The Earth’s atmosphere is the layer of gases held close to the planet by gravity, acting as a protective blanket that regulates temperature and shields life from the sun’s radiation. This gaseous envelope is responsible for the air we breathe and the weather systems that govern our world. The air is a mixture of various gases, but one component far outweighs all the others in sheer volume.
Nitrogen: The Atmosphere’s Primary Component
The most abundant gas in the atmosphere is molecular nitrogen (\(\text{N}_2\)), which consistently accounts for approximately 78% of the total volume of dry air. The sheer quantity of nitrogen means it is the primary constituent that gives the atmosphere its bulk and pressure.
Nitrogen gas is characterized by its chemical stability, which is due to the triple bond linking its two atoms. It does not readily combine with other elements or compounds in the air, meaning it acts mainly as a diluent for the more chemically active gases, like oxygen.
The Secondary and Trace Gases
The second most abundant gas is oxygen (\(\text{O}_2\)), which makes up about 21% of the atmosphere. Oxygen is highly reactive and is continuously supplied by biological processes, primarily photosynthesis, and is indispensable for the respiration of nearly all complex life forms. Together, nitrogen and oxygen account for roughly 99% of the atmosphere’s volume.
The remaining 1% of the atmosphere is composed of a mixture of other gases, which are often categorized as trace gases. The third most common gas is argon (Ar), an inert noble gas that constitutes about 0.93% of the atmosphere. Argon is chemically unreactive, much like nitrogen, and has no significant biological function.
Following these major components are variable gases, whose concentrations fluctuate based on location and time. Water vapor (\(\text{H}_2\text{O}\)) is the most significant of these variable components, ranging from nearly 0% in arid polar regions to as much as 4% in humid tropical areas. Carbon dioxide (\(\text{CO}_2\)), while only present at roughly 0.04% (or about 420 parts per million), is a powerful greenhouse gas that plays a significant role in regulating the planet’s temperature and is utilized by plants during photosynthesis.
Other trace components, such as neon (Ne), helium (He), methane (\(\text{CH}_4\)), and krypton (Kr), exist in minuscule quantities, often measured in parts per million or parts per billion. Despite their low concentrations, many of these trace gases, particularly the greenhouse gases, have disproportionately large effects on atmospheric chemistry and the Earth’s climate system.
The Critical Biological Role of Atmospheric Nitrogen
Despite its chemical inertness in the atmosphere, nitrogen is required for all forms of life on Earth. It is a building block for many organic molecules, including amino acids, which form proteins, and the nucleic acids DNA and RNA. Organisms cannot directly use the abundant \(\text{N}_2\) gas from the air because they lack the necessary biochemical machinery to break its strong triple bond.
This conundrum is solved through a process called nitrogen fixation, which converts atmospheric \(\text{N}_2\) into biologically usable forms, primarily ammonia (\(\text{NH}_3\)). This conversion is carried out naturally by specialized microorganisms, such as nitrogen-fixing bacteria, which possess the enzyme nitrogenase. These bacteria often live in symbiotic relationships within the root nodules of certain plants, like legumes, or exist freely in the soil.
Nitrogen fixation is the first step in the nitrogen cycle, ensuring the gas becomes available to the biosphere. Abiotic processes, such as lightning strikes, also contribute a small amount of fixed nitrogen by providing the intense energy needed to break the \(\text{N}_2\) bond. Once fixed into compounds like ammonia or nitrates, nitrogen can be absorbed by plants and then transferred to animals that consume them.