Air (Latin: aer) is a homogeneous mixture of gases (volatile elements and chemical compounds) and atmospheric areolae (microscopic solid particles and droplets of liquid suspended in a gaseous phase) forming the gaseous (entrusted) layer of the globe, whose chemical composition is balanced by the circulation of gases in the troposphere. However, the term ‘air’ is also sometimes used to refer to all spheres of the earth’s atmosphere (with different chemical compositions and physical properties).

In the pre-scientific natural philosophy, the classical concept of Empedocles, developed by Aristotle, air was regarded as one of the four elements.

Atmospheric air is a colourless, tasteless and odourless mixture that is slightly soluble in water and non-conductive.

The air that forms the earth’s atmosphere provides a protective coating for living organisms against sudden changes in temperature and harmful ultraviolet radiation (UV). Oxygen, carbon dioxide and nitrogen – the main components of air – have a significant influence on the life processes of living organisms (respiration, photosynthesis) and on the circulation of matter and the flow of energy in nature (oxygen, carbon and nitrogen cycles, the hydrological cycle).


The Earth’s atmosphere

The Earth’s atmosphere is the air (gas) envelope of the Earth made up of a homogeneous mixture of gases and atmospheric aerosols. This mixture is called atmospheric air. The atmosphere has no physical boundary. The Kármán line (100 km above sea level) – the boundary between this layer of the earth’s surface and outer space – is considered to be the conventional ending point of the atmosphere. The atmosphere as a heterogeneous system is characterised by the fact that with increasing altitude, its physical properties and chemical composition change, i.e. the pressure, temperature, air density change. The main layers of the Earth’s atmosphere are determined by the so-called stratification of the atmosphere – based on the thermal criterion, which takes into account the variation of the temperature of the atmospheric air with the increase of the height above sea level. The atmosphere consists of five concentric layers, which are separated by pauses.


Troposphere – the lowest layer of the Earth’s atmosphere, it extends 0–12 km above the Earth’s surface. As the altitude increases, the temperature of the troposphere decreases uniformly. This layer concentrates about 75% of the mass of the whole atmosphere and almost all of the mass of atmospheric water. It is also the zone where weather processes and meteorological phenomena take place. The next sphere, the stratosphere, extends from 15 to 50 km above sea level. It is characterised by a distinct thermal stratification – the lower part (cool layer) with a constant temp. (-55°C) and the upper (warm) part with the temperature increasing with the altitude up to 0°C. The increase of temperature is caused by absorption of UV (ultraviolet) radiation by the ozone layer (ozonosphere) with the highest concentration of ozone at the height of 20–30 km above sea level. The fourth sphere – the mesosphere – is located at a height of 50–80 km above sea level. Its characteristic feature is a drop in temperature in proportion to the increase in altitude, down to a value of -100°C in its highest parts. The troposphere, located at an altitude of 80–800 km above sea level, is characterised by an increase in temperature with increasing altitude (up to 1,500°C in the upper parts) and by a strong thinning of the air. The temperature increase is due to the absorption of UV radiation by the gas particles. This process is the cause of their photoionisation, leading to the formation of a large number of ions and free electrons that make up the ionosphere, located mainly in the lower part of the thermosphere. The outermost layer of the Earth’s atmosphere is the exosphere, located between 800 and 10,000 km above the sea level. The characteristic features of this sphere are very high temperature (up to 1,700°C) and strong dilution of the air.


Physical properties of air

Atmospheric air is a colourless mixture of gases and atmospheric aerosols which is tasteless and odourless, slightly soluble in water and not electrically conductive. The density of atmospheric air depends on the pressure, temperature and chemical composition of the air, as well as the presence of pollutants. Air density is directly proportional to pressure (it decreases with decreasing atmospheric pressure and increasing altitude) and inversely proportional to temperature (its value decreases with increasing temperature).

Deprived of water vapour – dry air – at a temperature of 0°C and under a pressure of 1013.24hPa, it undergoes condensation, changing into a very mobile, light blue liquid with a density of about 870 d/dm3 and freezing point reaching the value of -213°C. Liquefied air is an unstable liquid – its molecules quickly absorb heat energy from the environment and as a result change back to the gaseous state.

Air humidity is the amount of water vapour contained in a given volume of air. This characteristic is largely dependent on the air temperature. The amount of water vapour in the air increases as the surrounding temperature rises. Precipitation (e.g. rain) is the result of this property – once the air reaches its saturation state (maximum water vapour content at a given temperature).

Due to its physical properties (temperature and relative humidity) the atmospheric air circulating in the troposphere is divided into four main air masses shaping specific areas of the earth (source areas). Depending on the geographical location of the source area, we distinguish between equatorial air masses (in the equatorial belt), tropical air masses (in the tropics of the northern and southern hemispheres), polar air masses (in the temperate zone of the northern and southern hemispheres) and arctic air masses (in the polar zone of the northern and southern hemispheres). The main air masses are separated by narrow transition areas, so-called atmospheric fronts – the tropical front (separating the equatorial air masses from the tropical air masses), polar front (separating the tropical and polar air masses) and arctic front (separating the polar and arctic air masses). Air masses play a significant role in global atmospheric circulation, determining the pattern of precipitation and winds on Earth.


Composition of the air

The composition of the gas mixture that makes up the air has changed over millions of years in a wide range. The oxygen-containing atmosphere was formed gradually, after the evolutionary success of organisms capable of photosynthesis. In successive geological epochs, the concentration of the basic constituents of the air has fluctuated, laying the foundation for climatic changes.

Nowadays, the content of air components fluctuates only slightly (except for water vapour, ozone and carbon dioxide) in the lower atmospheric layers – the hemisphere (up to a height of about 90–100 km above sea level), i.e. in the troposphere, the stratosphere and the mesosphere. In the so-called heterosphere (higher layers of the atmosphere), the chemical composition of the atmospheric air changes with height, which is the result of ionisation of gas molecules under the influence of solar and cosmic radiation).

The main constituents of air are nitrogen (N2), oxygen (O2), argon (Ar), carbon dioxide (CO2) and water vapour (H2O). The noble phases helium (He), neon (Ne), krypton (Kr), xenon (Xe) as well as methane (CH4), hydrogen (H2), diazotrophic oxide (N2O), and ozone (O3) are also constituents of the air in small quantities. The air that forms the troposphere (the lowest layer of the Earth’s atmosphere) contains atmospheric aerosols, consisting of small mineral particles (e.g. soil particles, rocks), sea salt particles, volcanic ash and dust, and airborne microorganisms (e.g. bacteria, small arachnids and insects, etc.) and the spores of living organisms (seeds, bryophytes and ferns, pollen, fungal spores) – the so-called aeroplankton.

The atmospheric air also contains gaseous pollutants (consisting of gases and volatile vapours of chemical compounds) and their derivatives. The degradation of the earth’s atmosphere is also caused by secondary pollutants, which are the result of physical transformations and chemical reactions occurring between the components of the atmosphere and the pollutants emitted into it.


Biological significance of the atmospheric air

The atmospheric air, which makes up the earth’s atmosphere, constitutes a layer protecting living organisms from the harmful effects of UV radiation emitted by the sun. It is also a barrier against sudden daily temperature fluctuations. This is possible thanks to the slow process of heat transfer to the higher layers of the troposphere via the surface layer which absorbs solar radiation. Air also protects the globe from total heat loss (the so-called greenhouse effect). The carbon dioxide (CO2), water vapour (H2O), methane (CH4), and nitrous oxide (N2O), components of the atmosphere absorb most of the infrared radiation emitted by the Earth and reflect it back. Thanks to this thermal insulation, the global average surface temperature is about 15°C. If there was no such insulation, the surface temperature of the globe would be about -18°C. Life on earth in such conditions would be completely impossible. The main components of atmospheric air (nitrogen, oxygen, carbon dioxide and water vapour) and the physical phenomena occurring in the atmosphere have a gigantic influence on the life processes of living organisms (photosynthesis, respiration, etc.) and the flow of energy and circulation of matter between the atmosphere, hydrosphere, lithosphere and biosphere.