sulphur species. Reactive species are sulphur' dioxide (SO,) and reduced sulphur (H,S, 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 (H,S) (20-40×109 g S/year) and dimethyl sulphide (DMS) (35-55×109 kg S/year). DMS is primarily produced by biological productivity near the ocean surface. SO, emissions increased by about 50 per cent between 1940 and 1980. Volcanic activity releases approximately 109 kg S/year as sulphur dioxide. Because the lifetime of SO, and 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 H,S 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 S02 are precipitated as acid rain containing sulphuric acid (H2S04). 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, see Chapter 3F). In these areas and central southern China, rainfall deposits > 1 g nr2 of SO, annually.
The role of halogens of carbon (CFCs and HCFCs) in the destruction of ozone in the stratosphere is described in the next section.
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 S02 and natural H2S; ammonium salts from NH3; nitrogen from NO J. 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 (1012 g) 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 /am are highly effective in scattering solar radiation, and those of about 0.1 м m diameter are important in cloud condensation.
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Упражнение 2.
Прочитайте следующие слова и определите их соответствия
в русском языке:
Chemical, concentration, fermentation, anthropogenic, specific, reaction, molecule, gas, role, distance, condensation, central, mineral, spore, process, aerosol, ozone, methane, conversion, effect.
Упражнение 3.
Найдите в тексте из упражнения 1 слова с суффиксами ~ al/~ial, ~able, ~ive, ~le,~ant, ~ic. Определите, к какой части речи они относятся.
Упражнение 4.
Найдите в тексте примеры следующих частей речи.
| v. | n. | adj. | adv. | num. | prep. | part. |
Упражнение 5.
В правой колонке найдите русские эквиваленты следующих английских словосочетаний:
| 1. oxyden molecule 2. residence time 3. coal combustion 4. condensation nuclei 5. fog droplets 6. sea salt 7. soil grain 8. power station 9. weather processes | a. морская соль b. электростанция c. частица почвы d. погодные процессы e. молекула кислорода f. капли тумана g. сжигание угля h. ядра конденсации i. время жизни |
Упражнение 6.
Переведите следующие слова на русский язык:
Moreover, particularly, complex, conversely, primarily, through, almost, after, entirely, sharply, in addition to, despite, others.
Упражнение 7.
Прочитайте текст и найдите в нем ответы на следующие вопросы:
1. Почему водяной пар и озон влияют на тепловой баланс атмосферы?
2. Каковы основные источники водяного пара?
3. На какой высоте главным образом формируется озон?
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4. На какой высоте в низких и высоких широтах находится озоновый слой?
(Контрольное время – 7 минут)
Variation with height
The light gases (hydrogen and helium especially) might be expected to become more abundant in the upper atmosphere, but large-scale turbulent mixing of the atmosphere prevents such diffusive separation even at heights of many tens of kilometres above the surface. The height variations that do occur are related to the source-locations of the two major non-permanent gases – water vapour and ozone. Since both absorb some solar and terrestrial radiation, the heat budget and vertical temperature structure of the atmosphere are considerably affected by the distribution of these two gases.
Water vapour comprises up to 4 per cent of the atmosphere by volume (about 3 per cent by weight) near the surface, but only 3-6 ppmv (parts, per million by volume) above 10 to 12 km. It is supplied to the atmosphere by evaporation from surface water or by transpiration from plants and is transferred upwards by atmospheric turbulence. Turbulence is most effective below about 10 km and as the maximum possible water vapour density of cold air is anyway very low, there is little water vapour in the upper layers of the atmosphere.
Ozone (03) is concentrated mainly between 15 and 35 km. The upper layers of the atmosphere are irradiated by ultraviolet radiation from the sun, which causes the break-up of oxygen molecules at altitudes above 30 km (i.e. O, —> O + O). These separated atoms (O + O) may then combine individually with other oxygen molecules to create ozone, as illustrated by the simple photochemical scheme:
02 + 0 + M—>03+M
where M represents the energy and momentum balance provided by collision with a third atom or molecule. Such three-body collisions are rare at 80 to 100 km because of the very low density of the atmosphere, while below about 35 km most of the incoming ultraviolet radiation has already been absorbed at higher levels. Therefore ozone is mainly formed between 30 and 60 km, where collisions between O and O, are more likely. Ozone itself is unstable; its abundance is determined by three distinctly different photochemical interactions. Above 40 km odd oxygen is destroyed primarily by a cycle involving molecular oxygen; between 20 and 40 km NOx cycles are dominant; while below 20 km a hydrogen-oxygen radical (HO2) is responsible. Additional important cycles involve chlorine (CIO) and bromine (BrO) chains at-various altitudes. Collisions with monatomic oxygen may recreate oxygen, but ozone is mainly destroyed through cycles involving catalytic reactions, some of which are photochemical associated with longer wavelength ultraviolet radiation (2.3-2.9 |xm). The destruction of ozone involves a recombination with atomic oxygen, causing a net loss of the odd oxygen. This takes place through the catalytic effect of a radical such as OH (hydroxyl):
The odd hydrogen atoms and OH result from the dissociation of water vapour, molecular hydrogen and methane (CH4).
Stratospheric ozone is similarly destroyed in the presence of nitrogen oxides (NOx, i.e. NO, and NO) and chlorine radicals (CI, CIO). The source gas of the NOT is nitrous oxide (N20), which is produced by combustion and fertilizer use, while chlorofluoro-carbons (CFCs), manufactured for 'freon', give rise to the chlorines. These source gases are transported into the stratosphere from the surface and are converted by oxidation into NOх, and by UV photode- composition into chlorine radicals, respectively.
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The chlorine chain involves:
2 (CI + 03 CIO + O2)
CIO + CIO C1202
and
CI + 03 CIO + O2
OH + 03 HO, + 202
Both reactions result in a conversion of O3 to 02 and the removal of all odd oxygens. Another cycle may involve an interaction of the oxides of chlorine and bromine. It appears that the increases of CR and Br species during the decades 1970-90 are sufficient to explain the observed decrease of stratospheric ozone over Antarctica (see pp. 10-11). A mechanism that may enhance the catalytic process involves polar stratospheric clouds. These can form readily during the austral spring (October), when temperatures decrease to 185-195 K, permitting the formation of particles of nitric acid (HNO3) ice and water ice. It is apparent, however, that anthropogenic sources of the trace gases are
a primary factor in the ozone decline. Conditions in the Arctic are somewhat different as the stratosphere is warmer and there is more mixing of air from lower latitudes.
The constant metamorphosis of oxygen to ozone and from ozone back to oxygen involves a very complex set of photochemical processes, which tend to maintain an approximate equilibrium above about 40 km. However, the ozone mixing ratio is at its maximum at about 35 km, whereas maximum ozone concentration (see Note 1) occurs lower down, between 20 and 25 km in low latitudes and between 10 and 20 km in high latitudes. This is the result of some circulation mechanism transporting ozone downwards to levels where its destruction is less likely, allowing an accumulation of the gas to occur. Despite the importance of the ozone layer, it is essential to realize that if the atmosphere were compressed to sea level (at normal sea-level temperature and pressure) ozone would contribute only about 3 mm to the total atmospheric thickness of 8 km.
Упражнение 8.
Выберите из текста Variation with height 10–15 основных,
с точки зрения смысловой нагрузки, слов (ключевые слова). Определите, к каким частям речи они относятся.
Упражнение 10.
Переведите текст письменно. (Контрольное время – 30 минут)
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