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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).
Some sources of a trace gas are biogenic processes, outgassing from solid Earth, ocean emissions, industrial emissions, and in situ formation. A few examples of biogenic sources include photosynthesis, Manure, , Paddy field, and . 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 Hydroxyl 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
| Carbon dioxide | CO2 | 419 ppm ≈ppmv (May, 2021) | Increasing, See Note | Biological, oceanic, combustion, anthropogenic | photosynthesis |
| Neon | Ne | 18.18 ppmv | _________ | Volcanic | ________ |
| Helium | He | 5.24 ppmv | _________ | Radiogenic | ________ |
| Methane | CH4 | 1.89 ppm (May, 2021) | 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 |
| NOx | 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 |
The Intergovernmental Panel on Climate Change (IPCC) states that ''"no single atmospheric lifetime can be given"'' for CO2. 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.Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', '''11'''(4): 1783–1838. As of year 2014, fossil CO2 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|carbon sink]] 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 CO2 lifetime effectively increases as more fossil carbon is extracted by humans.
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.
The most influential greenhouse gas is water vapor. It frequently occurs in high concentrations, may transition to and from an aerosol (clouds), and is thus not generally classified as a trace gas. Regionally, water vapor can trap up to 80 percent of outgoing IR radiation. 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. It contributes about 20% of Earth's total greenhouse effect. The reason that 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.
In contrast, the most abundant gases (,, and ) in the atmosphere are not greenhouse gases. This 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 symmetric molecule with vibrational energy states that are almost totally unaffected at infrared frequencies.
Below is a table of some of the major trace greenhouse gases, their man-made sources, and an estimate of the relative contribution of those sources to the enhanced greenhouse effect that influences global warming.
Key Greenhouse Gases and Sources
| 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% |
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