Earths atmosphere is composed mainly of nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, water vapour, and many trace gases. In dry air near Earths surface, nitrogen accounts for about 78 percent by volume, oxygen for about 21 percent, argon for about 0.93 percent, and carbon dioxide for roughly 0.04 percent, although the exact carbon dioxide concentration changes over time and location.
Water vapour is highly variable and can range from almost zero to several percent of the atmosphere depending on temperature, humidity, altitude, and weather. Trace gases include neon, helium, methane, krypton, hydrogen, nitrous oxide, ozone, and others. These gases differ greatly in concentration but can have disproportionate effects because some participate strongly in radiative processes, atmospheric chemistry, biological cycles, or climate regulation.
Nitrogen is the most abundant gas in the atmosphere and exists primarily as diatomic nitrogen molecules containing two nitrogen atoms. Molecular nitrogen is relatively chemically stable because the two atoms are held together by a strong triple bond. Despite its abundance, most organisms cannot directly use atmospheric nitrogen gas for biological growth. Nitrogen fixation by certain microorganisms converts atmospheric nitrogen into chemically usable compounds, while lightning and industrial processes also contribute to nitrogen fixation.
Nitrogen compounds are essential components of proteins, nucleic acids, and other biological molecules. Nitrogen is also involved in atmospheric chemistry and nutrient cycling, although its dominant atmospheric form is comparatively unreactive. The large proportion of nitrogen provides the background composition of the atmosphere and influences its physical properties, including pressure and density.
Oxygen is the second most abundant major atmospheric gas and is present primarily as diatomic oxygen molecules. It is essential for aerobic respiration in many organisms and supports combustion and numerous oxidation reactions. Most atmospheric oxygen is maintained through interactions among photosynthesis, respiration, decomposition, and geological processes. Photosynthetic organisms use light energy to convert carbon dioxide and water into organic matter while releasing oxygen, whereas respiration consumes oxygen and produces carbon dioxide and water. The present atmospheric oxygen concentration reflects a long history of biological and geological processes, including the development and expansion of oxygen producing organisms. Oxygen also participates in atmospheric chemistry, including the formation and destruction of ozone and reactions involving pollutants and naturally occurring gases.
Argon is the third most abundant gas in dry air and is chemically inert under most ordinary atmospheric conditions. It is a noble gas, meaning that its outer electron structure makes it relatively unreactive compared with many other elements. Most atmospheric argon is argon 40, which is produced by the radioactive decay of potassium 40 within Earths crust and then enters the atmosphere through geological processes.
Because argon reacts very little chemically, its atmospheric abundance is useful in certain geochemical and atmospheric studies. Other noble gases, including neon, helium, krypton, and xenon, occur in much smaller concentrations. Their low reactivity means that they generally participate less directly in biological and chemical cycles than gases such as oxygen, carbon dioxide, methane, or water vapour.
Carbon dioxide is present at a much lower concentration than nitrogen and oxygen but is scientifically important because it is a greenhouse gas and a major component of the global carbon cycle. Plants and algae use carbon dioxide during photosynthesis, while respiration, decomposition, volcanic activity, ocean processes, and combustion release it. The oceans absorb and release large quantities of carbon dioxide, and dissolved carbon dioxide participates in seawater carbonate chemistry.
Human activities, particularly fossil fuel combustion, cement production, and land use changes, have increased atmospheric carbon dioxide substantially relative to preindustrial conditions. Because carbon dioxide absorbs infrared radiation, changes in its concentration influence Earths energy balance. Its concentration is therefore important far beyond what would be expected from its small percentage of atmospheric volume.
Water vapour is the most abundant variable gas in the atmosphere and can represent a much larger fraction of air than most trace gases under warm and humid conditions. Unlike nitrogen and oxygen, its concentration changes rapidly with temperature, evaporation, condensation, precipitation, and atmospheric circulation. Water vapour is a powerful greenhouse gas and is central to cloud formation, rainfall, storms, and the hydrological cycle.
Because warmer air can generally contain more water vapour before reaching saturation, changes in temperature can influence atmospheric moisture and amplify climate responses through water vapour feedback. Water vapour also participates in chemical reactions and transports latent heat when water changes between vapour, liquid, and solid phases. Its highly variable concentration is one reason atmospheric composition is often reported separately as dry air composition and humid air composition.
Other trace gases can have important atmospheric effects despite their low concentrations. Methane is a potent greenhouse gas and participates in atmospheric oxidation chemistry. Nitrous oxide is another greenhouse gas and also contributes to stratospheric chemical processes. Ozone occurs in very small concentrations but absorbs ultraviolet radiation in the stratosphere and contributes to air pollution near the ground. Hydrogen, carbon monoxide, nitrogen oxides, sulfur dioxide, and volatile organic compounds are present at low concentrations and can participate in atmospheric reactions. Some are emitted naturally, while others have important human sources. Atmospheric aerosols are not gases at all but are often discussed alongside atmospheric composition because they consist of suspended particles and droplets that affect radiation, clouds, chemistry, and air quality.
The relative abundance of atmospheric gases changes with altitude, location, season, weather, biological activity, pollution, and geological processes. The atmosphere is therefore not a perfectly fixed mixture. Water vapour varies most strongly, while gases such as carbon dioxide and methane can change over time because of natural cycles and human emissions. In the upper atmosphere, solar radiation can break molecules apart and create ions and radicals, causing chemical compositions and reactions to differ from those near the surface.
The concentrations of trace gases are often measured in parts per million, parts per billion, or even smaller units. Understanding atmospheric composition is important for meteorology, climate science, atmospheric chemistry, ecology, aviation, and air quality. The key principle is that abundance alone does not determine importance because gases with very small concentrations can strongly influence radiation, chemistry, biological processes, and climate.
