Ozone In The Stratosphere

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Ozone measurements were first made in the 1930s. Two properties are of interest: (i) the total ozone in an atmospheric column. This is measured with the Dobson spectrophotometer by comparing the solar radiation at a wavelength where ozone absorption occurs with that in another wavelength where such effects are absent; (ii) the vertical distribution of ozone. This can be measured by chemical soundings of the stratosphere, or calculated at the surface using the Umkehr method; here the effect of solar elevation angle on the scattering of solar radiation is measured. Ozone measurements, begun in the Antarctic during the International Geophysical Year, 1957-58, showed a regular annual cycle with an austral spring (October-November) peak as ozone-rich air from mid-latitudes was transported poleward as the winter polar vortex in the stratosphere broke down. Values declined seasonally from around 450 Dobson units (DU) in spring to about 300 DU in summer and continued about this level through the autumn and winter. Scientists of the British Antarctic Survey noted a different pattern at Halley Base beginning in the 1970s. In spring, with the return of sunlight, values began to decrease steadily between about 12 and 20 km altitude. Also in the 1970s, satellite sounders began mapping the spatial distribution of ozone over the polar regions. These revealed that low values formed a central core and the term "Antarctic ozone hole" came into use. Since the mid-1970s, values start decreasing in late winter and reach minima of around 100 DU in the austral spring.

Using a boundary of 220 DU (corresponding to a thin, 2.2-mm ozone layer, if all the gas were brought to sea level temperature and pressure), the extent of the Antarctic ozone hole at the end of September averaged 21 million km2, during 1990-99. This expanded to cover 27 million km2 by early September in 1999 and 2000.

In the Arctic, temperatures in the stratosphere are not as low as over the Antarctic, but in recent years ozone depletion has been large when temperatures fall well below normal in the winter stratosphere. In February 1996, for example, column totals averaging 330 DU for the Arctic vortex were recorded compared with 360 DU, or higher, in other years. A series of mini-holes was observed over Greenland, the northern North Atlantic and northern Europe with an absolute low over Greenland below 180 DU. An extensive ozone hole is less likely to develop in the Arctic because the more dynamic stratospheric circulation, compared with the Antarctic, transports ozone poleward from mid-latitudes.

spring (October), when temperatures decrease to 185 to 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 the 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. Nevertheless, ozone decreases are now observed in the boreal spring in the Arctic stratosphere.

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 a circulation mechanism transporting ozone downward 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 sealevel (at normal sea-level temperature and pressure) ozone would contribute only about 3 mm to the total atmospheric thickness of 8 km (Figure 2.3).

6 Variations with latitude and season

Variations of atmospheric composition with latitude and season are particularly important in the case of water vapour and stratospheric ozone.

Ozone content is low over the equator and high in subpolar latitudes in spring (see Figure 2.3). If the distribution were solely the result of photochemical

Ozone Pressure Latitude
Figure 2.3 Variation of total ozone with latitude and season in Dobson units (milliatmosphere centimeters) for two time intervals: (top) 1964-1980 and (bottom) 1984-1993. Values over 350 units are stippled.

Source: From Bojkov and Fioletov (1995). From Journal of Geophysical Research 100 (D), Fig. 15, pp. 16, 548. Courtesy of American Geophysical Union.

processes, the maximum would occur in June near the equator, so the anomalous pattern must result from a poleward transport of ozone. Apparently, ozone moves from higher levels (30 to 40 km) in low latitudes towards lower levels (20 to 25 km) in high latitudes during the winter months. Here the ozone is stored during the polar night, giving rise to an ozone-rich layer in early spring under natural conditions. It is this feature that has been disrupted by the stratospheric ozone 'hole' that now forms each spring in the Antarctic and in some recent years in the Arctic also (see Box 2.1). The type of circulation responsible for this transfer is not yet known with certainty, although it does not seem to be a simple, direct one.

The water vapour content of the atmosphere is related closely to air temperature (see B.2, this chapter, and Chapter 4B and C) and is therefore greatest in summer and in low latitudes. There are, however, obvious exceptions to this generalization, such as the tropical desert areas of the world.

The carbon dioxide content of the air (currently averaging 372 parts per million (ppm)) has a large seasonal range in higher latitudes in the northern hemisphere associated with photosynthesis and decay in the biosphere. At 50°N, the concentration ranges from about 365 ppm in late summer to 378 ppm in spring. The low summer values are related to the assimilation of CO2 by the cold polar seas. Over the year, a small net transfer of CO2 from low to high altitudes takes place to maintain an equilibrium content in the air.

7 Variations with time

The quantities of carbon dioxide, other greenhouse gases and particles in the atmosphere undergo long-term variations that may play an important role in the earth's radiation budget. Measurements of atmospheric trace gases show increases in nearly all of them since the Industrial Revolution began (Table 2.3). The burning of fossil fuels is the primary source of these increasing trace gas concentrations. Heating, transportation and industrial activities generate almost 5 X 1020 J/year of energy. Oil and natural gas consumption account for 60 per cent of global energy and coal about 25 per cent. Natural gas is almost 90 per cent methane (CH4), whereas the burning of coal and oil releases not only CO2 but also odd nitrogen (NOx), sulphur and carbon monoxide (CO). Other factors relating to agricultural practices (land clearance, farming, paddy cultivation and cattle raising) also contribute to modifying the atmospheric composition. The concentrations and sources of the most important greenhouse gases are considered in turn.

Carbon dioxide (CO2). The major reservoirs of carbon are in limestone sediments and fossil fuels. The atmosphere contains just over 775 X 1012 kg of carbon (C), corresponding to a CO2 concentration of 370 ppm (Figure 2.4). The major fluxes of CO2 are a result of solution/dissolution in the ocean and photosynthesis/ respiration and decomposition by biota. The average

Table 2.3 Anthropogenically induced changes in concentration of atmospheric trace gases.

Gas Concentration Annual Increase Sources

1850* 2000 1990s

Carbon dioxide

280 ppm

370 ppm

0.4

Fossil fuels

Methane

800 ppbv

I750ppbv

0.3

Rice paddies, cows,

wetlands

Nitrous oxide

280 ppbv

316 ppbv

0.25

Microbiological activity,

fertilizer, fossil fuel

CFC-11

0

0.27 ppbv

- 0

Freon+

HCFC-22

0

0.11 ppbv

5

CFC substitute

Ozone

?

10-50 ppbv

- 0

Photochemical reactions

(troposphere)

(troposphere)

Notes: * Pre-industrial levels are derived primarily from measurements in ice cores where air bubbles are trapped as snow accumulates on polar ice sheets. t Production began in the 1930s.

Source: Updated from Schimel et al. (1996), in Houghton et al. (1996).

Source: Updated from Schimel et al. (1996), in Houghton et al. (1996).

Figure 2.4 Global carbon reservoirs (gigatonnes of carbon (GtC): where 1 Gt = I09 metric tons = I012 kg) and gross annual fluxes (GtC yr-1). Numbers emboldened in the reservoirs suggest the net annual accumulation due to anthropogenic causes.

Source: Based on Sundquist, Trabalka, Bolin and Siegenthaler; after Houghton et al. (1990 and 2001).

Figure 2.4 Global carbon reservoirs (gigatonnes of carbon (GtC): where 1 Gt = I09 metric tons = I012 kg) and gross annual fluxes (GtC yr-1). Numbers emboldened in the reservoirs suggest the net annual accumulation due to anthropogenic causes.

Source: Based on Sundquist, Trabalka, Bolin and Siegenthaler; after Houghton et al. (1990 and 2001).

time for a CO2 molecule to be dissolved in the ocean or taken up by plants is about four years. Photosynthetic activity leading to primary production on land involves 50 X 1012 kg of carbon annually, representing 7 per cent of atmospheric carbon; this accounts for the annual oscillation in CO2 observed in the northern hemisphere due to its extensive land biosphere.

The oceans play a key role in the global carbon cycle. Photosynthesis by phytoplankton generates organic compounds of aqueous carbon dioxide. Eventually, some of the biogenic matter sinks into deeper water, where it undergoes decomposition and oxidation back into carbon dioxide. This process transfers carbon dioxide from the surface water and sequesters it in the ocean deep water. As a consequence, atmospheric concentrations of CO2 can be maintained at a lower level than otherwise. This mechanism is known as a 'biologic pump'; long-term changes in its operation may have caused the rise in atmospheric CO2 at the end of the last glaciation. Ocean biomass productivity is limited by the availability of nutrients and by light. Hence, unlike the land biosphere, increasing CO2 levels will not necessarily affect ocean productivity; inputs of fertilizers in river runoff may be a more significant factor. In the oceans, the carbon dioxide ultimately goes to produce carbonate of lime, partly in the form of shells and the skeletons of marine creatures. On land, the dead matter becomes humus, which may subsequently

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