Air Composition

ir is a mixture of gases - 78% nitrogen and 21% oxygen - with traces of water vapor, carbon dioxide, argon, and various other components. Air is usually modeled as a uniform (no variation or fluctuation) gas with properties averaged from the individual components.

Gas	Ratio compared to Dry Air (%)		Molecular Mass - M - (kg/kmol)	Chemical Symbol	Boiling Point
	By volume	By weight			(K)	(oC)
Oxygen	20.95	23.20	32.00	O2	90.2	-182.95
Nitrogen	78.09	75.47	28.02	N2	77.4	-195.79
Carbon Dioxide	0.03	0.046	44.01	CO2	194.7	-78.5
Hydrogen	0.00005	~ 0	2.02	H2	20.3	-252.87
Argon	0.933	1.28	39.94	Ar	84.2	-186
Neon	0.0018	0.0012	20.18	Ne	27.2	-246
Helium	0.0005	0.00007	4.00	He	4.2	-269
Krypton	0.0001	0.0003	83.8	Kr	119.8	-153.4
Xenon	9 10-6	0.00004	131.29	Xe	165.1	-108.1

• The water or vapor content in air varies. The maximum moisture carrying capacity of air depends primarily on temperature
• The composition of air is unchanged until elevation of approximately 10.000 m
• The average air temperature diminishes at the rate of 0.6^oC for each 100 m vertical height
• "One Standard Atmosphere" is defined as the pressure equivalent to that exerted by a 760 mm column of mercury at 0^oC sea level and at standard gravity (32.174 ft/sec²)

Other components in air

• Sulfur dioxide - SO₂ - 1.0 parts/million (ppm)
• Methane - CH₄ - 2.0 parts/million (ppm)
• Nitrous oxide - N₂O - 0.5 parts/million (ppm)
• Ozone - O₃ - 0 to 0.07 parts/million (ppm)
• Nitrogen dioxide - NO₂ - 0.02 parts/million (ppm)
• Iodine - I₂ - 0.01 parts/million (ppm)
• Carbon monoxide - CO - 0 to trace (ppm)
• Ammonia - NH₃ - 0 to trace (ppm)

Common Pressure Units frequently used as alternative to "one Atmosphere"

• 76 Centimeters (760 mm) of Mercury
• 29.921 Inches of Mercury
• 10.332 Meters of Water
• 406.78 Inches of Water
• 33.899 Feet of Water
• 14.696 Pound-Force per Square Inch
• 2116.2 Pounds-Force per Square Foot
• 1.033 Kilograms-Force per Square Centimeter
• 101.33 Kilopascal