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
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
(78.1%),
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
(20.9%), and
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
(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
Outgassing (sometimes called offgassing, particularly when in reference to indoor air quality) is the release of a gas that was dissolved, trapped, frozen, or absorbed in some material. Outgassing can include sublimation and evaporation (which a ...
from solid Earth, ocean emissions, industrial emissions, and
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
formation.
A few examples of biogenic sources include
photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...
,
animal excrements,
termite
Termites are small insects that live in colonies and have distinct castes (eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blattode ...
s,
rice paddies
A paddy field is a flooded field of arable land used for growing semiaquatic crops, most notably rice and taro. It originates from the Neolithic rice-farming cultures of the Yangtze River basin in southern China, associated with pre-Aust ...
, and
wetland
A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The ...
s. Volcanoes are the main source for trace gases from solid earth. The global
ocean
The ocean (also the sea or the world ocean) is the body of salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Earth's water. An ocean can also refer to any of the large bodies of water into which the wo ...
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
Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, ...
), fossil fuel mining,
biomass burning
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms biom ...
, 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
The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...
, gas-to-particle conversion forming
aerosols
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...
,
wet deposition
In the physics of aerosols, deposition is the process by which aerosol particles collect or deposit themselves on solid surfaces, decreasing the concentration of the particles in the air. It can be divided into two sub-processes: ''dry'' and ''w ...
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
The
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to advance scientific knowledge about climate change caused by human activities. The World Meteorological Organization (WMO) a ...
(IPCC) states that ''"no single atmospheric lifetime can be given"'' for CO
2.
This is mostly due to the high rate of growth and large cumulative magnitude of the disturbances to Earth's
carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...
by the geologic extraction and burning of fossil carbon. As of year 2014, fossil CO
2 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
2 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
The residence time of a fluid parcel is the total time that the parcel has spent inside a control volume (e.g.: a chemical reactor, a lake, a human body). The residence time of a set of parcels is quantified in terms of the frequency distribution ...
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
r, where fc is the
coefficient of variation
In probability theory and statistics, the coefficient of variation (CV), also known as relative standard deviation (RSD), is a standardized measure of dispersion of a probability distribution or frequency distribution. It is often expressed as ...
, τ
r 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.
Trace greenhouse gases
A few examples of the major
greenhouse gas
A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es are
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...
,
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
,
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
,
nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
,
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
, and
CFCs
Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are fully or partly halogenated hydrocarbons that contain carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F), produced as volatile derivatives of methane, ethane, and propan ...
. These gases can absorb
infrared radiation
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
from the Earth's surface as it passes through the atmosphere.
The most influential greenhouse gas is
water vapor
(99.9839 °C)
, -
, Boiling point
,
, -
, specific gas constant
, 461.5 J/( kg·K)
, -
, Heat of vaporization
, 2.27 MJ/kg
, -
, Heat capacity
, 1.864 kJ/(kg·K)
Water vapor, water vapour or aqueous vapor is the gaseous pha ...
. 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 greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
.
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
In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
.
Key Greenhouse Gases and Sources
References
External links
A description of atmospheric trace gasesOn trace gases and their role
{{Authority control
Gases
Microscale meteorology