Atmospheric composition

1. What atmospheric composition means

The atmosphere is a gaseous envelope retained by Earth's gravity. It contains gases, suspended solid and liquid particles called aerosols, and variable amounts of water vapour. Atmospheric composition is usually reported for dry air, meaning that water vapour has been excluded. Percentages quoted by volume are approximately equivalent to mole fractions for atmospheric gases; they should not be confused with percentages by mass. Because water vapour occupies part of the mixture, the percentages of other gases in moist air are slightly lower than their dry-air values.

Nitrogen accounts for about 78.08%, oxygen 20.95% and argon 0.934% of dry air by volume. Together, these gases constitute almost 99.96% of dry air. The remainder includes carbon dioxide, neon, helium, methane, krypton, hydrogen, nitrous oxide and ozone. Carbon dioxide represents slightly more than 0.04%, or over 420 parts per million, in the contemporary global atmosphere.

A trace gas is present in a small proportion, but this does not imply a small environmental effect. Ozone protects life from harmful ultraviolet radiation, while carbon dioxide and methane affect Earth's energy balance. Always distinguish a constituent's abundance from its chemical activity and ability to absorb radiation.

• One part per million equals 0.0001%; therefore, 400 ppm equals 0.04%.
• Dry air is not necessarily clean air: removing water vapour does not automatically remove pollutants or aerosols.

2. Major gases and their geographical significance

Nitrogen is the most abundant atmospheric gas. Molecular nitrogen is relatively unreactive because of its strong triple bond. Most organisms cannot directly use atmospheric nitrogen; biological fixation, lightning and industrial processes convert it into reactive forms. The nitrogen cycle connects the atmosphere with soils, oceans and living organisms. Nitrogen also dilutes oxygen, moderating the intensity of combustion compared with an oxygen-rich atmosphere.

Oxygen supports aerobic respiration and combustion. Its accumulation over geological time is closely associated with oxygenic photosynthesis. Oxygen is exchanged among the atmosphere, biosphere and oceans through photosynthesis, respiration and decomposition. Argon, the third most abundant constituent of dry air, is a chemically inert noble gas and plays little direct role in weather.

Carbon dioxide is supplied by respiration, decomposition, volcanic activity, fossil-fuel combustion and land-use change. Photosynthesis and ocean uptake remove part of it. Methane comes from sources including wetlands, livestock, rice cultivation, waste and fossil-fuel systems. Nitrous oxide is associated particularly with microbial processes in soils and oceans, with agricultural nitrogen inputs increasing emissions. These long-lived greenhouse gases influence climate despite their low concentrations.

The labels permanent and variable gases are relative: they describe stability over relevant atmospheric timescales, not absolute constancy throughout Earth's history.

3. Water vapour and aerosols

Water vapour is the invisible gaseous phase of water; clouds and fog consist of liquid droplets, ice crystals or both. Its concentration varies markedly with temperature, moisture supply and atmospheric circulation. Warm, humid tropical air generally contains more water vapour than cold polar air. Its abundance usually decreases rapidly with altitude because the lower atmosphere receives moisture through evaporation and transpiration, while cooling promotes condensation and precipitation.

Water vapour absorbs terrestrial infrared radiation and is a major contributor to the natural greenhouse effect. Evaporation consumes latent heat, whereas condensation releases it, redistributing energy and supporting cloud development and storms. In climate change, water vapour mainly acts as a feedback: warming increases the atmosphere's saturation vapour pressure, allowing greater moisture content where water is available.

Aerosols include mineral dust, sea salt, smoke, sulphates, black carbon, pollen and volcanic particles. Many serve as cloud condensation nuclei; some act as ice-nucleating particles. Their climatic effects depend on composition, size, altitude and the underlying surface. Sulphate particles generally scatter sunlight and cool the surface, while black carbon absorbs radiation. Deposition of black carbon on snow can reduce albedo and accelerate melting.

Rain and snow remove many aerosols, so most tropospheric particles have much shorter lifetimes than well-mixed greenhouse gases.

4. How composition varies with altitude

Atmospheric density and pressure decrease with altitude because progressively less air lies above a given level. This does not mean that the percentage of oxygen immediately declines at the same rate. Throughout the lower atmosphere, oxygen remains close to 21% of dry air, but its partial pressure decreases. High-altitude breathing difficulty therefore primarily reflects reduced oxygen availability per breath, not a sharply reduced oxygen percentage.

On the basis of composition, the atmosphere is divided into the homosphere and heterosphere. Turbulence and large-scale motion keep the major gases relatively well mixed in the homosphere, extending roughly to 80–100 km. Above the transition region, molecular diffusion becomes increasingly important. The heterosphere shows stronger separation according to molecular mass, with lighter species becoming relatively more abundant at greater heights.

These divisions differ from the troposphere, stratosphere, mesosphere and thermosphere, which are identified mainly by temperature trends. A well-mixed homosphere does not mean that every constituent is uniformly distributed: water vapour, ozone and aerosols have pronounced vertical variations. The cold tropical tropopause limits moisture entering the stratosphere, helping make it very dry. Upper-atmospheric composition is further modified by photodissociation and ionisation under energetic solar radiation.

5. Ozone, radiation and changing composition

Ozone is a three-atom form of oxygen. Roughly 90% of atmospheric ozone occurs in the stratosphere. Ultraviolet radiation splits oxygen molecules, and the resulting oxygen atoms combine with other oxygen molecules to form ozone. Repeated formation and destruction constitute a natural photochemical cycle. Ozone absorption of ultraviolet radiation protects organisms and contributes to the stratosphere's rise in temperature with altitude.

Near the ground, ozone is a harmful secondary pollutant formed through sunlight-driven reactions involving nitrogen oxides and volatile organic compounds. It damages respiratory health, crops and vegetation. Thus, the same molecule can be protective in one atmospheric region and harmful in another. The ozone hole refers to severe seasonal thinning, especially over Antarctica, rather than a literal absence of atmosphere.

Nitrogen and oxygen do not significantly absorb terrestrial infrared radiation under ordinary atmospheric conditions. Greenhouse gases absorb and emit radiation in particular wavelength bands, slowing Earth's loss of heat to space. The natural greenhouse effect makes Earth habitable; rising human-caused greenhouse gas concentrations strengthen it. Ozone depletion and global warming are distinct processes, although some substances influence both. The Montreal Protocol addresses ozone-depleting substances, while its Kigali Amendment targets climate-warming hydrofluorocarbons.

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