Atmosfera, Tempo e Clima

Atmosfera, Tempo e Clima

(Parte 5 de 9)

Articles

Browning, K. A. (1996) Current research in atmospheric sciences. Weather51, 167–72.

Grahame, N. S. (2000) The development of meteorology over the last 150 years as illustrated by historical weather charts. Weather55(4),108–16.

Hare, F. K. (1951) Climatic classification. In L. D. Stamp,

L. D. and Wooldridge, S. W. (eds) London Essays in Geography, Longman, London, p. 1–34.

This chapter describes the composition of the atmosphere – its major gases and impurities, their vertical distribution, and variations through time. The various greenhouse gases and their significance are discussed. It also examines the vertical distribution of atmospheric mass and the structure of the atmosphere, particularly the vertical variation of temperature.

1Primary gases

Air is a mechanical mixture of gases, not a chemical compound. Dry air, by volume, is more than 9 per cent composed of nitrogen and oxygen (Table 2.1). Rocket observations show that these gases are mixed in remarkably constant proportions up to about 100 km altitude. Yet, despite their predominance, these gases are of little climatic importance.

Atmospheric composition, mass and structure

Learning objectives When you have read this chapter you will:

Be familiar with the composition of the atmosphere – its gases and other constituents, Understand how and why the distribution of trace gases and aerosols varies with height, latitude and time, Know how atmospheric pressure, density and water vapour pressure vary with altitude, Be familiar with the vertical layers of the atmosphere, their terminology and significance.

Table 2.1Average composition of the dry atmosphere below 25 km.

ComponentSymbolVolume %Molecular (dry air)weight

Carbon dioxide CO2 0.037 4.0 ‡Neon Ne 0.0018 20.18

*‡Helium He 0.005 4.0

‡Krypton Kr 0.01

‡Xenon Xe 0.00009

§Methane CH4 0.00017

Notes:* Decay products of potassium and uranium.

† Recombination of oxygen. ‡ Inert gases. § At surface.

In spite of their relative scarcity, the so-called greenhouse gasesplay 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:

1Carbon 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.

2Methane (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 CO2and H2O by a complex photochemical reaction system.

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

3Nitrous 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).

4Ozone (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.

5Chlorofluorocarbons (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. 6Hydrogenated 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

7Water 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).

In addition to the greenhouse gases, important reactive gas speciesare 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 109kg N/year. Biomass burning and lightning activity are other impor- tant 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 enters 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 109kg S/year), hydrogen sulphide (H2S) (20–40 109g S/year) and dimethyl sulphide (DMS) (35–5 109kg S/year).

DMS is produced primarily by biological productivity near the ocean surface. SO2emissions increased by about 50 per cent between 1940 and 1980, but declined in the 1990s. 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 pre- cipitated 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 depositionincludes 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.

from sea-salt. The background level of acidity in rainfall is about pH 4.8 to 5.6, because atmospheric CO2reacts 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:

and

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 SO2emissions occur from elevated stacks, SO2is readily transported by the low-level winds. NOxemissions, by contrast, are primarily from automobiles and thus NO3– is deposited mainly locally. SO2and NOxhave atmospheric resident times of one to three days. SO2is 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 NO2by 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.

There are significant quantities of aerosolsin 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 SO2and natural H2S; ammonium salts from NH3; nitrogen from

NOx). 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 2.2Aerosol production estimates, less than 5 µm radius (109 kg/year) and typical concentrations near the surface (µg m–3).

Concentration Production Remote Urban

Natural Primary production

Sea salt23005–10 Mineral particles 900–1500 0.5–5* Volcanic 20 Forest fires and biological debris50 Secondary production (gas →particle):

Nitrates from NOx22 Converted plant hydrocarbons25

Total natural3600

Anthropogenic Primary production:

Mineral particles 0–600

Industrial dust50

Combustion (black carbon)10}100–500† (organic carbon)50 Secondary production (gas →particle):

Nitrates from NOx300.20.5 Biomass combustion

(organics) 20 Total anthropogenic 290–890

Notes:*10–60 µg m–3during dust episodes from the Sahara over the Atlantic.

† Total suspended particles. 109kg = 1 Tg

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

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 500 to 1000 Aitken particles per cm3in 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 2.1A). Over Europe, 2000 to 3500 such particles per cm3are measured. Particles with diameters <10 µm (PM10), originating especially from mechanical breakdown processes, are now often documented separately. Particles with diameters of 0.1 to 1.0 µm are highly effective in scattering solar radiation (Chapter 3B.2), and those of about 0.1 µm 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.

(Parte 5 de 9)

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