A trace gas is a gas which makes up less than 1% by volume of the Earth's atmosphere, and it includes all gases except nitrogen (78.1%) and oxygen (20.9%). The most abundant trace gas at 0.934% is argon. Water vapor also occurs in the atmosphere with highly variable abundance.
Several atmospheric trace gases such as ozone O
3, sulfur dioxide SO
2 and nitrogen oxides NO
x are anthropogenic, chemically reactive factors of air quality at a regional level. Others such as carbon dioxide CO
2 and methane CH
4 are important greenhouse gases and are produced anthropogenically but mainly by plants, microorganisms and from natural geothermal sources.. However, volcanic activity is an important source of trace gases as well.
The Earth's climate is sensitive to changes in trace gas concentrations and temperature of the upper troposphere - lower stratosphere region of the atmosphere (UTLS). This zone extends from 6 to 25 km above sea level where the interaction between radiation and trace gases, clouds and aerosols can lead to important climate feedback. Trace gases can be short lived or long lived. The first category includes several greenhouse gases (e.g. CO2, CH4, N2O or CFCs), whilst the latter encompasses species with a lifetime equal to or less than a few months (O3, nitrogen oxides, VOCs, CO etc).
Trace gas exchanges between the upper troposphere and the lower stratosphere occur due to large air mass movements. Among these, one of the most important is the Asian monsoon circulation pattern. This way, pollutants (and among them, trace gases) from Asia, India and Indonesia reach the stratosphere.  Regarding the horizontal spread, the extratropical air in the stratosphere, is mixed towards the equator at the outer edge of the Asian monsoon anticyclone, thus affecting the concentration of trace gases in the area .
Trace gases can react with each other, leading to imbalances that can deeply affect the life on Earth. For example, certain halogen species, such as odd-chlorine species (ClO, HCl, HOCl, ClONO2) or iodine and bromine species are involved in the destruction of ozone, through chemical reactions. On the other hand, nitrogen compounds can play a part both in the destruction and creation of ozone and a balanced can be reached.
- Flowing-afterglow mass spectrometry
- ExoMars Trace Gas Orbiter
- Infrared gas analyzer
- Dobson unit
- Dobson ozone spectrophotometer
- Differential optical absorption spectroscopy
- Stratospheric Aerosol and Gas Experiment
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