Density of Air

The pore-air plays an important role in the behavior of unsaturated soils. For example, the density of air acts as a driving force for the vapor flow in the soils. The understanding of vapor flow is important in nuclear waste repositories and other geotechnical applications involving partially saturated soils. The density of air also plays an important role in the equilibrium between pore-air and free water or pore-water and the atmospheric air. The density of air is the mass of air, M , per unit volume of the air, V a . Air density depends on other state variables, namely, temperature and pressure of the atmosphere. Density also varies significantly with the composition of air. The natural air is comprised of dissimilar gaseous mixture, namely, oxygen, nitrogen, and other trace gases. The composition of these mixtures in the air, by volume, is: oxygen = 20.95%, nitrogen = 78.09%, and trace gases = 0.96% as illustrated in Fig.2.5.

Fig. 2.5. Illustration showing the composition of air

The molecular mass of different components of air is presented in Table 1.