Trace gases are gases that are present in small amounts within an environment such as a planet's atmosphere. Trace gases in Earth's atmosphere are gases other than nitrogen (78.1%), oxygen (20.9%), and argon (0.934%) which, in combination, make up 99.934% of its 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 sources of a trace gas are biogenic processes, outgassing from solid Earth, ocean emissions, industrial emissions, and in situ formation.
|Increasing,<br>See Note
|Biological, oceanic, combustion, anthropogenic
|photosynthesis
|-
|Neon
|Ne
|18.18 ppmv
|_________
|Volcanic
|________
|-
|Helium
|He
|5.24 ppmv
|_________
|Radiogenic
|________
|-
|Methane
|CH<sub>4</sub>
|1.89 ppm<br>(May, 2021)
|9 years
|Biological, anthropogenic
|OH
|-
|Hydrogen
|H<sub>2</sub>
|0.56 ppmv
|~ 2 years
|Biological, HCHO photolysis
|soil uptake
|-
|Nitrous oxide
|N<sub>2</sub>O
|0.33 ppmv
|150 years
|Biological, anthropogenic
|O(<sup>1</sup>D) in stratosphere
|-
|Carbon monoxide
|CO
|40 – 200 ppbv
|~ 60 days
|Photochemical, combustion, anthropogenic
|OH
|-
|Ozone
|O<sub>3</sub>
|10 – 200 ppbv (troposphere)
|Days – months
|Photochemical
|photolysis
|-
|Formaldehyde
|HCHO
|0.1 – 10 ppbv
|~ 1.5 hours
|Photochemical
|OH, photolysis
|-
|Nitrogen species
|NO<sub>x</sub>
|10 pptv – 1 ppmv
|Variable
|Soils, anthropogenic, lightning
|OH
|-
|Ammonia
|NH<sub>3</sub>
|10 pptv – 1 ppbv
|2 – 10 days
|Biological
|gas-to-particle conversion
|-
|Sulfur dioxide
|SO<sub>2</sub>
|10 pptv – 1 ppbv
|Days
|Photochemical, volcanic, anthropogenic
|OH, water-based oxidation
|-
|Dimethyl sulfide
|(CH<sub>3</sub>)<sub>2</sub>S
|several pptv – several ppbv
|Days
|Biological, ocean
|OH
|}
The Intergovernmental Panel on Climate Change (IPCC) states that "no single atmospheric lifetime can be given" for CO<sub>2</sub>. This is mostly due to the high rate of growth and large cumulative magnitude of the disturbances to Earth's carbon cycle by the geologic extraction and burning of fossil carbon. As of year 2014, fossil CO<sub>2</sub> emitted as a theoretical 10 to 100 GtC pulse on top of the existing atmospheric concentration was expected to be 50% removed by land vegetation and ocean sinks in less than about a century. A substantial fraction (20-35%) was also projected to remain in the atmosphere for centuries to millennia, where fractional persistence increases with pulse size. Thus CO<sub>2</sub> lifetime effectively increases as more fossil carbon is extracted by humans.
Mixing and lifetime
The overall abundance of man-made trace gases in Earth's atmosphere is growing. Most originate from industrial activity in the more populated northern hemisphere. Time-series data from measurement stations around the world indicate that it typically takes 1–2 years for their concentrations to become well-mixed throughout the troposphere.
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/τ<sub>r</sub>, where fc is the coefficient of variation, τ<sub>r</sub> 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. Globally, water vapor is responsible for about half of Earth's total greenhouse effect.
The second most important greenhouse gas, and the most important trace gas affected by man-made sources, is carbon dioxide.