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Composition
CHAPTER 2 The Atmosphere

The Chemical Composition of the Atmosphere


Composition

In the lowest 25 km of the atmosphere we find a mixture of permanent gases, certain variable gases, and solid and liquid particles. Water vapor is the most important variable gas and can account for as much as 4% of the atmosphere by volume. This "minor" constituent plays a most important role in the dynamics of the atmosphere. We can divide the atmosphere into that part in which the gaseous composition is nearly constant and a second part (above the first) in which the composition varies with altitude and time of year. The homosphere is the bottom part of the atmosphere (below 80 km) where there is general homogeneity of atmospheric composition (excluding water vapor whichis hightly variable). The heterosphere is the top part of the atmosphere (above 80 km) where there is general heterogeneity of atmospheric composition (the mean molecular weight varies with altitude above 80 km).

The average composition of dry, pure air in the layer of the atmosphere up to an altitude of 25 km is summarized in Table 3. Traces of other gases (carbon monoxide [CO], radon [Rd], nitrous oxide [N2O], methane [CH4], ammonia [NH3], and various hydrocarbons) occur also and are important beyond their low concentrations. Many of these are greenhouse gases and play a significant role in the radiative equilibrium of the Earth-Atmosphere system. In fact, the most important gases from a meteorological standpoint are the minor gases, with nitrogen and oxygen providing little but a bulk pressurizing service and service to life.

Above 80 km oxygen is dissociated on absorption of ultraviolet radiation from the sun. At higher levels nitrogen is also dissociated. While the dissociation products are not significant from the stand point of the dynamics of the atmosphere they are critical to the layering of the atmosphere. The dissociated ions form the layers off which we bounce our radio waves for long distance communication.

Dust, smoke, salt particles, condensed liquid and solid water are the most abundant nongaseous material found in the atmosphere. These materials affect the transmission of incoming solar radiation as well as outgoing terrestrial radiation. These particles are of critical importance in cloud processes and in the realization of precipitation from these clouds.


Climate Dynamics - 05 FEB 96
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