Geoscience Reference
In-Depth Information
2.1 Ozone in the stratosphere
Ozone measurements were first made in the 1930s. Two properties are of interest: (1) 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; (2) 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 1957-1958,
showed a regular annual cycle with an austral spring (October to 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 300DU in
summer and continued at 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 20km 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 now reach minima of 95-100DU
in the austral spring.
Using a boundary of 220DU (corresponding to a thin, 2.2mm 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 km
2
during 1990-1999. This expanded to cover 27 million km
2
by early September in 1999 and 2000 and continued at this level through spring 2006.
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 330DU for the Arctic vortex
were recorded compared with 360DU, 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 180DU. 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.
To combat the ozone decreases, the Montreal Protocol was agreed internationally in 1987 to
phase out the production of substances thought to be responsible for ozone depletion.
Subsequently, the concentrations of the most important chlorofluorocarbons (CFCs) have either
leveled off or decreased, but the size of the ozone hole has not yet responded.
composition. The concentrations and sources
of the most important greenhouse gases are
considered in turn.
fuels. The atmosphere contains about 800
×
10
12
kg of carbon (C), corresponding to a CO
2
concentration of 387ppm (
Figure 2.4
). The
major fluxes of CO
2
are a result of solution/
dissolution in the ocean and photosynthesis/
respiration and decomposition by biota. The
•
Carbon dioxide
(CO
2
). The major reservoirs of
carbon are in limestone sediments and fossil
