Ozone

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Ozone
IUPAC name[hide] Trioxygen
Identifiers
CAS number	10028-15-6
PubChem	24823
ChemSpider	23208
UNII	66H7ZZK23N
EC number	233–069–2
MeSH	Ozone
ChEBI	CHEBI:25812
RTECS number	RS8225000
Gmelin Reference	1101
Jmol-3D images	Image 1 Image 2
SMILES [show]
InChI [show]
Properties
Molecular formula	O3
Molar mass	48.00 g mol−1
Appearance	Pale blue gas
Density	2.144 mg cm−3 (at 0 °C)
Melting point	−192.2 °C; −313.9 °F; 81.0 K
Boiling point	−112 °C; −170 °F; 161 K
Solubility in water	1.05 g L−1 (at 0 °C)
Solubility	very soluble in CCl4, sulfuric acid
Refractive index (nD)	1.2226 (liquid)
Structure
Space group	C2v
Coordination geometry	Digonal
Molecular shape	Dihedral
Hybridisation	sp2 for O1
Dipole moment	0.53 D
Thermochemistry
Std molar entropy So298	238.92 J K−1 mol−1
Std enthalpy of formation ΔfHo298	142.67 kJ mol−1
Hazards
EU classification	O Xi
NFPA 704	0 4 4 OX
Related compounds
Related compounds	Sulfur dioxide Trisulfur Disulfur monoxide Cyclic ozone
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Ozone /ˈoʊzoʊn/ (systematically named 1λ¹,3λ¹-trioxidane and μ-oxidodioxygen), or trioxygen, is an inorganic molecule with the chemical formula O
2(μ-O) (also written [O(μ-O)O] or O
3). 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 O
2, breaking down in the lower atmosphere to normal dioxygen. Ozone is formed from dioxygen by the action of ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the Earth's atmosphere. In total, ozone makes up only 0.6 ppm of the atmosphere.

Ozone's odor is sharp, reminiscent of chlorine, and detectable by many people at concentrations of as little as 10 ppb in air. Ozone's O₃ formula was determined in 1865. The molecule was later proven to have a bent structure and to be diamagnetic. In standard conditions, ozone is a pale blue gas that condenses at progressively cryogenic temperatures to a dark blue liquid and finally a violet-black solid. Ozone's instability with regard to more common dioxygen is such that both concentrated gas and liquid ozone may decompose explosively.^[1] It is therefore used commercially only in low concentrations.

Ozone is a powerful oxidant (far more so than dioxygen) and has many industrial and consumer applications related to oxidation. This same high oxidizing potential, however, causes ozone to damage mucus and respiratory tissues in animals, and also tissues in plants, above concentrations of about 100 ppb. This makes ozone a potent respiratory hazard and pollutant near ground level. However, the so-called ozone layer (a portion of the stratosphere with a higher concentration of ozone, from two to eight ppm) is beneficial, preventing damaging ultraviolet light from reaching the Earth's surface, to the benefit of both plants and animals.

Nomenclature

The trivial name ozone is the most commonly used and preferred IUPAC name. The systematic names 1λ¹,3λ¹-trioxidane and μ-oxidodioxygen, valid IUPAC names, are constructed according to the substitutive and additive nomenclatures, respectively. The name ozone derives from ozein (ὄζειν), the Greek word for smell (verb), referring to ozone's distinctive smell.

In appropriate contexts, ozone can be viewed as trioxidane with two hydrogen atoms removed, and as such, trioxidanylidene may be used as a context-specific systematic name, according to substitutive nomenclature. By default, these names pay no regard to the radicality of the ozone molecule. In even more specific context, this can also name the non-radical singlet ground state, whereas the diradical state is named trioxidanediyl.

Trioxidanediyl (or ozonide) is used, non-systematically, to refer to the substituent group (-OOO-). Care should be taken to avoid confusing the name of the group for the context-specific name for ozone given above.

History

ozonometer, 1865

Ozone, the first allotrope of any chemical element to be recognized, was proposed as a distinct chemical substance by Christian Friedrich Schönbein in 1840, who named it after the Greek verb ozein (ὄζειν, "to smell"), from the peculiar odor in lightning storms.^[2][3] The formula for ozone, O₃, was not determined until 1865 by Jacques-Louis Soret^[4] and confirmed by Schönbein in 1867.^[2][5]

For much of the second half of the nineteenth century and well into the twentieth, ozone was considered a healthy component of the environment by naturalists and health-seekers. The City of Beaumont in California had as its official slogan "Beaumont: Zone of Ozone," as evidenced on postcards and Chamber of Commerce letterhead.^[6] Naturalists working outdoors often considered the higher elevations beneficial because of their ozone content. "There is quite a different atmosphere [at higher elevation] with enough ozone to sustain the necessary energy [to work]," wrote naturalist Henry Henshaw, working in Hawaii.^[7] Seaside air was considered to be healthy because of its "ozone" content but the smell giving rise to this belief is in reality that of rotting seaweed.^[8]

Physical properties

Ozone is colourless or slightly bluish gas (blue when liquified), slightly soluble in water and much more soluble in inert non-polar solvents such as carbon tetrachloride or fluorocarbons, where it forms a blue solution. At 161 K (−112 °C; −170 °F), it condenses to form a dark blue liquid. It is dangerous to allow this liquid to warm to its boiling point, because both concentrated gaseous ozone and liquid ozone can detonate. At temperatures below 80 K (−193.2 °C; −315.7 °F), it forms a violet-black solid.^[9]

Most people can detect about 0.01 μmol/mol of ozone in air where it has a very specific sharp odor somewhat resembling chlorine bleach. Exposure of 0.1 to 1 μmol/mol produces headaches, burning eyes and irritation to the respiratory passages.^[10] Even low concentrations of ozone in air are very destructive to organic materials such as latex, plastics and animal lung tissue.

Ozone is diamagnetic, which means that its electrons are all paired. In contrast, O₂ is paramagnetic, containing two unpaired electrons.

Structure

According to experimental evidence from microwave spectroscopy, ozone is a bent molecule, with C_2v symmetry (similar to the water molecule). The O – O distances are 127.2 pm (1.272 Å). The O – O – O angle is 116.78°.^[11] The central atom is sp² hybridized with one lone pair. Ozone is a polar molecule with a dipole moment of 0.53 D.^[12] The bonding can be expressed as a resonance hybrid with a single bond on one side and double bond on the other producing an overall bond order of 1.5 for each side.

Reactions

Ozone is a powerful oxidizing agent, far stronger than O₂. It is also unstable at high concentrations, decaying to ordinary diatomic oxygen. It has a varying length half-life (meaning half as concentrated, or half-depleted), depending upon atmospheric conditions (temperature, humidity, and air movement). In a sealed chamber, with fan moving the gas, ozone has a half-life of approximately a day at room temperature^[13] Some claims have been stated that ozone can have a half life as short as a half an hour in atmospheric conditions, although this claim is not verified by this reference:^[14]

2 O
3 → 3 O
2

This reaction proceeds more rapidly with increasing temperature and increased pressure. Deflagration of ozone can be triggered by a spark, and can occur in ozone concentrations of 10 wt% or higher.^[15]

With metals

Ozone will oxidize most metals (except gold, platinum, and iridium) to oxides of the metals in their highest oxidation state. For example:

2 Cu+ + 2 H
3O+ + O
3 → 2 Cu2+ + 3 H
2O + O
2

With nitrogen and carbon compounds

Ozone also oxidizes nitric oxide to nitrogen dioxide:

NO + O
3 → NO
2 + O
2

This reaction is accompanied by chemiluminescence. The NO
2 can be further oxidized:

NO
2 + O
3 → NO₃ + O
2

The NO
3 formed can react with NO
2 to form N
2O
5:

Solid nitronium perchlorate can be made from NO₂, ClO₂, and O
3 gases:

2 NO
2 + 2 ClO₂ + 2 O
3 → 2 NO₂ClO₄ + O
2

Ozone does not react with ammonium salts, but it oxidizes ammonia to ammonium nitrate:

2 NH
3 + 4 O
3 → NH
4NO
3 + 4 O
2 + H
2O

Ozone reacts with carbon to form carbon dioxide, even at room temperature:

C + 2 O₃ → CO₂ + 2 O₂

With sulfur compounds

Ozone oxidizes sulfides to sulfates. For example, lead(II) sulfide is oxidised to lead(II) sulfate:

PbS + 4 O₃ → PbSO₄ + 4 O₂

Sulfuric acid can be produced from ozone, water and either elemental sulfur or sulfur dioxide:

S + H₂O + O₃ → H₂SO₄

3 SO₂ + 3 H₂O + O₃ → 3 H₂SO₄

In the gas phase, ozone reacts with hydrogen sulfide to form sulfur dioxide:

H₂S + O₃ → SO₂ + H₂O

In an aqueous solution, however, two competing simultaneous reactions occur, one to produce elemental sulfur, and one to produce sulfuric acid:

H₂S + O₃ → S + O₂ + H₂O

3 H₂S + 4 O₃ → 3 H₂SO₄

With alkenes and alkynes

Alkenes can be oxidatively cleaved by ozone, in a process called ozonolysis, giving alcohols, aldehydes, ketones, and carboxylic acids, depending on the second step of the workup.



Usually ozonolysis is carried out in a solution of dichloromethane, at a temperature of −78^oC. After a sequence of cleavage and rearrangement, an organic ozonide is formed. With reductive workup (e.g. zinc in acetic acid or dimethyl sulfide), ketones and aldehydes will be formed, with oxidative workup (e.g. aqueous or alcoholic hydrogen peroxide), carboxylic acids will be formed.^[16]

Other substrates

All three atoms of ozone may also react, as in the reaction of tin(II) chloride with hydrochloric acid and ozone:

3 SnCl₂ + 6 HCl + O
3 → 3 SnCl₄ + 3 H₂O

Iodine perchlorate can be made by treating iodine dissolved in cold anhydrous perchloric acid with ozone:

I₂ + 6 HClO₄ + O₃ → 2 I(ClO₄)₃ + 3 H₂O

Combustion

Ozone can be used for combustion reactions and combusting gases; ozone provides higher temperatures than combusting in dioxygen (O₂). The following is a reaction for the combustion of carbon subnitride which can also cause higher temperatures:

3 C
4N
2 + 4 O
3 → 12 CO + 3 N
2

Ozone can react at cryogenic temperatures. At 77 K (−196.2 °C; −321.1 °F), atomic hydrogen reacts with liquid ozone to form a hydrogen superoxide radical, which dimerizes:^[17]

H + O
3 → HO₂ + O

2 HO₂ → H
2O
4

Reduction to ozonides

Reduction of ozone gives the ozonide anion, O−
3. Derivatives of this anion are explosive and must be stored at cryogenic temperatures. Ozonides for all the alkali metals are known. KO₃, RbO₃, and CsO₃ can be prepared from their respective superoxides:

KO₂ + O₃ → KO₃ + O₂

Although KO₃ can be formed as above, it can also be formed from potassium hydroxide and ozone:^[18]

2 KOH + 5 O₃ → 2 KO₃ + 5 O₂ + H₂O

NaO₃ and LiO₃ must be prepared by action of CsO₃ in liquid NH₃ on an ion exchange resin containing Na⁺ or Li⁺ ions:^[19]

CsO₃ + Na⁺ → Cs⁺ + NaO₃

A solution of calcium in ammonia reacts with ozone to give to ammonium ozonide and not calcium ozonide:^[17]

3 Ca + 10 NH₃ + 6 O
3 → Ca·6NH₃ + Ca(OH)₂ + Ca(NO₃)₂ + 2 NH₄O₃ + 2 O₂ + H₂

Applications

Ozone can be used to remove manganese from water, forming a precipitate which can be filtered:

2 Mn²⁺ + 2 O
3 + 4 H₂O → 2 MnO(OH)₂ (s) + 2 O₂ + 4 H⁺

Ozone will also detoxify cyanides by converting them to cyanates, which are a thousand times less toxic.^{[citation needed]}

CN⁻ + O₃ → CNO− + O₂

Ozone will also completely decompose urea:^[20]

(NH₂)₂CO + O₃ → N₂ + CO₂ + 2 H₂O

Ozone in Earth's atmosphere

The distribution of atmospheric ozone in partial pressure as a function of altitude

Concentration of ozone as measured by the Nimbus-7 satellite

Total ozone concentration in June 2000 as measured by EP-TOMS satellite instrument

The standard way to express total ozone levels (the amount of ozone in a vertical column) in the atmosphere is by using Dobson units. Point measurements are reported as mole fractions in nmol/mol (parts per billion, ppb) or as concentrations in μg/m³. The study of ozone concentration in the atmosphere started in the 1920s.^[21]

Ozone layer

Location and production

The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer between about 10 km and 50 km above the surface (or between about 6 and 31 miles). However, even in this "layer" the ozone concentrations are only two to eight parts per million, so most of the oxygen there remains of the dioxygen type.

Ozone in the stratosphere is mostly produced from short-wave ultraviolet rays (in the UVC band) but it can be also produced from x-rays reacting with oxygen:

O
2 + photon (radiation λ < 240 nm) → 2 O

O + O
2 + M → O
3 + M

α + β⁻ + O
2 → He + O
3

where "M" denotes the third body that carries off the excess energy of the reaction. The thus produced ozone is destroyed by the reaction with atomic oxygen:

O
3 + O → 2 O
2

The latter reaction is catalysed by the presence of certain free radicals, of which the most important are hydroxyl (OH), nitric oxide (NO) and atomic chlorine (Cl) and bromine (Br). In recent decades the amount of ozone in the stratosphere has been declining mostly because of emissions of chlorofluorocarbons (CFC) and similar chlorinated and brominated organic molecules, which have increased the concentration of ozone-depleting catalysts above the natural background.

See also: Ozone-oxygen cycle and Ozone depletion

Importance to surface-dwelling life on Earth

Levels of ozone at various altitudes and blocking of different bands of ultraviolet radiation. Essentially all UVC (100–280 nm) is blocked by dioxygen (at 100–200 nm) or by ozone (at 200–280 nm) in the atmosphere. The shorter portion of this band and even more energetic UV causes the formation of the ozone layer, when single oxygen atoms produced by UV photolysis of dioxygen (below 240 nm) react with more dioxygen. The ozone layer itself then blocks most, but not quite all, sunburn-producing UVB (280–315 nm). The band of UV closest to visible light, UVA (315–400 nm), is hardly affected by ozone, and most of it reaches the ground.

Ozone in the ozone layer filters out sunlight wavelengths from about 200 nm UV rays to 315 nm, with ozone peak absorption at about 250 nm.^[22] This ozone UV absorption is important to life, since it extends the absorption of UV by ordinary oxygen and nitrogen in air (which absorb all wavelengths < 200 nm) through the lower UV-C (200–280 nm) and the entire UV-B band (280–315 nm). The small unabsorbed part that remains of UV-B after passage through ozone causes sunburn in humans, and direct DNA damage in living tissues in both plants and animals. Ozone's effect on mid-range UV-B rays is illustrated by its effect on UV-B at 290 nm, which has a radiation intensity 350 million times as powerful at the top of the atmosphere as at the surface. Nevertheless, enough of UV-B radiation at similar frequency reaches the ground to cause some sunburn, and these same wavelengths are also among those responsible for the production of vitamin D in humans.

The ozone layer has little effect on the longer UV wavelengths called UV-A (315–400 nm), but this radiation does not cause sunburn or direct DNA damage, and while it probably does cause long-term skin damage in certain humans, it is not as dangerous to plants and to the health of surface-dwelling organisms on Earth in general (see ultraviolet for more information on near ultraviolet).

Low level ozone

Low level ozone (or tropospheric ozone) is an atmospheric pollutant.^[23] It is not emitted directly by car engines or by industrial operations, but formed by the reaction of sunlight on air containing hydrocarbons and nitrogen oxides that react to form ozone directly at the source of the pollution or many kilometers down wind.
Ozone reacts directly with some hydrocarbons such as aldehydes and thus begins their removal from the air, but the products are themselves key components of smog. Ozone photolysis by UV light leads to production of the hydroxyl radical HO• and this plays a part in the removal of hydrocarbons from the air, but is also the first step in the creation of components of smog such as peroxyacyl nitrates, which can be powerful eye irritants. The atmospheric lifetime of tropospheric ozone is about 22 days; its main removal mechanisms are being deposited to the ground, the above-mentioned reaction giving HO•, and by reactions with OH and the peroxy radical HO₂•.^[24]

There is evidence of significant reduction in agricultural yields because of increased ground-level ozone and pollution which interferes with photosynthesis and stunts overall growth of some plant species.^[25][26] The United States Environmental Protection Agency is proposing a secondary regulation to reduce crop damage, in addition to the primary regulation designed for the protection of human health.

Certain examples of cities with elevated ozone readings are Houston, Texas, and Mexico City, Mexico. Houston has a reading of around 41 nmol/mol, while Mexico City is far more hazardous, with a reading of about 125 nmol/mol.^[26]

Ozone cracking

Ozone cracking in natural rubber tubing

Ozone gas attacks any polymer possessing olefinic or double bonds within its chain structure, such as natural rubber, nitrile rubber, and styrene-butadiene rubber. Products made using these polymers are especially susceptible to attack, which causes cracks to grow longer and deeper with time, the rate of crack growth depending on the load carried by the rubber component and the concentration of ozone in the atmosphere. Such materials can be protected by adding antiozonants, such as waxes, which bond to the surface to create a protective film or blend with the material and provide long term protection. Ozone cracking used to be a serious problem in car tires for example, but the problem is now seen only in very old tires.^{[clarification needed][citation needed]} On the other hand, many critical products like gaskets and O-rings may be attacked by ozone produced within compressed air systems. Fuel lines made of reinforced rubber are also susceptible to attack, especially within the engine compartment, where some ozone is produced by electrical components. Storing rubber products in close proximity to a DC electric motor can accelerate ozone cracking. The commutator of the motor creates sparks which in turn produce ozone.

Ozone as a greenhouse gas

Although ozone was present at ground level before the Industrial Revolution, peak concentrations are now far higher than the pre-industrial levels, and even background concentrations well away from sources of pollution are substantially higher.^[27][28] Ozone acts as a greenhouse gas, absorbing some of the infrared energy emitted by the earth. Quantifying the greenhouse gas potency of ozone is difficult because it is not present in uniform concentrations across the globe. However, the most widely accepted scientific assessments relating to climate change (e.g. the Intergovernmental Panel on Climate Change Third Assessment Report)^[29] suggest that the radiative forcing of tropospheric ozone is about 25% that of carbon dioxide.

The annual global warming potential of tropospheric ozone is between 918–1022 tons carbon dioxide equivalent/tons tropospheric ozone. This means on a per-molecule basis, ozone in the troposphere has a radiative forcing effect roughly 1,000 times as strong as carbon dioxide. However, tropospheric ozone is a short-lived greenhouse gas, which decays in the atmosphere much more quickly than carbon dioxide. This means that over a 20 year horizon, the global warming potential of tropospheric ozone is much less, roughly 62 to 69 tons carbon dioxide equivalent / tons tropospheric ozone.^[30]

Because of its short-lived nature, tropospheric ozone does not have strong global effects, but has very strong radiative forcing effects on regional scales. In fact, there are regions of the world where tropospheric ozone has a radiative forcing up to 150% of carbon dioxide.^[31]

Health effects

Ozone air pollution

Red Alder leaf, showing discolouration caused by ozone pollution^[32]

Signboard in Gulfton, Houston indicating an ozone watch

Ozone precursors are a group of pollutants, predominantly those emitted during the combustion of fossil fuels. Ground-level ozone pollution (tropospheric ozone) is created near the Earth's surface by the action of daylight UV rays on these precursors. The ozone at ground level is primarily from fossil fuel precursors, but methane is a natural precursor, and the very low natural background level of ozone at ground level is considered safe. This section examines health impacts of fossil fuel burning, which raises ground level ozone far above background levels.

There is a great deal of evidence to show that ground level ozone can harm lung function and irritate the respiratory system.^[23][33] Exposure to ozone and the pollutants that produce it is linked to premature death, asthma, bronchitis, heart attack, and other cardiopulmonary problems.^[34][35]

Long-term exposure to ozone has been shown to increase risk of death from respiratory illness. A study of 450,000 people living in United States cities showed a significant correlation between ozone levels and respiratory illness over the 18-year follow-up period. The study revealed that people living in cities with high ozone levels such as Houston or Los Angeles had an over 30% increased risk of dying from lung disease.^[36][37]

Air quality guidelines such as those from the World Health Organization, the United States Environmental Protection Agency (EPA) and the European Union are based on detailed studies designed to identify the levels that can cause measurable ill health effects.

According to scientists with the US EPA, susceptible people can be adversely affected by ozone levels as low as 40 nmol/mol.^[35][38][39] In the EU, the current target value for ozone concentrations is 120 µg/m³ which is about 60 nmol/mol. This target applies to all member states in accordance with Directive 2008/50/EC.^[40] Ozone concentration is measured as a maximum daily mean of 8 hour averages and the target should not be exceeded on more than 25 calendar days per year, starting from January 2010. Whilst the directive requires in the future a strict compliance with 120 µg/m³ limit (i.e. mean ozone concentration not to be exceeded on any day of the year), there is no date set for this requirement and this is treated as a long-term objective. ^[41]

In the USA, the Clean Air Act directs the EPA to set National Ambient Air Quality Standards for several pollutants, including ground-level ozone, and counties out of compliance with these standards are required to take steps to reduce their levels. In May 2008, under a court order, the EPA lowered its ozone standard from 80 nmol/mol to 75 nmol/mol. The move proved controversial, since the Agency's own scientists and advisory board had recommended lowering the standard to 60 nmol/mol.^[35] Many public health and environmental groups also supported the 60 nmol/mol standard,^[42] and the World Health Organization recommends 51 nmol/mol.

On January 7, 2010, the U.S. Environmental Protection Agency (EPA) announced proposed revisions to the National Ambient Air Quality Standard (NAAQS) for the pollutant ozone, the principal component of smog:

... EPA proposes that the level of the 8-hour primary standard, which was set at 0.075 μmol/mol in the 2008 final rule, should instead be set at a lower level within the range of 0.060 to 0.070 μmol/mol, to provide increased protection for children and other ‘‘at risk’’ populations against an array of O
3 - related adverse health effects that range from decreased lung function and increased respiratory symptoms to serious indicators of respiratory morbidity including emergency department visits and hospital admissions for respiratory causes, and possibly cardiovascular-related morbidity as well as total non- accidental and cardiopulmonary mortality....^[43]

The EPA has developed an Air Quality Index (AQI) to help explain air pollution levels to the general public. Under the current standards, eight-hour average ozone mole fractions of 85 to 104 nmol/mol are described as "unhealthy for sensitive groups," 105 nmol/mol to 124 nmol/mol as "unhealthy," and 125 nmol/mol to 404 nmol/mol as "very unhealthy."^[44]

Ozone can also be present in indoor air pollution, partly as a result of electronic equipment such as photocopiers. A connection has also been known to exist between the increased pollen, fungal spores, and ozone caused by thunderstorms and hospital admissions of asthma sufferers.^[45]

In the Victorian era, one British folk myth held that the smell of the sea was caused by ozone. In fact, the characteristic "smell of the sea" is caused by dimethyl sulfide a chemical generated by phytoplankton. Victorian British folk considered the resulting smell "bracing," but in high concentrations, dimethyl sulfide is actually toxic.^[46]

Heat waves

Ozone production rises during heat waves, because plants absorb less ozone. It is estimated that curtailed ozone absorption by plants is responsible for the loss of 460 lives in the UK in the hot summer of 2006.^[47] A similar investigation to assess the joint effects of ozone and heat during the European heat waves in 2003, concluded that these appear to be additive.^[48]

Physiology

Ozone, along with reactive forms of oxygen such as superoxide, singlet oxygen, hydrogen peroxide, and hypochlorite ions, is naturally produced by white blood cells and other biological systems (such as the roots of marigolds) as a means of destroying foreign bodies. Ozone reacts directly with organic double bonds. Also, when ozone breaks down to dioxygen it gives rise to oxygen free radicals, which are highly reactive and capable of damaging many organic molecules. Moreover, it is believed that the powerful oxidizing properties of ozone may be a contributing factor of inflammation. The cause-and-effect relationship of how the ozone is created in the body and what it does is still under consideration and still subject to various interpretations, since other body chemical processes can trigger some of the same reactions. A team headed by Dr. Paul Wentworth Jr. of the Department of Chemistry at the Scripps Research Institute has shown evidence linking the antibody-catalyzed water-oxidation pathway of the human immune response to the production of ozone. In this system, ozone is produced by antibody-catalyzed production of trioxidane from water and neutrophil-produced singlet oxygen.^[49]
When inhaled, ozone reacts with compounds lining the lungs to form specific, cholesterol-derived metabolites that are thought to facilitate the build-up and pathogenesis of atherosclerotic plaques (a form of heart disease). These metabolites have been confirmed as naturally occurring in human atherosclerotic arteries and are categorized into a class of secosterols termed atheronals, generated by ozonolysis of cholesterol's double bond to form a 5,6 secosterol^[50] as well as a secondary condensation product via aldolization.^[51]

Ozone has been implicated to have an adverse effect on plant growth: "... ozone reduced total chlorophylls, carotenoid and carbohydrate concentration, and increased 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production. In treated plants, the ascorbate leaf pool was decreased, while lipid peroxidation and solute leakage were significantly higher than in ozone-free controls. The data indicated that ozone triggered protective mechanisms against oxidative stress in citrus."^[52]

Safety regulations

Due to the strongly oxidizing properties of ozone, ozone is a primary irritant, affecting especially the eyes and respiratory systems and can be hazardous at even low concentrations. The Canadian Center for Occupation Safety and Health reports that:

"Even very low concentrations of ozone can be harmful to the upper respiratory tract and the lungs. The severity of injury depends on both by the concentration of ozone and the duration of exposure. Severe and permanent lung injury or death could result from even a very short-term exposure to relatively low concentrations." ^[53]

To protect workers potentially exposed to ozone, U.S. Occupational Safety and Health Administration has established a permissible exposure limit (PEL) of 0.1 μmol/mol (29 CFR 1910.1000 table Z-1), calculated as an 8 hour time weighted average. Higher concentrations are especially hazardous and NIOSH has established an Immediately Dangerous to Life and Health Limit (IDLH) of 5 μmol/mol.^[54] Work environments where ozone is used or where it is likely to be produced should have adequate ventilation and it is prudent to have a monitor for ozone that will alarm if the concentration exceeds the OSHA PEL. Continuous monitors for ozone are available from several suppliers.

Elevated ozone exposure can occur on passenger aircraft, with levels depending on altitude and atmospheric turbulence.^[55] United States Federal Aviation Authority regulations set a limit of 250 nmol/mol with a maximum four-hour average of 100 nmol/mol.^[56] Some planes are equipped with ozone converters in the ventilation system to reduce passenger exposure.^[55]

Production

Ozone often forms in nature under conditions where O₂ will not react.^[10] Ozone used in industry is measured in μmol/mol (ppm, parts per million), nmol/mol (ppb, parts per billion), μg/m³, mg/h (milligrams per hour) or weight percent. The regime of applied concentrations ranges from 1 to 5% in air and from 6 to 14% in oxygen for older generation methods. New electrolytic methods can achieve up 20 to 30% dissolved ozone concentrations in output water.

Temperature and humidity plays a large role in how much ozone is being produced using traditional generation methods such as corona discharge and ultraviolet light. Old generation methods will produce less than 50% its nominal capacity if operated with humid ambient air than when it operates in very dry air. New generators using electrolytic methods can achieve higher purity and dissolution through using water molecules as the source of ozone production.

Corona discharge method

This is the most common type of ozone generator for most industrial and personal uses. While variations of the "hot spark" coronal discharge method of ozone production exist, including medical grade and industrial grade ozone generators, these units usually work by means of a corona discharge tube.^[57] They are typically cost-effective and do not require an oxygen source other than the ambient air to produce ozone concentrations of 3–6%. Fluctuations in ambient air, due to weather or other environmental conditions, cause variability in ozone production. However, they also produce nitrogen oxides as a by-product. Use of an air dryer can reduce or eliminate nitric acid formation by removing water vapor and increase ozone production. Use of an oxygen concentrator can further increase the ozone production and further reduce the risk of nitric acid formation by removing not only the water vapor, but also the bulk of the nitrogen.

Ultraviolet light

UV ozone generators, or vacuum-ultraviolet (VUV) ozone generators, employ a light source that generates a narrow-band ultraviolet light, a subset of that produced by the Sun. The Sun's UV sustains the ozone layer in the stratosphere of Earth.^[58]

While standard UV ozone generators tend to be less expensive,^{[clarification needed]} they usually produce ozone with a concentration of about 0.5% or lower. Another disadvantage of this method is that it requires the air (oxygen) to be exposed to the UV source for a longer amount of time, and any gas that is not exposed to the UV source will not be treated. This makes UV generators impractical for use in situations that deal with rapidly moving air or water streams (in-duct air sterilization, for example). Production of ozone is one of the potential dangers of ultraviolet germicidal irradiation. VUV ozone generators are used in swimming pool and spa applications ranging to millions of gallons of water. VUV ozone generators, unlike corona discharge generators, do not produce harmful nitrogen by-products and also unlike corona discharge systems, VUV ozone generators work extremely well in humid air environments. There is also not normally a need for expensive off-gas mechanisms, and no need for air driers or oxygen concentrators which require extra costs and maintenance.

Cold plasma

In the cold plasma method, pure oxygen gas is exposed to a plasma created by dielectric barrier discharge. The diatomic oxygen is split into single atoms, which then recombine in triplets to form ozone.

Cold plasma machines utilize pure oxygen as the input source and produce a maximum concentration of about 5% ozone. They produce far greater quantities of ozone in a given space of time compared to ultraviolet production. However, because cold plasma ozone generators are very expensive, they are found less frequently than the previous two types.

The discharges manifest as filamentary transfer of electrons (micro discharges) in a gap between two electrodes. In order to evenly distribute the micro discharges, a dielectric insulator must be used to separate the metallic electrodes and to prevent arcing.

Some cold plasma units also have the capability of producing short-lived allotropes of oxygen which include O₄, O₅, O₆, O₇, etc. These species are even more reactive than ordinary O
3.^{[citation needed]}

Electrolytic

Electrolytic ozone generation (EOG) splits water molecules into H₂, O₂, and O₃. In most EOG methods, the hydrogen gas will be removed to leave oxygen and ozone as the only reaction products. Therefore, EOG can achieve higher dissolution in water without other competing gases found in corona discharge method, such as nitrogen gases present in ambient air. This method of generation can achieve concentrations of 20–30% and is independent of air quality because water is used as the source material. Production of ozone electrolytically is typically unfavorable because of the high overpotential required to produce ozone as compared to oxygen. This is why ozone is not produced during typical water electrolysis. However, it is possible to increase the overpotential of oxygen by careful catalyst selection such that ozone is preferentially produced under electrolysis. Catalysts typically chosen for this approach are lead dioxide^[59] or boron-doped diamond.^[60]

Special considerations

Ozone cannot be stored and transported like other industrial gases (because it quickly decays into diatomic oxygen) and must therefore be produced on site. Available ozone generators vary in the arrangement and design of the high-voltage electrodes. At production capacities higher than 20 kg per hour, a gas/water tube heat-exchanger may be utilized as ground electrode and assembled with tubular high-voltage electrodes on the gas-side. The regime of typical gas pressures is around 2 bars (200 kPa) absolute in oxygen and 3 bars (300 kPa) absolute in air. Several megawatts of electrical power may be installed in large facilities, applied as one phase AC current at 50 to 8000 Hz and peak voltages between 3,000 and 20,000 volts. Applied voltage is usually inversely related to the applied frequency.

The dominating parameter influencing ozone generation efficiency is the gas temperature, which is controlled by cooling water temperature and/or gas velocity. The cooler the water, the better the ozone synthesis. The lower the gas velocity, the higher the concentration (but the lower the net ozone produced). At typical industrial conditions, almost 90% of the effective power is dissipated as heat and needs to be removed by a sufficient cooling water flow.

Because of the high reactivity of ozone, only a few materials may be used like stainless steel (quality 316L), titanium, aluminium (as long as no moisture is present), glass, polytetrafluorethylene, or polyvinylidene fluoride. Viton may be used with the restriction of constant mechanical forces and absence of humidity (humidity limitations apply depending on the formulation). Hypalon may be used with the restriction that no water come in contact with it, except for normal atmospheric levels. Embrittlement or shrinkage is the common mode of failure of elastomers with exposure to ozone.
Ozone cracking is the common mode of failure of elastomer seals like O-rings.

Silicone rubbers are usually adequate for use as gaskets in ozone concentrations below 1 wt%, such as in equipment for accelerated aging of rubber samples.

Incidental production

Ozone may be formed from O
2 by electrical discharges and by action of high energy electromagnetic radiation. Unsuppressed arcing breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts [O
2 → 2O]. Free radicals of oxygen in and around the arc recombine to create ozone [O
3].^[61] Certain electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages, such as ionic air purifiers, laser printers, photocopiers, tasers and arc welders. Electric motors using brushes can generate ozone from repeated sparking inside the unit. Large motors that use brushes, such as those used by elevators or hydraulic pumps, will generate more ozone than smaller motors. Ozone is similarly formed in the Catatumbo lightning storms phenomenon on the Catatumbo River in Venezuela, which helps to replenish ozone in the upper troposphere. It is the world's largest single natural generator of ozone, lending calls for it to be designated a UNESCO World Heritage Site.^[62]

Laboratory production

In the laboratory, ozone can be produced by electrolysis using a 9 volt battery, a pencil graphite rod cathode, a platinum wire anode and a 3 molar sulfuric acid electrolyte.^[63] The half cell reactions taking place are:

3 H₂O → O₃ + 6 H⁺ + 6 e⁻ (ΔE^o = −1.53 V)

6 H⁺ + 6 e⁻ → 3 H₂ (ΔE^o = 0 V)

2 H₂O → O₂ + 4 H⁺ + 4 e⁻ (ΔE^o = −1.23 V)

In the net reaction, three equivalents of water are converted into one equivalent of ozone and three equivalents of hydrogen. Oxygen formation is a competing reaction.

It can also be "prepared" by high voltage arc. This can be done with an apparatus consisting of two concentric glass tubes sealed together at the top, with in and out spigots at the top and bottom of the outer tube. The inner core should have a length of metal foil inserted into it connected to one side of the power source. The other side of the power source should be connected to another piece of foil wrapped around the outer tube. Dry O
2 should be run through the tube in one spigot. As the O
2 is run through one spigot into the apparatus and high voltage is applied to the foil leads, electricity will discharge between the dry dioxygen in the middle and form O
3 and O
2 out the other spigot. The reaction can be summarized as follows:^[10]

3 O
2 — electricity → 2 O
3

Applications

Industry

The largest use of ozone is in the preparation of pharmaceuticals, synthetic lubricants, and many other commercially useful organic compounds, where it is used to sever carbon-carbon bonds.^[10] It can also be used for bleaching substances and for killing microorganisms in air and water sources.^[64] Many municipal drinking water systems kill bacteria with ozone instead of the more common chlorine.^[65] Ozone has a very high oxidation potential.^[66] Ozone does not form organochlorine compounds, nor does it remain in the water after treatment. Ozone can form the suspected carcinogen bromate in source water with high bromide concentrations. The Safe Drinking Water Act mandates that these systems introduce an amount of chlorine to maintain a minimum of 0.2 μmol/mol residual free chlorine in the pipes, based on results of regular testing. Where electrical power is abundant, ozone is a cost-effective method of treating water, since it is produced on demand and does not require transportation and storage of hazardous chemicals. Once it has decayed, it leaves no taste or odor in drinking water.

Although low levels of ozone have been advertised to be of some disinfectant use in residential homes, the concentration of ozone in dry air required to have a rapid, substantial effect on airborne pathogens exceeds safe levels recommended by the U.S. Occupational Safety and Health Administration and Environmental Protection Agency. Humidity control can vastly improve both the killing power of the ozone and the rate at which it decays back to oxygen (more humidity allows more effectiveness). Spore forms of most pathogens are very tolerant of atmospheric ozone in concentrations where asthma patients start to have issues.

Industrially, ozone is used to:

• Disinfect laundry in hospitals, food factories, care homes etc.;^[67]
• Disinfect water in place of chlorine^[10]
• Deodorize air and objects, such as after a fire. This process is extensively used in fabric restoration
• Kill bacteria on food or on contact surfaces;^[68]
• Sanitize swimming pools and spas
• Kill insects in stored grain^[69]
• Scrub yeast and mold spores from the air in food processing plants;
• Wash fresh fruits and vegetables to kill yeast, mold and bacteria;^[68]
• Chemically attack contaminants in water (iron, arsenic, hydrogen sulfide, nitrites, and complex organics lumped together as "colour");
• Provide an aid to flocculation (agglomeration of molecules, which aids in filtration, where the iron and arsenic are removed);
• Manufacture chemical compounds via chemical synthesis^[70]
• Clean and bleach fabrics (the former use is utilized in fabric restoration; the latter use is patented);^{[citation needed]}
• Assist in processing plastics to allow adhesion of inks;
• Age rubber samples to determine the useful life of a batch of rubber;
• Eradicate water borne parasites such as Giardia lamblia and Cryptosporidium in surface water treatment plants.

Ozone is a reagent in many organic reactions in the laboratory and in industry. Ozonolysis is the cleavage of an alkene to carbonyl compounds.

Many hospitals around the world use large ozone generators to decontaminate operating rooms between surgeries. The rooms are cleaned and then sealed airtight before being filled with ozone which effectively kills or neutralizes all remaining bacteria.^[71]

Ozone is used as an alternative to chlorine or chlorine dioxide in the bleaching of wood pulp.^[72] It is often used in conjunction with oxygen and hydrogen peroxide to eliminate the need for chlorine-containing compounds in the manufacture of high-quality, white paper.^[73]

Ozone can be used to detoxify cyanide wastes (for example from gold and silver mining) by oxidizing cyanide to cyanate and eventually to carbon dioxide.^[74]

Consumers

Devices generating high levels of ozone, some of which use ionization, are used to sanitize and deodorize uninhabited buildings, rooms, ductwork, woodsheds, and boats and other vehicles.
One company has been successfully selling a CPAP sanitizer for the CPAP gear used by sleep apnea patients. This sanitizer works by pumping high concentration levels of electrically-generated ozone into the unit's humidification water tank (with or without water in it) and out through the hose into the mask, which is enclosed and sealed in an ozone-capturing receptacle (that also contains the ozone generator and pump that pushes it into the water tank), which completes a closed-loop system. This closed-loop system prevents the high levels of ozone from escaping while effectively sanitizing the CPAP equipment, as the CPAP equipment is prone to developing bacterial infestations and harboring viruses and other pathogens because of the constant moisture generated by the CPAP system's humidifier. The sanitizing unit has a two-hour cycle, it pumps the ozone for 6–10 minutes (user-designated) and then resting for two hours while maintaining the sealed closed-circuit loop as the ozone decays back into oxygen and finishes the sanitizing effect.^[75]

In the U.S., air purifiers emitting low levels of ozone have been sold. This kind of air purifier is sometimes claimed to imitate nature's way of purifying the air without filters and to sanitize both it and household surfaces. The United States Environmental Protection Agency (EPA) has declared that there is "evidence to show that at concentrations that do not exceed public health standards, ozone is not effective at removing many odor-causing chemicals" or "viruses, bacteria, mold, or other biological pollutants." Furthermore, its report states that "results of some controlled studies show that concentrations of ozone considerably higher than these [human safety] standards are possible even when a user follows the manufacturer’s operating instructions."^[76] A couple kept repeating health claims for the generator they sold, without supporting scientific studies. In 1998 a federal jury convicted them, among others things, of illegally distributing an ozone generator and of wire fraud.^[77]

Ozonated water is used to launder clothes and to sanitize food, drinking water, and surfaces in the home. According to the U.S. Food and Drug Administration (FDA), it is "amending the food additive regulations to provide for the safe use of ozone in gaseous and aqueous phases as an antimicrobial agent on food, including meat and poultry." Studies at California Polytechnic University demonstrated that 0.3 μmol/mol levels of ozone dissolved in filtered tapwater can produce a reduction of more than 99.99% in such food-borne microorganisms as salmonella, E. coli 0157:H7 and Campylobacter. This quantity is 20,000 times the WHO-recommended limits stated above.^[68][78] Ozone can be used to remove pesticide residues from fruits and vegetables.^[79][80]

Ozone is used in homes and hot tubs to kill bacteria in the water and to reduce the amount of chlorine or bromine required by reactivating them to their free state. Since ozone does not remain in the water long enough, ozone by itself is ineffective at preventing cross-contamination among bathers and must be used in conjunction with halogens. Gaseous ozone created by ultraviolet light or by corona discharge is injected into the water.^[81]

Ozone is also widely used in treatment of water in aquariums and fish ponds. Its use can minimize bacterial growth, control parasites, eliminate transmission of some diseases, and reduce or eliminate "yellowing" of the water. Ozone must not come in contact with fish's gill structures. Natural salt water (with life forms) provides enough "instantaneous demand" that controlled amounts of ozone activate bromide ion to hypobromous acid, and the ozone entirely decays in a few seconds to minutes. If oxygen fed ozone is used, the water will be higher in dissolved oxygen, fish's gill structures will atrophy and they will become dependent on higher dissolved oxygen levels.

Aquaculture

Ozonation - a process of infusing water with ozone - can be used in aquaculture to facilitate organic breakdown. Ozone is also added to recirculating systems to reduce nitrite levels^[82] through conversion into nitrate. If nitrite levels in the water are high, nitrites will also accumulate in the blood and tissues of fish, where it interferes with oxygen transport (it causes oxidation of the heme-group of haemoglobin from ferrous (Fe2+) to ferric (Fe3+), making haemoglobin unable to bind O
2).^[83]
Despite these apparent positive effects, ozone use in recirculation systems has been linked to reducing the level of bioavailable iodine in salt water systems, resulting in iodine deficiency symptoms such as goitre and decreased growth in Senegalese sole (Solea senegalensis) larvae.^[84]

Ozonate seawater is used for surface disinfection of haddock and Atlantic halibut eggs against nodavirus. Nodavirus is a lethal and vertically transmitted virus which causes severe mortality in fish. Haddock eggs should not be treated with high ozone level as eggs so treated did not hatch and died after 3–4 days.^[85]

Agriculture

Ozone application on freshly cut pineapple and banana shows increase in flavonoids and total phenol contents when exposure is up to 20 minutes. Decrease in ascorbic acid (one form of vitamin C) content is observed but the positive effect on total phenol content and flavonoids can overcome the negative effect.^[86] Tomatoes upon treatment with ozone shows an increase in β-carotene, lutein and lycopene.^[87] However, ozone application on strawberries in pre-harvest period shows decrease in ascorbic acid content.^[88]

Ozone facilitates the extraction of some heavy metals from soil using EDTA. EDTA forms strong, water-soluble coordination compounds with some heavy metals (Pb, Zn) thereby making it possible to dissolve them out from contaminated soil. If contaminated soil is pre-treated with ozone, the extraction efficacy of Pb, Am and Pu increases by 11.0–28.9%,^[89] 43.5%^[90] and 50.7%^[90] respectively.

Stratosphere

From Wikipedia, the free encyclopedia

Space Shuttle Endeavour appears to straddle the stratosphere and mesosphere in this photo. "The orange layer is the troposphere, where all of the weather and clouds which we typically watch and experience are generated and contained. This orange layer gives way to the whitish Stratosphere and then into the Mesosphere."^[1] (The shuttle is actually orbiting at more than 200 miles in altitude, far above this transition layer.)

Atmosphere diagram showing stratosphere. The layers are to scale: from Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.

This image shows the temperature trend in the Lower Stratosphere as measured by a series of satellite-based instruments between January 1979 and December 2005. The Lower Stratosphere is centered around 18 kilometers above Earth's surface. The stratosphere image is dominated by blues and greens, which indicates a cooling over time. Source: [1]

The stratosphere /ˈstrætəsfɪər/ is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the troposphere and stratosphere, the tropopause, is marked by where this inversion begins, which in terms of atmospheric thermodynamics is the equilibrium level. At moderate latitudes the stratosphere is situated between about 10–13 km (33,000–43,000 ft; 6.2–8.1 mi) and 50 km (160,000 ft; 31 mi) altitude above the surface, while at the poles it starts at about 8 km (26,000 ft; 5.0 mi) altitude, and near the equator it may start at altitudes as high as 18 km (59,000 ft; 11 mi).

Ozone and temperature

Within this layer, temperature increases as altitude increases (see temperature inversion); the top of the stratosphere has a temperature of about 270 K (−3°C or 26.6°F), just slightly below the freezing point of water.^[2] The stratosphere is layered in temperature because ozone (O₃) here absorbs high energy UVB and UVC energy waves from the Sun and is broken down into atomic oxygen (O) and diatomic oxygen (O₂). Atomic oxygen is found prevalent in the upper stratosphere due to the bombardment of UV light and the destruction of both ozone and diatomic oxygen. The mid stratosphere has less UV light passing through it, O and O₂ are able to combine, and is where the majority of natural ozone is produced. It is when these two forms of oxygen recombine to form ozone that they release the heat found in the stratosphere. The lower stratosphere receives very low amounts of UVC, thus atomic oxygen is not found here and ozone is not formed (with heat as the byproduct).^{[verification needed]} This vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere. The top of the stratosphere is called the stratopause, above which the temperature decreases with height.

Methane (CH₄), while not a direct cause of ozone destruction in the stratosphere, does lead to the formation of compounds that destroy ozone. Monoatomic oxygen (O) in the upper stratosphere reacts with methane (CH₄) to form a hydroxyl radical (OH·). This hydroxyl radical is then able to interact with non-soluble compounds like chlorofluorocarbons, and UV light breaks off chlorine radicals (Cl·). These chlorine radicals break off an oxygen atom from the ozone molecule, creating an oxygen molecule (O₂) and a hypochloryl radical (ClO·). The hypochloryl radical then reacts with an atomic oxygen creating another oxygen molecule and another chlorine radical, thereby preventing the reaction of monoatomic oxygen with O₂ to create natural ozone.

Aircraft flight

Commercial airliners typically cruise at altitudes of 9–12 km (30,000–39,000 ft) in temperate latitudes (in the lower reaches of the stratosphere).^[3] This optimizes fuel burn, mostly due to the low temperatures encountered near the tropopause and low air density, reducing parasitic drag on the airframe. (Stated another way, it allows the airliner to fly faster for the same amount of drag.) It also allows them to stay above hard weather (extreme turbulence).

Concorde would cruise at mach 2 at about 18,000 m (59,000 ft), and the SR-71 would cruise at mach 3 at 26,000 m (85,000 ft), all still in the stratosphere.

Because the temperature in the tropopause and lower stratosphere remains constant (or slightly decreases) with increasing altitude, very little convective turbulence occurs at these altitudes. Though most turbulence at this altitude is caused by variations in the jet stream and other local wind shears, areas of significant convective activity (thunderstorms) in the troposphere below may produce convective overshoot.

Although a few gliders have achieved great altitudes in the powerful thermals in thunderstorms,^{[citation needed]} this is dangerous. Most high altitude flights by gliders use lee waves from mountain ranges and were used to set the current record of 15,447 m (50,679 ft).

On October 14, 2012, Felix Baumgartner became the record holder for both reaching the altitude record for a manned balloon and highest skydive ever from 39.04 km (128,100 ft).^[4]

Circulation and mixing

The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which the horizontal mixing of gaseous components proceeds much more rapidly than in vertical mixing.

An interesting feature of stratospheric circulation is the quasi-biennial oscillation (QBO) in the tropical latitudes, which is driven by gravity waves that are convectively generated in the troposphere. The QBO induces a secondary circulation that is important for the global stratospheric transport of tracers, such as ozone^[5] or water vapor.

In northern hemispheric winter, sudden stratospheric warmings, caused by the absorption of Rossby waves in the stratosphere, can be observed in approximately half of winters when easterly winds develop in the stratosphere. These events often precede unusual winter weather ^[6] and may even be responsible for the cold European winters of the 1960s.^[7]

Life

Bacteria

Bacterial life survives in the stratosphere, making it a part of the biosphere.^[8] In 2001 an Indian experiment, involving a high-altitude balloon, was carried out at a height of 41 kilometres and a sample of dust was collected with bacterial material inside.^[9]

Birds

Also, some bird species have been reported to fly at the lower levels of the stratosphere. On November 29, 1975, a Rüppell's Vulture was ingested into a jet engine 11,552 m (37,900 ft) above the Ivory Coast, and Bar-headed geese reportedly overfly Mount Everest's summit, which is 8,848 m (29,029 ft).^[10][11]

Troposphere

From Wikipedia, the free encyclopedia

Space Shuttle Endeavour sillouetted against the atmosphere. The orange layer is the troposphere, the white layer is the stratosphere and the blue layer the mesosphere.^[1] (The shuttle is actually orbiting at an altitude of more than 200 miles, far above all three layers.)

Earth's atmosphere diagram showing the exosphere and other layers. The layers are to scale. From Earth's surface to the top of the stratosphere (50km) is just under 1% of Earth's radius.

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapour and aerosols.^[2] The average depth of the troposphere is approximately 17 km (11 mi) in the middle latitudes. It is deeper in the tropics, up to 20 km (12 mi), and shallower near the polar regions, approximately 7 km (4.3 mi) in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred metres to 2 km (1.2 mi) deep depending on the landform and time of day. The border between the troposphere and stratosphere, called the tropopause, is a temperature inversion.^[3]

The word troposphere derives from the Greek: tropos for "change" reflecting the fact that turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena we associate with day-to-day weather occur in the troposphere.^[3]

Pressure and temperature structure

A view of Earth's troposphere from an airplane.

Atmospheric circulation shown with three large cells.

Composition

The chemical composition of the troposphere is essentially uniform, with the notable exception of water vapor. The source of water vapour is at the surface through the processes of evaporation and transpiration. Furthermore the temperature of the troposphere decreases with height, and saturation vapor pressure decreases strongly as temperature drops, so the amount of water vapor that can exist in the atmosphere decreases strongly with height. Thus the proportion of water vapour is normally greatest near the surface and decreases with height.

Pressure

The pressure of the atmosphere is maximum at sea level and decreases with higher altitude. This is because the atmosphere is very nearly in hydrostatic equilibrium, so that the pressure is equal to the weight of air above a given point. The change in pressure with height, therefore can be equated to the density with this hydrostatic equation:^[4]

where:

• g_n is the standard gravity
• ρ is the density
• z is the altitude
• p is the pressure
• R is the gas constant
• T is the thermodynamic (absolute) temperature
• m is the molar mass

Since temperature in principle also depends on altitude, one needs a second equation to determine the pressure as a function of height, as discussed in the next section.*

Temperature

This image shows the temperature trend in the Middle Troposphere as measured by a series of satellite-based instruments between January 1979 and December 2005. The middle troposphere is centered around 5 kilometers above the surface. Oranges and yellows dominate the troposphere image, indicating that the air nearest the Earth’s surface warmed during the period.Source: [1]

The temperature of the troposphere generally decreases as altitude increases. The rate at which the temperature decreases, , is called the environmental lapse rate (ELR). The ELR is nothing more than the difference in temperature between the surface and the tropopause divided by the height. The reason for this temperature difference is that most absorption of the sun's energy occurs at the ground which then heats the lower levels of the atmosphere, and the radiation of heat occurs at the top of the atmosphere cooling the earth, this process maintaining the overall heat balance of the earth.

As parcels of air in the atmosphere rise and fall, they also undergo changes in temperature for reasons described below. The rate of change of the temperature in the parcel may be less than or more than the ELR. When a parcel of air rises, it expands, because the pressure is lower at higher altitudes. As the air parcel expands, it pushes on the air around it, doing work; but generally it does not gain heat in exchange from its environment, because its thermal conductivity is low (such a process is called adiabatic). Since the parcel does work and gains no heat, it loses energy, and so its temperature decreases. (The reverse, of course, will be true for a sinking parcel of air.) ^[3]

Since the heat exchanged is related to the entropy change by , the equation governing the temperature as a function of height for a thoroughly mixed atmosphere is

where S is the entropy. The rate at which temperature decreases with height under such conditions is called the adiabatic lapse rate.

For dry air, which is approximately an ideal gas, we can proceed further. The adiabatic equation for an ideal gas is ^[5]

where is the heat capacity ratio (=7/5, for air). Combining with the equation for the pressure, one arrives at the dry adiabatic lapse rate,^[6]

If the air contains water vapor, then cooling of the air can cause the water to condense, and the behavior is no longer that of an ideal gas. If the air is at the saturated vapor pressure, then the rate at which temperature drops with height is called the saturated adiabatic lapse rate. More generally, the actual rate at which the temperature drops with altitude is called the environmental lapse rate. In the troposphere, the average environmental lapse rate is a drop of about 6.5 °C for every 1 km (1,000 meters) in increased height.^[3]

The environmental lapse rate (the actual rate at which temperature drops with height, ) is not usually equal to the adiabatic lapse rate (or correspondingly, ). If the upper air is warmer than predicted by the adiabatic lapse rate (), then when a parcel of air rises and expands, it will arrive at the new height at a lower temperature than its surroundings. In this case, the air parcel is denser than its surroundings, so it sinks back to its original height, and the air is stable against being lifted. If, on the contrary, the upper air is cooler than predicted by the adiabatic lapse rate, then when the air parcel rises to its new height it will have a higher temperature and a lower density than its surroundings, and will continue to accelerate upward.^[3][4]

The troposphere is heated from below by latent heat, longwave radiation, and sensible heat. Surplus heating and vertical expansion of the troposphere occurs in the tropics. At middle latitudes, tropospheric temperatures decrease from an average of 15°C at sea level to about -55°C at the tropopause. At the poles, tropospheric temperature only decreases from an average of 0ºC at sea level to about -45°C at the tropopause. At the equator, tropospheric temperatures decrease from an average of 20ºC at sea level to about -70 to -75°C at the tropopause. The troposphere is thinner at the poles and thicker at the equator. The average thickness of the tropical tropopause is roughly 7 kilometers greater than the average tropopause thickness at the poles. ^[7]

Tropopause

The tropopause is the boundary region between the troposphere and the stratosphere.
Measuring the temperature change with height through the troposphere and the stratosphere identifies the location of the tropopause. In the troposphere, temperature decreases with altitude. In the stratosphere, however, the temperature remains constant for a while and then increases with altitude. The region of the atmosphere where the lapse rate changes from positive (in the troposphere) to negative (in the stratosphere), is defined as the tropopause.^[3] Thus, the tropopause is an inversion layer, and there is little mixing between the two layers of the atmosphere.

Atmospheric flow

The flow of the atmosphere generally moves in a west to east direction. This however can often become interrupted, creating a more north to south or south to north flow. These scenarios are often described in meteorology as zonal or meridional. These terms, however, tend to be used in reference to localised areas of atmosphere (at a synoptic scale). A fuller explanation of the flow of atmosphere around the Earth as a whole can be found in the three-cell model.

Zonal Flow

A zonal flow regime is the meteorological term meaning that the general flow pattern is west to east along the Earth's latitude lines, with weak shortwaves embedded in the flow.^[7] The use of the word "zone" refers to the flow being along the Earth's latitudinal "zones". This pattern can buckle and thus become a meridional flow.

Meridional flow

Meridional Flow pattern of October 23, 2003. Note the amplified troughs and ridges in this 500 hPa height pattern.

When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional) direction, and thus the term "meridional flow" arises. Meridional flow patterns feature strong, amplified troughs and ridges, with more north-south flow in the general pattern than west-to-east flow.^[8]

Three-cell model

The three cells model attempts to describe the actual flow of the Earth's atmosphere as a whole. It divides the Earth into the tropical (Hadley cell), mid latitude (Ferrel cell), and polar (polar cell) regions, dealing with energy flow and global circulation. Its fundamental principle is that of balance - the energy that the Earth absorbs from the sun each year is equal to that which it loses back into space, but this however is not a balance precisely maintained in each latitude due to the varying strength of the sun in each "cell" resulting from the tilt of the Earth's axis in relation to its orbit. It demonstrates that a pattern emerges to mirror that of the ocean - the tropics do not continue to get warmer because the atmosphere transports warm air poleward and cold air equatorward, the effect of which appears to be that of heat and moisture distribution around the planet.^[9]

Synoptic scale observations and concepts

Forcing

Forcing is a term used by meteorologists to describe the situation where a change or an event in one part of the atmosphere causes a strengthening change in another part of the atmosphere. It is usually used to describe connections between upper, middle or lower levels (such as upper-level divergence causing lower level convergence in cyclone formation), but can sometimes also be used to describe such connections over distance rather than height alone. In some respects, teleconnections could be considered a type of forcing.

Divergence and convergence

An area of convergence is one in which the total mass of air is increasing with time, resulting in an increase in pressure at locations below the convergence level (recall that atmospheric pressure is just the total weight of air above a given point). Divergence is the opposite of convergence - an area where the total mass of air is decreasing with time, resulting in falling pressure in regions below the area of divergence. Where divergence is occurring in the upper atmosphere, there will be air coming in to try to balance the net loss of mass (this is called the principle of mass conservation), and there is a resulting upward motion (positive vertical velocity). Another way to state this is to say that regions of upper air divergence are conducive to lower level convergence, cyclone formation, and positive vertical velocity. Therefore, identifying regions of upper air divergence is an important step in forecasting the formation of a surface low pressure area.