Trace gases
Trace gases are atmospheric gases that exist in very low concentrations, typically less than one part per thousand. While the majority of the atmosphere is composed of permanent gases like nitrogen, oxygen, and argon, trace gases include a variety of compounds containing elements such as carbon, sulfur, nitrogen, and hydrogen. These gases, although making up less than 0.05 percent of the atmosphere, have significant implications for climate and environmental health. Major greenhouse gases like carbon dioxide, methane, and ozone are classified as trace gases, yet they play a crucial role in the greenhouse effect by trapping heat within the atmosphere.
Trace gases also influence the chemical reactions that determine the lifespan of pollutants and the thermal structure of the atmosphere. For instance, while permanent gases do not absorb heat, trace gases do, particularly in the near-infrared spectrum. This characteristic makes them important in regulating the Earth's temperature and climate. Additionally, the presence of trace gases can affect air quality and ecosystem health, as pollutants can impact plant life and soil nitrogen availability over considerable distances from their sources. Understanding trace gases is essential for comprehending their effects on climate change and environmental sustainability.
Trace gases
Definition
Trace gases are gases that exist at low concentrations in an atmosphere, generally less than one part per thousand. The bulk of Earth’s atmosphere is made up of “permanent” gases—molecular oxygen and nitrogen—and argon, as well as the “variable” gas water vapor. In addition to these gases, thousands of trace gases are present, including many compounds of such common elements as carbon, sulfur, nitrogen, oxygen, and hydrogen. The atmosphere also contains small amounts of the halogens (fluorine, chlorine, bromine, and iodine) and the noble gases (helium, neon, krypton, and radon), as well as very small quantities of some metallic species such as mercury.
Each of the elements carbon, nitrogen, sulfur, and the halogens exists in numerous forms, ranging from fully reduced (that is, bound to the maximum possible number of hydrogen atoms) to completely oxidized (that is, bound to the maximum possible number of oxygen atoms). Carbon, for example, may exist within methane (CH4, its fully reduced form) or carbon dioxide (CO2, its fully oxidized form). Between these two extremes, the element is found in such other species as formaldehyde (H2CO) and methanol (CH3OH). Carbon can also be found within larger species containing two or more carbon atoms, including volatile organic compounds.
Hydrogen sulfide (H2S, the gas associated with the smell of rotten eggs) is the fully reduced form of sulfur, and sulfuric acid (H2SO4) is the most oxidized form that is stable in the atmosphere. Nitrogen can be found as ammonia (NH3, the most reduced form), nitric oxide (NO), nitrogen dioxide (NO2), the nitrate radical (NO3, the most oxidized) or other species such as nitric acid (HNO3). Likewise, the halogens can be found bound to hydrogen or oxygen, as in hydrochloric acid (HCl), chlorine dioxide (ClO2), and chlorine monoxide (ClO), which play a key role in destruction of the stratospheric ozone layer. (While molecular oxygen, O2, is a permanent gas, ozone, O3, is a highly variable and important trace gas.)
Most metals are present in gaseous form only very briefly, if at all. Because they have very low volatility, if metals are released in their gas phase (usually in a hot combustion plume), they very quickly enter the aerosol phase. A notable exception is mercury, which has a sufficiently high volatility that its gas phase concentration typically exceeds its aerosol concentration. Most trace gases have both natural and human sources.
Significance for Climate Change
Numerous trace gases have impacts on the Earth’s changing climate, some large and others small. All the major greenhouse gases (GHGs) released by human activities—including CO2, methane, ozone, and the chlorofluorocarbons—are trace gases. Trace gases make up a tiny fraction of the gases in the atmosphere, currently less than 0.05 percent combined, although this value is creeping up as CO2 levels rise. However, they have large and disproportionate impacts on several properties of the atmosphere. Trace gases contribute significantly to the strength of the greenhouse effect, the degree to which the atmosphere traps heat received from the Sun. They also influence the rate at which pollutants emitted from the Earth’s surface react and are removed from the atmosphere, the thermal structure of the atmosphere, and the healthfulness of the air.
Although present only at very low concentrations, trace species can have such significant effects because the permanent gases behave as though they are not there. Because molecular nitrogen and oxygen are both pairs of identical atoms and argon is a single atom, the permanent gases cannot absorb heat (far infrared light). As a result, all the energy released by the Earth is available to be absorbed by the trace gases, as well as water vapor. The permanent gases also do not absorb visible light, although most of the trace gases do not either, so this distinction has limited climate importance. However, more than half of the incoming sunlight falls in the region of the electromagnetic spectrum just beyond red (near infrared), which the permanent gases absorb only slightly, but CO2, ozone, and water vapor all absorb strongly.
Nitrogen and argon are unreactive, and oxygen is only mildly reactive, so the chemical reactions of the trace gases are free to dominate the chemistry of the atmosphere. This situation has many climate implications because it controls the concentrations and lifetimes of most of the pollutants released into the atmosphere.
Throughout most of the lower 90 kilometers of the atmosphere, the average temperature is controlled by atmospheric pressure. At higher altitudes, the pressure decreases. As it does, the air becomes markedly colder by an average of 9.6° Celsius per kilometer increase in altitude. This trend reverses briefly in the stratosphere, however. In this region, the atmosphere becomes warmer with altitude as a result of the stratospheric ozone layer. There is sufficient ozone in this layer to trap a large fraction of the incoming sunlight, heating the air with the Sun’s energy. Above the stratospheric is the mesosphere, where the atmosphere again cools with rising altitude.
Toxic air contaminants impact climate indirectly, but in some instances significantly. Urban ozone, acid rain, dry acid deposition, and other toxic pollutants can affect forest health for hundreds of kilometers downwind of their sources. Forest ecosystems may die off as a direct result of airborne pollutants. They may also be weakened by pollution, decreasing their ability to resist pests, which in turn may result in the loss of forest biomass, either directly or by increasing susceptibility to fires. Further, the transport and eventual deposition of nitrogen from combustion sources can change substantially the quantity of available nitrogen in soils, which in turn can cause changes in species distributions, net biomass, and fire frequency—again, for hundreds of kilometers downwind of major sources.
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