Air is a mixture of "permanent" gases (N2, O2) in constant ratio together with minor constituents (see Table 1.2). The molecular weight of the mixture that makes up air is 28.97, so that 22.4 liters of air at standard temperature and pressure (STP; T = 273 K and p = 1013 h Pa) weighs 28.97 g.
The composition of air is a direct consequence of the supply of elements from the Earth's interior and the presence of life on the surface. Photosynthesis by plants makes O2; nitrogenous compounds from living organisms are returned to the atmosphere as N2 from metabolism. Lightning converts N2 into usable molecules for life. Two of the most important minor constituents are H2O and CO2; they play a central role in controlling the temperature of the Earth's surface (see Chapter 2) and sustaining life (living material is primarily composed of C, H and O).
Atmospheric water vapor is present in variable amounts (typically 0.5% by volume). It is primarily the result of evaporation from the ocean's surface. Unlike N2 and O2, water vapor—and to a lesser degree CO2—is of great importance in radiative transfer (the passage of radiation through the atmosphere), because it strongly absorbs and emits in the infrared, the region of the spectrum (wavelengths about 10 ^m)
Atlas Mis. I
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