Without an atmosphere, human life one Earth would not be possible as we know it. There would be no oxygen and we would not be able to breathe. There would be no greenhouse effect; nothing to protect us from the harmful effects of the sun’s rays. We would be constantly bombarded by an incessant rain of meteorites that would not disintegrate before crashing on earth. So, what is our atmosphere really made of?
Before we discuss the terrestrial atmosphere’s composition, let’s first answer the question of what is an atmosphere? An atmosphere is the gaseous envelope surrounding certain celestial bodies, such as the Earth, Venus or Mars. The gases are held around these celestial bodies by the gravitational force that holds them and prevents them from escaping to space.
Like Pluto, Triton or Titan, the Earth has an atmosphere but the latter is rather original when compared to other planets in the solar system. The atmospheres of Venus and Mars, for example, are very rich in carbon dioxide CO2 — respectively 97% and 95% — while the Earth’s atmosphere contains very little. Indeed, the Earth’s atmosphere consists of molecular nitrogen or nitrogen (N2) and oxygen (O2) at 78 and 21%. The remaining 1% is composed of rare gases that are not very chemically reactive, such as argon, neon, helium, krypton, xenon and radon.
In the lower layers of the atmosphere is also found water, which plays a very special role: water vapor is the only gas to have a very variable concentration in time and space. The other gases, they have a relatively stable and homogeneous concentration, even if there are obviously fluctuations.
The proportions mentioned above, specifically, are rather constant in what is called dry air, that is to say without water vapor. However, depending on the layers of the atmosphere, this water vapor is present in varying proportions. This is especially the case of the lower layers of the atmosphere such as the troposphere (where we live), where we also find carbon dioxide (CO2) and other aerosols and particularly polluting components.
The dynamics of the atmosphere are also highly impacted by water, which is present in three forms, namely gaseous, solid and liquid. When water moves from one state to another, it releases or absorbs energy. Water is even the most important greenhouse gas, easily transporting heat over great distances and strongly absorbing sunlight. On the other hand, while dry air is rather rare, researchers use another notion to study the behavior of the atmosphere: the perfect gas.
This notion of perfect gas considers the components as independent of each other, so that there is the same relationship between temperature, volume and pressure, regardless of the nature of the gas. It is a model with an interesting approximation to calculate the dynamics of the Earth’s atmosphere.
Ozone (O3) also plays a crucial role in the Earth’s atmosphere and in our survival despite its very low concentration. Its importance lies in its ability to absorb much of the sun’s ultraviolet radiation and to protect the Earth’s surface from this dangerous radiation. It is mainly found at high altitude, with a peak concentration around 82,000 feet, forming what is referred to as the famous “ozone layer”, which unfortunately has fluctuating holes, appearing and disappearing over the poles according to the time of the year.
There are also in the atmosphere what meteorologists call aerosols: they are solid or liquid particles (excluding droplets or water crystals) suspended in the air. Some aerosols are of natural origin, such as volcanic ash, flower pollen, plant spores or sea salt crystals; others are produced by human activity such as industrial discharges.
The presence of these aerosols in the atmosphere is not without effect: indeed, some of them have the property of facilitating the formation of water droplets or ice crystals that form clouds.