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Notes: * Decay products of potassium and uranium. t Recombination of oxygen. t Inert gases. ยง At surface.

2 Greenhouse gases

In spite of their relative scarcity, the so-called greenhouse gases play a crucial role in the thermodynamics of the atmosphere. They trap radiation emitted by the earth, thereby producing the greenhouse effect (see Chapter 3C). Moreover, the concentrations of these trace gases are strongly affected by human (i.e. anthropogenic) activities:

1 Carbon dioxide (CO2) is involved in a complex global cycle (see 2A.7). It is released from the earth's interior and produced by respiration of biota, soil microbia, fuel combustion and oceanic evaporation. Conversely, it is dissolved in the oceans and consumed by plant photosynthesis. The imbalance between emissions and uptake by the oceans and terrestrial biosphere leads to the net increase in the atmosphere.

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.

Almost two-thirds of the total production is related to anthropogenic activity.

Methane is oxidized to CO2 and H2O by a complex photochemical reaction system.

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

3 Nitrous oxide (N2O) is produced primarily by nitrogen fertilizers (50-75 per cent) and industrial processes. Other sources are transportation, biomass burning, cattle feed lots and biological mechanisms in the oceans and soils. It is destroyed by photochemical reactions in the stratosphere involving the production of nitrogen oxides (NOx).

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

5 Chlorofluorocarbons (CFCs: chiefly CFCl3 (F-12) and CF2Cl2 (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 to 130 years.

6 Hydrogenated halocarbons (HFCs and HCFCs) are also entirely anthropogenic gases. They have increased sharply in the atmosphere over the past few decades, following their use as substitutes for CFCs. Trichloroethane (C2H3Cl3), 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. The role of halogens of carbon (CFCs and HCFCs) in the destruction of ozone in the stratosphere is described below

7 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 (see Chapter 3).

3 Reactive gas species

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. Their primary significance is as a catalyst for tropospheric ozone formation. Fossil fuel combustion (approximately 40 per cent for transportation and 60 per cent for other energy uses) is the primary source of NOx (mainly NO) accounting for ~25 X 109kg N/year. Biomass burning and lightning activity are other important sources. NOx emissions increased by some 200 per cent between 1940 and 1980. The total source of NOx is about 40 X 109 kg N/year. About 25 per cent of this enters the stratosphere, where it undergoes photochemical dissociation. It is also removed as nitric acid (HNO3) in snowfall. Odd nitrogen is also released as NH by ammonia oxidation in fertilizers and by domestic animals (6-10 X 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 X 109 kg S/year), hydrogen sulphide (H2S) (20-40 X 109 g S/year) and dimethyl sulphide (DMS) (35-55 X 109 kg S/year). DMS is produced primarily by biological productivity near the ocean surface. SO2 emissions increased by about 50 per cent between 1940 and 1980, but declined in the 1990s. Volcanic activity releases approximately 109 kg S/year as sulphur dioxide. Because the lifetime of SO2 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 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 SO2 are 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.

Acid deposition includes both acid rain and snow (wet deposition) and dry deposition of particulates. Acidity of precipitation represents an excess of positive hydrogen ions [H+] in a water solution. Acidity is measured on the pH scale (1 - log[H+]) ranging from 1 (most acid) to 14 (most alkaline), 7 is neutral (i.e. the hydrogen cations are balanced by anions of sulphate, nitrate and chloride). Peak pH readings in the eastern United States and Europe are <4.3.

Over the oceans, the main anions are Cl- and SO42-from sea-salt. The background level of acidity in rainfall is about pH 4.8 to 5.6, because atmospheric CO2 reacts with water to form carbonic acid. Acid solutions in rainwater are enhanced by reactions involving both gas-phase and aqueous-phase chemistry with sulphur dioxide and nitrogen dioxide. For sulphur dioxide, rapid pathways are provided by:

HOSO

The OH radical is an important catalyst in gas-phase reaction and hydrogen peroxide (H2O2) in the aqueous phase.

Acid deposition depends on emission concentrations, atmospheric transport and chemical activity, cloud type, cloud microphysical processes, and type of precipitation. Observations in northern Europe and eastern North America in the mid-1970s, compared with the mid-1950s, showed a twofold to threefold increase in hydrogen ion deposition and rainfall acidity. Sulphate concentrations in rainwater in Europe increased over this twenty-year period by 50 per cent in southern Europe and 100 per cent in Scandinavia, although there has been a subsequent decrease, apparently associated with reduced sulphur emissions in both Europe and North America. The emissions from coal and fuel oil in these regions have high sulphur content (2-3 per cent) and, since major SO2 emissions occur from elevated stacks, SO2 is readily transported by the low-level winds. NO^ emissions, by contrast, are primarily from automobiles and thus NO3- is deposited mainly locally. SO2 and NO^ have atmospheric resident times of one to three days. SO2 is not dissolved readily in cloud or raindrops unless oxidized by OH or H2O2, but dry deposition is quite rapid. NO is insoluble in water, but it is oxidized to NO2 by reaction with ozone, and ultimately to HNO3 (nitric acid), which dissolves readily.

In the western United States, where there are fewer major sources of emission, H+ ion concentrations in rainwater are only 15 to 20 per cent of levels in the east, while sulphate and nitrate anion concentrations are one-third to one-half of those in the east. In China, high-sulphur coal is the main energy source and rainwater sulphate concentrations are high; observations in southwest China show levels six times those in New York City. In winter, in Canada, snow has been found to have more nitrate and less sulphate than rain, apparently because falling snow scavenges nitrate faster and more effectively. Consequently, nitrate accounts for about half of the snowpack acidity. In spring, snow-melt runoff causes an acid flush that may be harmful to fish populations in rivers and lakes, especially at the egg or larval stages.

In areas with frequent fog, or hill cloud, acidity may be greater than with rainfall; North American data indicate pH values averaging 3.4 in fog. This is a result of several factors. Small fog or cloud droplets have a large surface area, higher levels of pollutants provide more time for aqueous-phase chemical reactions, and the pollutants may act as nuclei for fog droplet condensation. In California, pH values as low as 2.0 to 2.5 are not uncommon in coastal fogs. Fog water in Los Angeles usually has high nitrate concentrations due to automobile traffic during the morning rush-hour.

The impact of acid precipitation depends on the vegetation cover, soil and bedrock type. Neutralization may occur by addition of cations in the vegetation canopy or on the surface. Such buffering is greatest if there are carbonate rocks (Ca, Mg cations); otherwise the increased acidity augments normal leaching of bases from the soil.

4 Aerosols

There are 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 (Table 2.2). Some originate as particles - soil grains and mineral dust from dry surfaces, carbon soot from coal fires and biomass burning, and volcanic dust. Figure 2.1B shows their size distributions. Others are converted into particles from inorganic gases (sulphur from anthropogenic SO2 and natural H2S; ammonium salts from NH3; nitrogen from NO^). Sulphate aerosols, two-thirds of which come from coal-fired power station emissions, now play an important role in countering global warming effects by

Table 2.2 Aerosol production estimates, less than 5 jUm radius (I09 kg/year) and typical concentrations near the surface (Ug m-3).

Concentration Production Remote Urban

Natural

Primary production

Sea salt 2300

Mineral particles 900-1500

Volcanic 20

Forest fires and biological debris 50 Secondary production (gas ^ particle):

Sulphates from H2S 70

Nitrates from NO 22

Converted plant hydrocarbons 25

Total natural 3600

Anthropogenic Primary production:

Mineral particles 0-600

Industrial dust 50

Combustion (black carbon) 10

(organic carbon) 50 Secondary production (gas ^ particle):

Sulphate from SO2 140

Nitrates from NO 30

Biomass combustion

(organics) 20

Total anthropogenic 290-890

Notes: *I0-60 jug m 3 during dust episodes from the Sahara over the Atlantic. t Total suspended particles. 109 kg = I Tg

Sources: Ramanathan et al. (2001), Schimel et al. (1996), Bridgman (1990).

Figure 2.1 Atmospheric particles. (A) Mass distribution, together with a depiction of the surface-atmosphere processes that create and modify atmospheric aerosols, illustrating the three size modes. Aitken nuclei are solid and liquid particles that act as condensation nuclei and capture ions, thus playing a role in cloud electrification. (B) Distribution of surface area per unit volume.

Figure 2.1 Atmospheric particles. (A) Mass distribution, together with a depiction of the surface-atmosphere processes that create and modify atmospheric aerosols, illustrating the three size modes. Aitken nuclei are solid and liquid particles that act as condensation nuclei and capture ions, thus playing a role in cloud electrification. (B) Distribution of surface area per unit volume.

Sources: (A) After Glenn E. Shaw, University of Alaska, Geophysics Institute. (B) After Slinn (1983).

reflecting incoming solar radiation (see Chapter 13). Other aerosol sources are sea-salt and organic matter (plant hydrocarbons and anthropogenically derived). Natural sources are several times larger than anthropogenic ones on a global scale, but the estimates are wide-ranging. Mineral dust is particularly hard to estimate due to the episodic nature of wind events and the considerable spatial variability. For example, the wind picks up some 1500 Tg (1012g) of crustal material annually, about half from the Sahara and the Arabian

Peninsula (see Plate 5). Most of this is deposited downwind over the Atlantic. There is similar transport from western China and Mongolia eastward over the North Pacific Ocean. Large particles originate from mineral dust, sea salt spray, fires and plant spores (Figure 2.1A); these sink rapidly back to the surface or are washed out (scavenged) by rain after a few days. Fine particles from volcanic eruptions may reside in the upper stratosphere for one to three years.

Small (Aitken) particles form by the condensation of gas-phase reaction products and from organic molecules and polymers (natural and synthetic fibres, plastics, rubber and vinyl). There are 5 00 to 1000 Aitken particles per cm3 in air over Europe. Intermediate-sized (accumulation mode) 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 (Figure 2.1 A). Over Europe, 2000 to 3500 such particles per cm3 are measured. Particles with diameters <10 |am (PM10), originating especially from mechanical breakdown processes, are now often documented separately. Particles with diameters of 0.1 to 1.0 |am are highly effective in scattering solar radiation (Chapter 3B.2), and those of about 0.1 |am diameter are important in cloud condensation.

Having made these generalizations about the atmosphere, we now examine the variations that occur in composition with height, latitude and time.

5 Variations 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 up to at least 100 km 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 affected considerably 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 to 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 or 15 km and as the maximum possible water vapour density of cold air is very low anyway (see B.2, this chapter), there is little water vapour in the upper layers of the atmosphere.

Ozone (O3) is concentrated mainly between 15 and 35 km. The upper layers of the atmosphere are irradiated by ultraviolet radiation from the sun (see C.1, this chapter), which causes the breakup of oxygen molecules at altitudes above 30 km (i.e. O2 ^ 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:

The odd hydrogen atoms and OH result from the dissociation of water vapour, molecular hydrogen and methane (CH4).

Figure 2.2 Schematic illustrations of (A) the Chapman cycle of ozone formation and (B) ozone destruction. X is any ozone-destroying species (e.g. H, OH, NO, CR, Br).

Source: After Hales (1996), from Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

where M represents the energy and momentum balance provided by collision with a third atom or molecule; this Chapman cycle is shown schematically in Figure 2.2A. 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 formed mainly between 30 and 60 km, where collisions between O and O2 are more likely. Ozone itself is unstable; its abundance is determined by three 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 (ClO) and bromine (BrO) chains at various altitudes. Collisions with monatomic oxygen may recreate oxygen (see Figure 2.2B), but ozone is destroyed mainly through cycles involving catalytic reactions, some of which are photochemical associated with longer wavelength ultraviolet radiation (2.3 to 2.9 ^m). 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):

Figure 2.2 Schematic illustrations of (A) the Chapman cycle of ozone formation and (B) ozone destruction. X is any ozone-destroying species (e.g. H, OH, NO, CR, Br).

Source: After Hales (1996), from Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

Stratospheric ozone is similarly destroyed in the presence of nitrogen oxides (NOx, i.e. NO2 and NO) and chlorine radicals (Cl, ClO). The source gas of the NOx is nitrous oxide (N2O), which is produced by combustion and fertilizer use, while chlorofluorocarbons (CFCs), manufactured for 'freon', give rise to the chlorines. These source gases are transported up to the stratosphere from the surface and are converted by oxidation into NOx, and by UV photodecomposition into chlorine radicals, respectively.

The chlorine chain involves:

Both reactions result in a conversion of O3 to O2 and the removal of all odd oxygens. Another cycle may involve an interaction of the oxides of chlorine and bromine (Br). It appears that the increases of Cl and Br species during the years 1970 to 1990 are sufficient to explain the observed decrease of stratospheric ozone over Antarctica (see Box 2.1). A mechanism that may enhance the catalytic process involves polar stratospheric clouds. These can form readily during the austral

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