Atmospheric air is a mixture of nitrogen and oxygen being the earth atmosphere. Main components of air which are practically the same throughout the globe are nitrogen (78.08 volume per cent) and oxygen (20.95 v.%). Along with them air contains 0.94 v.% of inert gases and 0.03 v.% of carbon dioxide. The air of such a composition is named dry. Its molecular mass is regarded to be M = 28.96 g/mole.
In the lower atmosphere strata the air contains also water vapor, its concentration is substantially variable depending on the partial water vapor pressure at the appropriate temperature and relative humidity. For instance at 20°C and relative humidity 80% air contains about 0.02 v.% of water vapor. In the air layers adjacent to the earth surface other components may be present being in most cases of antropogenic origin.
At ambient pressure and temperature air can be regarded as a perfect gas, its properties may be described by equations:
where v denotes specific volume; u is specific internal energy; R is the gas constant for air.
At low temperatures the air is liquified. The normal (at 0.1013 MPa) boiling (condensation) temperature of the oxygen is equal— 183°C, that of the nitrogen -195.8°C. Liquid air at atmospheric pressure behaves practically as an ideal solution following the Raoult's Law. The normal condensation temperature of air is -191.4°C, the normal boiling temperature -194°C.
At elevated temperatures air undergoes some physicochemical transformations. The nitrogen reacts with oxygen producing various oxides: N2O, NO, NO2, NO3. Their equilibrium concentration can be derived from the isotherm equations of the respective reactions.
At temperatures higher than 2000 K and moderate pressures the nitrogen and oxygen start to dissociate, and at temperatures exceeding 4000 K and atmospheric pressure the ionization of oxygen, nitrogen, and other components becomes evident. This implies the transition of air into the plasma state. The equilibrium dissociation degree can be calculated according to the Saha equation.
The enthalpy is taken as zero at an arbitrary point. The entropy is taken zero for the solid air at 0K.
Air is a mixture mainly consisting of diatomic gases. Therefore its heat capacity at close to normal temperatures and pressures may with good accuracy be taken equal to
With increasing temperature the heat capacity slightly increases due to exciting of the vibrational degrees of freedom in the oxygen and nitrogen molecules. Table 3 gives air heat capacity values for a wide range of temperatures and pressures.
As for all pure substances in the supercritical region, the isobars and isotherms of the heat capacity cp have maximums the steeper the closer to the critical point.
The temperature dependence of the viscosity of air is qualitatively the same as for pure substances: in the liquid phase the viscosity decreases with temperature following an approximately exponential function; in the gas phase at low pressures the viscosity increases according to equation:
with increasing pressure at constant temperature the viscosity increases. This dependence is most strong in the vicinity of the critical point. Air viscosity values at various temperatures and pressures are given in Table 4.
The behavior of the thermal conductivity of air is similar to the viscosity: in the liquid phase with growing temperature the heat conductivity decreases whereas in the gas phase-increases. At low pressures the temperature dependence is described by the equation:
Along the isotherm with increasing pressure the thermal conductivity increases. In Table 5 the air thermal conductivity is given at various temperatures and pressures.
At low pressures and high temperatures the thermal conductivity sharply increases due to dissociation. With growing temperature the thermal conductivity goes through maximums which are connected with maximum heat transfer by the heats of respective reactions. Thermal conductivities of air at dissociation conditions are given in Table 6.
Additional information about air properties can be found in the following literature: Handbook, edited by V. P. Glushko (1978) "Nauka" Publishing House, Moskow (in Russian).
Wassermann, A. A. and Rabinovitch, V. A. (1968) Thermophysical properties of liquid air and its components. Standarts Publishing House, Moscow (in Russian).
Handbook Thermophysical Properties of Gases and Liquids, edited by N. B. Vargaftic (1972) "Nauka" Publishing House, Moscow (in Russian).