The atmosphere is the gaseous envelope surrounding the Earth. Its composition is generally described in terms of dry air, since water vapor (H₂O), which typically ranges from about 1% to 5% by volume, is highly variable depending on meteorological conditions.
Atmospheric concentrations are commonly expressed in ppmv (parts per million by volume), where 1 ppmv corresponds to 10−6, or 0.0001%.
Dry air is mainly composed (Figure 1) [1] of:
- 78% nitrogen (N₂) and 21% oxygen (O₂);
- approximately 1% noble gases, primarily argon (Ar; 0.93%), as well as neon (Ne; 18 ppmv), helium (He; 5.2 ppmv), krypton (Kr; 1.1 ppmv), and xenon (Xe; 0.09 ppmv).
Figure 1: Composition of the atmosphere. Dry air is predominantly composed of nitrogen and oxygen. Noble gases and trace gases represent a very small fraction of the atmosphere but can have significant effects on the Earth’s climate system. Carbon dioxide concentration corresponds to values observed in 2023.
The atmosphere also contains trace gases, including fluorinated compounds, which are present in very low concentrations but can have a significant impact on the climate system.
Fluorinated gases are synthetic, human-made compounds belonging to the fluorocarbon family. They are widely used in various applications such as refrigeration and air conditioning systems, heat pumps, electronics, and industrial processes. These gases can be classified into several main categories: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF₆).
These gases exhibit two major environmental impacts:
- they contribute significantly to the greenhouse effect due to their very high Global Warming Potential (GWP), as illustrated in Figure 2;
- some of them (notably CFCs and HCFCs) are responsible for the depletion of the stratospheric ozone layer, which protects life on Earth from harmful ultraviolet radiation (Figure 2).
It should be noted, however, that not all fluorinated gases affect the ozone layer: HFCs, for instance, do not contribute to ozone depletion but remain powerful greenhouse gases.
Figure 2: Vertical structure of the atmosphere. The ozone layer is mainly located in the stratosphere, between approximately 15 and 35 km altitude. Greenhouse gases are primarily concentrated in the troposphere, where they influence the Earth’s radiative balance.
Bibliography
[1] Lead, C.-O., Ramaswamy, A., Boucher, O., Haigh, J., Hauglustaine, D., Haywood, J., Myhre, G., Nakajima, T., Shi, G., Contributing, S., Betts, R., Charlson, R., Chuang, C., Daniel, J., Genio, A., Van Dorland, R., Feichter, J., Fuglestvedt, J., De, P., & Forster, F. (n.d.). 6 Radiative Forcing of Climate Change. https://www.ipcc.ch/site/assets/uploads/2018/03/TAR-06.pdf