Atmospheric air contains many gaseous components as well as water vapor and miscellaneous contaminants (e.g., smoke, pollen, and gaseous pollutants not normally present in free air far from pollution sources).
Dry air is atmospheric air with all water vapor and contaminants removed. Its composition is relatively constant, but slight variations in the amounts of individual components occur with time, geographic location, and altitude.
Harrison(1965) lists the approximate percentage composition of dry air by volume as:
Nitrogen, 78.084;
Oxygen, 20.9476;
Argon, 0.934;
Neon, 0.001818;
Helium, 0.000524;
Methane, 0.00015;
Sulfur Dioxide, 0 to 0.0001;
Hydrogen, 0.00005;
and minor components such as Krypton, Xenon, and Ozone, 0.0002.
Harrison(1965) and Hyland and Wexler(1983a) used a value of 0.0314 (circa 1955) for Carbon Dioxide. Carbon dioxide reached 0.0379 in 2005, is currently increasing by 0.00019 percent per year and is projected to reach 0.0438 in 2036 (Gatley et al. 2008; Keeling and Whorf 2005a, 2005b). Increases in carbon dioxide are offset by decreases in oxygen; consequently, the oxygen percentage in 2036 is projected to be 20.9352. Using the projected changes, the relative molecular mass for dry air for at least the first half of the 21st century is 28.966, based on the carbon-12 scale.
The gas constant for dry air using the Mohr and Taylor (2005) value for the universal gas constant is
Rda = 8314.472/28.966 = 287.042 J/(kgda ·K)
Moist air is a binary (two-component) mixture of dry air and water vapor. The amount of water vapor varies from zero (dry air) to a maximum that depends on temperature and pressure.
Saturation is a state of neutral equilibrium between moist air and the condensed water phase (liquid or solid); unless otherwise stated, it assumes a flat interface surface between moist air and the condensed phase. Saturation conditions change when the interface radius is very small (e.g., with ultrafine water droplets).
According to the Industrial Formulation IAPWS-IF97 (R7-97 2012), the relative molecular mass of water is 18.015 257.
The gas constant for water vapor is
Rw = 8314.472/18.015 257 = 461.524 J/(kgw ·K)