Air is everywhere even if we cannot see, taste or touch it. It does not have a smell of its own, but it is full of smells and scents that it carries to our noses: it is the air that allows us to smell the fragrance of flowers or the foul odour of car exhaust! Air enabled the development of life on Earth. It contains the oxygen needed for humans and animals to breathe and the carbon dioxide needed for plants to carry out photosynthesis. Air also enables the Earth to retain some of the heat from the Sun and repel the sun's harmful rays from living beings.
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 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.
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.
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.
The air completely surrounds the Earth and literally wraps it up in a shroud that, under the effect of gravity and centrifugal force caused by the Earth’s rotation, takes a spheroid shape (think of the shape of an egg), flatter at the poles and bulging at the Equator. This is why, even if its boundaries with the interplanetary spaces cannot be accurately identified, it has been called atmosphere (from the Greek atmòs = steam, and sfaira = sphere). A number of layers (spheres) can be identified in the atmosphere; these layers are concentric with the Earth and have different temperatures and chemical properties. Starting from the Earth’s surface (the soil) and going up, we find: the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere.
The troposphere is the first layer of the atmosphere, the one we live on. The heat that comes from the Earth’s surface warms this layer up and therefore the temperature diminishes when it goes upwards. In the troposphere the air always moves. The troposphere is characterized by atmospheric phenomena like the wind, cloud formation, precipitations, etc. Immediately above it, at a height that varies from the 8 km above the Poles and 18 km above the Equator, there is the tropopause, that represents that passage to the stratosphere.
Origin of the atmosphere
The atmosphere surrounding the Earth is very different from the atmosphere on our planet when it was created, 4.5 billion years ago. The origin of the present atmosphere, as well as that of other planets' atmospheres, is still widely debated by scientists. What is certain is that the current composition of the Earth's atmosphere is the result of a long evolution, which began at the time of the creation of planet Earth: volcanic activity, photosynthesis, the action of solar radiation, oxidative processes and microbial activity have modified its composition over time until the current equilibrium was reached.
Reasons why the atmosphere is useful
The atmosphere protects Earth from harmful solar radiations and regulates heating provided by the Sun. The first function is made possible by ozone existing in the stratosphere, the second function is achieved by a mix of gases existing in the whole atmosphere called “greenhouse gases”. Ozone accumulating in the stratosphere (called “stratospheric ozone”) is crucial for the survival of life on Earth as it constitutes a sort of natural screen for a part of solar radiations that are invisible to humans and are located in the ultraviolet region (frequency from 100 to 400 nm).