Chemical Composition of Air

There is no chemical formula for air because the Earth’s atmosphere consists of a mixture of gases. Just three gases account for 99% of dry air: nitrogen, oxygen, and argon. If you figure water vapor into the composition, then 99.9% of air consists of five gases: nitrogen, oxygen, argon, water vapor, and carbon dioxide.

Composition of Air

This table lists the chemical composition of air. Water vapor comprises anywhere from 0% to 5% of air, averaging about 0.4% (about the same as the amount of carbon dioxide). The amount of water vapor in air largely depends on temperature. So, when it’s cold, there is very little water vapor. In hot, humid climates, water is more abundant than argon.

While the percentages of nitrogen, oxygen, and argon are fairly stable, the increasing amount of carbon dioxide in air means older tables list a lower percentage. For example, the 1996 CRC Handbook of Chemistry and Physics listed the carbon dioxide percentage as 0.0314%, while the 2019 number was 0.0497% and the 2020 number is closer to 0.0415%! Any time the percentage of carbon dioxide changes, it alters the relative percentages of other trace gases (e.g., neon, helium, methane, etc.). So, don’t get too caught up in the numbers. The relative abundance is what matters. So, there is nearly three times more neon than helium, which is about three times more abundant than methane.

Also, note that levels of carbon oxides, nitrogen oxides, and sulfur oxides may be higher near local sources of combustion or volcanic activity. Air also contains particulates, such as soot, spores, pollen, dust, and volcanic ash.

Composition of Other Layers of the Atmosphere

The composition of air refers to the chemical composition of the troposphere. The troposphere is the lowest layer of the atmosphere, extending from the surface to about 12 km or 39000 ft. The troposphere contains about 80% of the mass of the Earth’s atmosphere. Nearly all of the atmosphere’s water vapor exists in this layer.

The stratosphere is above the troposphere. The tropopause separates the two layers. The stratosphere extends to an altitude of 50 to 55 km (164000 to 180000 ft). It contains very little water. The ozone layer is within the stratosphere. Most of the atmosphere’s ozone resides in this layer.

The mesosphere extends from the stratopause to 80 to 85 km (260000 to 280000 ft). It contains very little water or ozone.

The thermosphere extends from the mesopause to the thermopause, at about 500 to 1000 km (1,600,000 to 3,300,000 ft). It contains the ionosphere. There is no water here. This layer contains gas molecules, which may become ionized.

The outer layer of the Earth’s atmosphere is the exosphere, which merges into the solar wind around 10,000 km (33,000,000 ft). This layer consists of hydrogen, helium, nitrogen, oxygen, and carbon dioxide. Molecular forms of the elements only occur toward the base of the layer. The density of “air” here is too thin for it to behave as a gas. The effect of the solar wind ionizes atoms to form plasma.

References

• Cox, Arthur N., ed. (2000). Allen’s Astrophysical Quantities (4th ed.). AIP Press. pp. 258–259. ISBN 0-387-98746-0.
• Haynes, H. M., ed. (2016–2017). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. ISBN 978-1-4987-5428-6.
• Lide, David R. (1996) CRC Handbook of Chemistry and Physics. Boca Raton, FL: 14–17.