Trace gases are those gases in the atmosphere other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of the gases in the atmosphere (not including water vapor).
Abundance, sources and sinks
The abundance of a trace gas can range from a few parts per trillion (ppt) by volume to several hundred parts per million by volume (ppmv).  When a trace gas is added into the atmosphere, that process is called a source. There are two possible types of sources - natural or anthropogenic. Natural sources are caused by processes that occur in nature. In contrast, anthropogenic sources are caused by human activity. Some of the sources of a trace gas are biogenic, solid Earth (outgassing), the ocean, industrial activities, or in situ formation.  A few examples of biogenic sources include photosynthesis, animal excrements, termites, rice paddies, and wetlands. Volcanoes are the main source for trace gases from solid earth. The global ocean is also a source of several trace gases, in particular sulfur-containing gases. In situ trace gas formation occurs through chemical reactions in the gas-phase.  Anthropogenic sources are caused by human related activities such as fossil fuel combustion (e.g. in transportation), fossil fuel mining, biomass burning, and industrial activity.
In contrast, a sink is when a trace gas is removed from the atmosphere. Some of the sinks of trace gases are chemical reactions in the atmosphere, mainly with the OH radical, gas-to-particle conversion forming aerosols, wet deposition and dry deposition.  Other sinks include microbiological activity in soils.
Below is a chart of several trace gases including their abundances, atmospheric lifetimes, sources, and sinks.
Trace gases – taken at pressure 1 atm
|Gas||Chemical Formula||Fraction of Volume of Air by the Species||Residence Time or Lifetime||Major Sources||Major Sinks|
|Carbon Dioxide||CO2||405 ppmv (in 2018)||3 – 4 years||Biological, oceanic, combustion, anthropogenic||photosynthesis|
|Methane||CH4||1.8 ppmv||9 years||Biological, anthropogenic||OH|
|Hydrogen||H2||0.56 ppmv||~ 2 years||Biological, HCHO photolysis||soil uptake|
|Nitrous Oxide||N2O||0.33 ppmv||150 years||Biological, anthropogenic||O(1D) in stratosphere|
|Carbon Monoxide||CO||40 – 200 ppbv||~ 60 days||Photochemical, combustion, anthropogenic||OH|
|Ozone||O3||10 – 200 ppbv (troposphere)||Days - Months||Photochemical||photolysis|
|Formaldehyde||HCHO||0.1 – 10 ppbv||~ 1.5 hours||Photochemical||OH, photolysis|
|Nitrogen Species||NOx||10 pptv – 1 ppmv||variable||Soils, anthropogenic, lightning||OH|
|Ammonia||NH3||10 pptv - 1 ppbv||2 – 10 days||Biological||gas-to-particle conversion|
|Sulfur Dioxide||SO2||10 pptv – 1 ppbv||Days||Photochemical, volcanic, anthropogenic||OH, water-based oxidation|
|Dimethyl sulfide||(CH3)2S||several pptv – several ppbv||Days||Biological, ocean||OH|
A few examples of the major greenhouse gases are water, carbon dioxide, methane, nitrous oxide, ozone, and CFCs. These gases can absorb infrared radiation from the Earth's surface as it passes through the atmosphere. The most important greenhouse gas is water vapor because it can trap about 80 percent of outgoing IR radiation.  The second most important greenhouse gas, and the most important one affected by man-made sources into the atmosphere, is carbon dioxide.  The reason for why greenhouse gases can absorb infrared radiation is their molecular structure. For example, carbon dioxide has two basic modes of vibration that create a strong dipole-moment, which causes its strong absorption of infrared radiation.  Below is a table of some of the major greenhouse gases with man-made sources and their contribution to the enhanced greenhouse effect.
Key Greenhouse Gases and Sources
|Gas||Chemical Formula||Major Human Sources||Contribution to Increase (Estimated)|
|Carbon Dioxide||CO2||fossil fuel combustion, deforestation||55%|
|Methane||CH4||rice fields, cattle and dairy cows, landfills, oil and gas production||15%|
|Nitrous Oxide||N2O||fertilizers, deforestation||6%|
In contrast, the most abundant gases in the atmosphere are not greenhouse gases. The main reasons is because they cannot absorb infrared radiation as they do not have vibrations with a dipole moment.  For instance, the triple bonds of atmospheric dinitrogen make for a highly symmetric molecule that is very inert in the atmosphere.
The residence time of a trace gas depends on the abundance and rate of removal. The Junge (empirical) relationship describes the relationship between concentration fluctuations and residence time of a gas in the atmosphere. It can expressed as fc = b/τr, where fc is the coefficient of variation, τr is the residence time in years, and b is an empirical constant, which Junge originally gave as 0.14 years.  As residence time increases, the concentration variability decreases. This implies that the most reactive gases have the most concentration variability because of their shorter lifetimes. In contrast, more inert gases are non-variable and have longer lifetimes. When measured far from their sources and sinks, the relationship can be used to estimate tropospheric residence-times of gases. 
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