The lowest layer of atmosphere in which man along with other living organisms live, is called troposphere. The troposphere is roughly 10 km thick, being somewhat thicker in the equatorial region than it is at the poles. It is characterized by a steady decrease in temperature and is a mixture of several different gases, most of which are fairly constant in their abundance.
The composition of troposphere excepting water vapours and air-borne particles (dust), is shown in the table 81. Both water vapour and dust occur in the troposphere in extremely variable concentrations. The concentration of water vapour in the troposphere ranges from virtually 0 per cent to more than 4 per cent. As a rule, its abundance is a function of altitude and temperature.
It is most common at the lowest levels of the atmosphere, being almost entirely absent above about 8-10 km, and it is more abundant in warm air than it is in cold air. Dust is even more variable component of troposphere. It is even more limited to the lower levels of the atmosphere than is water vapour, and it has no specific relationship with any other feature of the atmosphere.
Troposphere is the layer of sulphates and is the region of strong air movements and cloud formation, i.e. it is the locus of most weather phenomena which affect different ecosystems of the biosphere.
Table 8 1. Composition of the dry atmosphere (air of troposphere) by volume (Clapham, Jr., 1973):
Carbon dioxide (CO2)0.0314*
Nitrous oxide (N2O)0.00005
(*These components are highly variable.)
The stratosphere can be defined as the air mass extending from the or uppermost level of the troposphere, to the stratosphere uppermost level of the stratosphere), about 56 km, above the surface of the earth. In stratosphere, the temperature rises from a minimum of about — 55°C to a maximum of about 5°C.
The stratosphere exhibits several significant differences from the troposphere. Water vapour is virtually absent. The only clouds found in the stratosphere are very thin wispy clouds formed of tiny ice crystals. Ozone (O3) is present there in significant quantities and there it forms a well marked ozone layer called ozonosphere within the stratosphere.
Ozone is formed from oxygen by a photochemical reaction in which energy from the sun (here symbolized by hv) splits apart the oxygen molecule (O2) to form atomic oxygen:
O2 + hv > 2O
The atomic oxygen then combines with molecular oxygen to form ozone:
O2 + O > O3
Both of these reactions are reversible, but the ozone content of the stratosphere is sufficiently constant, i.e., ozone is clearly in chemical equilibrium with the rest of the air, which means that ozone is being produced from oxygen as fast as it is broken down to molecular oxygen.
This is important because ozone absorbs ultraviolet radiation from the sun. In fact, the reason that the stratosphere becomes with increasing distance from the earth is that the ultraviolet energy absorbed by the ozone is transforming into heat (Craig, 1968).
For the ecosystem, the ozone umbrella is of vital importance because its absorption of ultraviolet radiation prevents the latter from reaching the surface of the earth, where it would be lethal to most living organisms. Moreover, because of its extra heat ozonosphere also acts like a blanket that reduces the cooling rate of the earth and thus adds to the effect of water vapour.
Above the stratosphere is the mesosphere which is characterized by cold temperatures and very low atmospheric pressure. In fact, the temperature reversal starts from the stratopause, i.e., temperature begins to drop, reaching a minimum about — 95?C at a level some 80—90 km above the earth’s surface. This level is termed the mesopause.
Above the mesosphere is the thermosphere which extends up to 500 km above the earth’s surface and is characterized by steady temperature increase with height from mesopause. The thermosphere includes the region in which ultraviolet radiation and cosmic rays cause ionization of molecules like oxygen and nitric oxide. This region is called the ionosphere. In the thermosphere molecules of air are so widely spaced that high frequency audible sounds are not carried by the atmosphere.
The region of atmosphere above the thermosphere is called exosphere or outer space which lacks atoms except that of hydrogen and helium and extends up to 32,190 km from the planet. Exosphere has very high temperature due to solar radiations. The earth’s magnetic field becomes more important than gravity in the distribution of atomic particles in the exosphere.
All the layers of the atmosphere are of interest to the ecologists since together they form the total blanket of air in the biosphere. Air is an important ecological factor and also serves as a medium of living for some organisms.
Moreover, sometime the ionosphere is affected by magnetic storms on the sun, with an increase in the number of free electrons present during periods of intense activity. Population cycles have been attributed to these storms and subsequent electrical activity in the ionosphere A. N. Moen (1973) has found that white-tailed deer seems to respond to unidentified factors, with an increase in nervousness, activity and heart rate.