The air we breathe consists of a mixture of gases and solid and liquid particles. Its composition is not constant, it can in fact vary from place to place and over time. Leaving aside the water vapour, atmospheric dust and other variable components, we find that the composition of the air is practically constant. Nitrogen and oxygen, equal to 78% and 20.95% by volume, respectively, are the two main components of the atmosphere. Nitrogen is a colourless and odourless gas and is inert, since it does not participate in vital processes, unlike oxygen, necessary for living beings to breathe. The oxygen in the air is almost entirely of biological origin, since it is produced by autotrophic organisms through photosynthesis.

The remaining 1% consists of:

• argon, equal to 0.93% by volume, an inert gas like nitrogen;
• carbon dioxide (CO2), equal to 0.03%, of natural and anthropogenic (i.e. generated by the activities carried out by man, such as combustion processes) origin, plays a key role in the greenhouse effect;
• other gases such as neon, krypton, xenon, hydrogen and others, which together make-up only 0.01% by volume of the atmosphere.

One of the most important components of the atmosphere is water vapour, which is the result of evaporation of the water of lakes, seas and rivers. In addition to being at the origin of clouds and precipitation, like carbon dioxide, water vapour has the ability to absorb the thermal energy radiated from the Earth. To learn more, visit the climate change section. The amount of water vapour present in the air can vary from almost zero up to a maximum of approx. 4% by volume.

Atmospheric dust
Atmospheric dust consists of microscopic solid and liquid particles present in small quantities and characterised by variable dimensions and physical and chemical characteristics. The dust may be of biological origin, such as pollen and spores, generally the result of biological processes, of geological origin (for example, the particulate coming from volcanic eruptions or erosion phenomena) or of human origin, such as the fine particles produced by the exhaust gases of cars. Atmospheric dust plays an important role in the process of cloud and fog formation since the surface of some of its particles promotes the condensation of water vapour. 

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Air for life
The atmosphere is an essential component for life on Earth, since it contains the oxygen that living organisms need to breathe. In addition, it filters out the harmful solar radiations, reflecting them and preventing them from reaching the soil, and allows the Earth’s surface to maintain the right temperature for plants and animals to live. The atmosphere is also the place where the main weather phenomena (wind, rain, snow, etc.), that compose the climate, occur.

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The study of the atmosphere
The first scientific studies on the composition of the atmosphere started in the 18th century. The nature of air, infact, remained a mistery for a long time and only after 1770 Joseph Priestley (1733-1804), known for his pioneristic studies on the atmosphere, demonstrated that air contains something indispensable for the life of animals, oxygen, which will be defined with this word by Lavoisier some years after its discovery. Priestley also discovered that animals and humans “consume air” and that plants can renew and purify it.

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