What do we really breathe in everyday life?

The Earth's atmosphere is mainly composed of nitrogen. There is also oxygen and chemically neutral rare gases. In the lower layers, water is very present and plays a big role, as well as carbon dioxide and some pollutants.

Theair of the'atmosphere terrestrial consists essentially ofnitrogen molecular (N2) – like that of Titan, of Triton and of Pluto — and of dioxygen (O2) in respective proportions of approximately 78% and 21%. There are also some " gas rare ”, a chemical name justified by their very low concentrations. These proportions are more or less constant in "dry air", that is to say without taking into account water vapor. This is however present in variable proportions, particularly high in the lower layers, where the carbon dioxide, or carbon dioxide (CO2), as well as many aerosols and other components.

Water plays a special role. It is present in gaseous form, liquid and solid. Transitions from one state to another (phase changes) absorb or releaseenergy, which locally influences the dynamics of the atmosphere. In addition, water easily transports the heat over long distances and it strongly absorbs rays infrared. It’s even the main greenhouse gas of the Earth's atmosphere.

Dry air and perfect gas: two theoretical concepts

Note that the term "saturation" used in meteorology denotes the maximum water vapor concentration that a mass of water may contain, and which depends on the temperature and the pressure. Beyond that, the water condenses into droplets and it forms fog or rain. Thus, air "90% saturated" does not contain 90% water but its concentration of water vapour reaches nine-tenths of the maximum, and the fog is not far away. This is why the concentrations of the components of the air are always given for "dry air", very rare in the Earth's atmosphere.

To study the behavior of the atmosphere, scientists use the so-called “laws of perfect gas " The components are considered to be independent of each other (they do not attract or repel each other). There is then the same relation between pressure, temperature and volume, whatever the nature of the gas. All other things being equal, a number of molecules nitrogen, for example, will occupy the same volume as the same number of oxygen molecules or a mixture of the two. This model gives a good approximation to calculate the dynamics of the Earth's atmosphere.

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