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Carbon dioxide

2007 Schools Wikipedia Selection. Related subjects: Chemical compounds

   Carbonwawawadin mamma din mamma din mamma din mamma mother dioxide
   Carbon dioxide
   Carbon dioxide
   Other names Carbonic acid gas,
   Carbonic anhydride,
   dry ice (solid)
   Molecular formula CO[2]
   Molar mass 44.0095(14) g/mol
   Solid state Dry ice, carbonia
   Appearance colorless gas
   CAS number [124-38-9]
   Properties
   Density and phase 1600 kg/m³, solid
   1.98 kg/m³, gas at 298 K
   Solubility in water 1.45 kg/m³
   Latent heat of
   sublimation 25.13 kJ/mol
   Melting point −57 °C (216 K), pressurized
   Boiling point −78 °C (195 K), sublimes
   Acidity (pK[a]) 6.35 and 10.33
   Viscosity 0.07 c P at −78 °C
   Structure
   Molecular shape linear
   Crystal structure quartz-like
   Dipole moment zero
   Hazards
   MSDS External MSDS
   Main hazards asphyxiant, irritant
   NFPA 704

   0
   0
   0

   (liquid)
   R-phrases R: As, Fb
   S-phrases S9, S23, S36 (liquid)
   RTECS number FF6400000
   Supplementary data page
   Structure & properties n, ε[r], etc.
   Spectral data UV, IR, NMR, MS
   Related compounds
   Related oxides carbon monoxide
   carbon suboxide
   dicarbon monoxide
   carbon trioxide
   Except where noted otherwise, data are given for
   materials in their standard state (at 25 °C, 100 kPa)
   Infobox disclaimer and references

   Carbon dioxide is a chemical compound composed of one carbon and two
   oxygen atoms. It is often referred to by its formula CO[2]. It is
   present in the Earth's atmosphere at a low concentration and acts as a
   greenhouse gas. In its solid state, it is called dry ice. It is a major
   component of the carbon cycle.

Origins of CO2

   Atmospheric carbon dioxide derives from multiple natural sources
   including volcanic outgassing, the combustion of organic matter, and
   the respiration processes of living aerobic organisms; man-made sources
   of carbon dioxide come mainly from the burning of various fossil fuels
   for power generation and transport use. It is also produced by various
   microorganisms from fermentation and cellular respiration. Plants
   convert carbon dioxide to oxygen during a process called
   photosynthesis, using both the carbon and the oxygen to construct
   carbohydrates. In addition, plants also release oxygen to the
   atmosphere, which is subsequently used for respiration by heterotrophic
   organisms, forming a cycle.

Chemical and physical properties

   Carbon dioxide is a colorless gas which, when inhaled at high
   concentrations (a dangerous activity because of the associated
   asphyxiation risk), produces a sour taste in the mouth and a stinging
   sensation in the nose and throat. These effects result from the gas
   dissolving in the mucous membranes and saliva, forming a weak solution
   of carbonic acid. You may notice this sensation if you attempt to
   stifle a burp after drinking a carbonated beverage.

   Its density at 25 °C is 1.98 kg m^-3, about 1.65 times that of air. The
   carbon dioxide molecule (O=C=O) contains two double bonds and has a
   linear shape. It has no electrical dipole. As it is fully oxidized, it
   is not very reactive and, in particular, not flammable.

   At temperatures below −78 °C, carbon dioxide changes directly from a
   gas to a white solid called dry ice through a process called
   deposition. Liquid carbon dioxide forms only at pressures above 5.1
   atm; at atmospheric pressure, it passes directly between the solid
   phase and the gaseous phase in a process called sublimation.

   Carbon dioxide is soluble in water, in which it spontaneously
   interconverts between CO[2] and H[2]CO[3] ( carbonic acid). The
   relative concentrations of CO[2], H[2]CO[3], and the deprotonated forms
   HCO[3]^- ( bicarbonate) and CO[3]^2-( carbonate) depend on pH. In
   neutral or slightly alkaline water (pH > 6.5), the bicarbonate form
   predominates (>50%) becoming the most prevalent (>95%) at the pH of
   seawater, while in very alkaline water (pH > 10.4) the predominant
   (>50%) form is carbonate. The bicarbonate and carbonate forms are very
   soluble, such that air-equilibrated ocean water (mildly alkaline with
   typical pH = 8.2-8.5) contains about 120 mg of bicarbonate per liter.

Uses

General

   Carbon dioxide is used to produce carbonated soft drinks and soda
   water. Traditionally, the carbonation in beer and sparkling wine comes
   about through natural fermentation, but some manufacturers carbonate
   these drinks artificially. A candy called Pop Rocks is pressurized with
   carbon dioxide gas at about 40 bar (600 psi). When placed in the mouth,
   it dissolves (just like other hard candy) and releases the gas bubbles
   with an audible "pop."

   Leavening agents produce carbon dioxide to cause dough to rise. Baker's
   yeast produces carbon dioxide by fermentation within the dough, while
   chemical leaveners such as baking powder and baking soda release carbon
   dioxide when heated or if exposed to acids.
   A carbon dioxide laser.
   Enlarge
   A carbon dioxide laser.

   Carbon dioxide is often used as an inexpensive, nonflammable
   pressurized gas. Life jackets often contain canisters of pressured
   carbon dioxide for quick inflation. Steel capsules are also sold as
   supplies of compressed gas for airguns, paintball markers, for
   inflating bicycle tires, and for making seltzer. Rapid vaporization of
   liquid CO[2] is used for blasting in coal mines.

   Carbon dioxide is the most commonly used compressed gas for pneumatic
   systems in combat robots. Carbon dioxide is ideal for this application
   because at room temperature it becomes a liquid at a pressure of 60
   bar. A tank of liquid carbon dioxide provides a constant 60 bar
   pressure until the tank is close to being empty. A tank of compressed
   air would gradually reduce in pressure as it was used.

   Carbon dioxide extinguishes flames, and some fire extinguishers,
   especially those designed for electrical fires, contain liquid carbon
   dioxide under pressure. Carbon dioxide also finds use as an atmosphere
   for welding, although in the welding arc, it reacts to oxidize most
   metals. Use in the automotive industry is common despite significant
   evidence that welds made in carbon dioxide are brittler than those made
   in more inert atmospheres, and that such weld joints deteriorate over
   time because of the formation of carbonic acid. It is used as a welding
   gas primarily because it is much less expensive than more inert gases
   such as argon or helium.

   Liquid carbon dioxide is a good solvent for many organic compounds, and
   is used to remove caffeine from coffee. First, the green coffee beans
   are soaked in water. The beans are placed in the top of a column
   seventy feet (21 meters) high. The carbon dioxide fluid at about 93
   degrees Celsius enters at the bottom of the column. The caffeine
   diffuses out of the beans and into the carbon dioxide.

   Carbon dioxide has begun to attract attention in the pharmaceutical and
   other chemical processing industries as a less toxic alternative to
   more traditional solvents such as organochlorides. It's used by some
   dry cleaners for this reason. (See green chemistry.)

   Plants require carbon dioxide to conduct photosynthesis, and
   greenhouses may enrich their atmospheres with additional CO[2] to boost
   plant growth. It has been proposed that carbon dioxide from power
   generation be bubbled into ponds to grow algae that could then be
   converted into biodiesel fuel . High levels of carbon dioxide in the
   atmosphere effectively exterminate many pests. Greenhouses will raise
   the level of CO[2] to 10,000 ppm (1%) for several hours to eliminate
   pests such as whiteflies, spider mites, and others.

   In medicine, up to 5% carbon dioxide is added to pure oxygen for
   stimulation of breathing after apnea and to stabilize the O[2]/CO[2]
   balance in blood.

   A common type of industrial gas laser, the carbon dioxide laser, uses
   carbon dioxide as a medium.

   Carbon dioxide can also be combined with limonene from orange peels or
   other epoxides to create polymers and plastics.

   Carbon dioxide is commonly injected into or adjacent to producing oil
   wells. It will act as both a pressurizing agent and, when dissolved
   into the underground crude oil, will significantly reduce its
   viscosity, enabling the oil to flow more rapidly through the earth to
   the removal well. In mature oil fields, extensive pipe networks are
   used to carry the carbon dioxide to the injection points.

Refrigerant

   Liquid and solid carbon dioxide are important refrigerants, especially
   in the food industry, where they are employed during the transportation
   and storage of ice cream and other frozen foods. Solid carbon dioxide
   is called "dry ice" and is used for small shipments where refrigeration
   equipment is not practical.

   Liquid carbon dioxide was used as a refrigerant prior to the discovery
   of R-12 and may be enjoying something of a renaissance due to
   environmental concerns. Its physical properties are not favorable,
   having a low critical temperature of 88°F/31°C (the maximum temperature
   at which it will condense from gas to liquid) and high critical
   pressure of 1070 psi (the pressure required for phase change at the
   critical temperature). These properties necessitate the use of very
   strong refrigeration plumbing to contain the operating pressure of
   ~1400 psi, in contrast to pressures of ~300 psi for R-134a systems.
   Although carbon dioxide is non-inflammable and non-toxic it is an
   asphyxiant, which raises safety concerns in the case of leaks in
   enclosed spaces or system rupture in the case of vehicle accident.
   Despite these issues Coca-Cola has fielded CO[2]-based beverage coolers
   and the US Army and others have expressed interest .

Solid CO[2]

Dry ice

   Dry ice is a genericized trademark for solid ("frozen") carbon dioxide.
   The term was coined in 1925 by Prest Air Devices, founded in Long
   Island City, New York in 1923. The name refers to the fact that under
   normal atmospheric pressure, solid CO[2] sublimates, or changes
   directly into a gas without passing through a "wet" liquid phase. As a
   general rule, dry ice will sublimate at a rate of five to ten pounds
   every 24 hours in a typical ice chest.

   Dry ice is produced by compressing carbon dioxide gas to a liquid form,
   removing the heat produced by the compression (see Charles's law), and
   then letting the liquid carbon dioxide expand quickly. This expansion
   causes a drop in temperature so that some of the CO[2] freezes into
   "snow", which is then compressed into pellets or blocks. The freezing
   point of CO[2] is -109.3°F or -78.5°C.

   Dry ice has many industrial uses, including
   Dry ice used to cool drinks in Central Park.(New York City, New York,
   U.S.)
   Enlarge
   Dry ice used to cool drinks in Central Park.
   (New York City, New York, U.S.)
     * Cooling foodstuffs, biological samples, and other perishable items,
       particularly for shipment.
     * Producing "dry ice fog" for special effects. When dry ice is put
       into contact with water, the frozen carbon dioxide sublimates into
       a mixture of cold carbon dioxide gas and cold humid air. This
       causes condensation and the formation of fog (see: fog machine).
       The use of warm water speeds up sublimation and leads to more
       vigorous production of fog.
     * Tiny pellets of dry ice ( instead of sand) are shot at a surface to
       be cleaned. Dry ice is not as hard as sand, but it speeds
       processing by sublimating to a gas and does not produce nearly as
       much lung-damaging dust.
     * Increasing precipitation from existing clouds or decreasing cloud
       thickness by cloud seeding.
     * Producing carbon dioxide gas as needed in such systems as the fuel
       tank inerting system in the B-47 aircraft.
     * Brass or other metallic bushings are buried in dry ice to shrink
       them so they will fit inside a machined hole. When the bushing
       warms back up, it expands and makes an extremely tight fit.
     * As a cooling supplement for overclocking a central processing unit,
       a graphics processing unit, or other types of computer hardware.
     * A rudimentary cloud chamber can be built using dry ice to supercool
       alcohol vapor.

   Dry ice requires special precautions when handling. It is extremely
   cold, requiring proper insulating gloves to handle. It constantly
   produces carbon dioxide gas, so it cannot be stored in a light duty
   sealed container as the pressure buildup will quickly cause the
   container to explode (see dry ice bomb). The sublimated gas must be
   ventilated; otherwise, it may fill the enclosed space and create a
   suffocation hazard. Special care for ventilating vehicles is needed as
   well because of the small space. People who handle dry ice should also
   be aware that carbon dioxide is heavier than air and will sink to the
   floor. Some markets require those purchasing dry ice to be of 18 years
   of age or older.

Solid amorphous CO[2]

   In June 2006, an Italian-French science team announced the synthetic
   production of solid amorphous CO[2] glass. This form of glass, called
   carbonia, was produced by supercooling heated CO[2] at extreme pressure
   (40–48 GPa or about 400,000 atmospheres) in a diamond anvil. This
   discovery confirmed the theory that carbon dioxide could exist in a
   glass state similar to other members of its elemental family, like
   silicon ( silica glass) and germanium. Unlike silica and germanium
   oxide glasses, however, carbonia glass is not stable at normal
   pressures and reverts back to gas when pressure is released.

Biology

   Carbon dioxide is an end product in organisms that obtain energy from
   breaking down sugars or fats with oxygen as part of their metabolism,
   in a process known as cellular respiration. This includes all plants,
   animals, many fungi and some bacteria. In higher animals, the carbon
   dioxide travels in the blood from the body's tissues to the lungs where
   it is exhaled. In plants using photosynthesis, carbon dioxide is
   absorbed from the atmosphere.

   Carbon dioxide content in fresh air varies and is between 0.03% (300
   ppm) to 0.06% (600 ppm), depending on location and in exhaled air
   approximately 4.5%. When inhaled in high concentrations (greater than
   5% by volume), it is immediately dangerous to the life and health of
   plants, humans and other animals. The current threshold limit value
   (TLV) or maximum level that is considered safe for healthy adults for
   an 8-hour work day is 0.5% (5000 ppm). The maximum safe level for
   infants, children, the elderly and individuals with cardio-pulmonary
   health issues would be significantly less. Acute carbon dioxide
   toxicity is sometimes known as choke damp, an old mining industry term,
   and was the cause of death at Lake Nyos in Cameroon, where an upwelling
   of CO[2]-laden lake water in 1986 covered a wide area in a blanket of
   the gas, killing nearly 2000. The lowering of carbon dioxide in the
   atmosphere is largely due to absorption by plants, which convert it to
   sugars through photosynthesis. Phytoplankton photosynthesis absorbs
   dissolved CO[2] in the upper ocean and thereby promotes the absorption
   of CO[2] from the atmosphere (Falkowski P. Scholes RJ. Boyle E.
   Canadell J. Canfield D. Elser J. Gruber N. Hibbard K. Hogberg P. Linder
   S. Mackenzie FT. Moore B 3rd. Pedersen T. Rosenthal Y. Seitzinger S.
   Smetacek V. Steffen W. The global carbon cycle: a test of our knowledge
   of earth as a system. [Review] [65 refs] [Journal Article. Review]
   Science. 290(5490):291-6, 2000.

   Hemoglobin, the main oxygen-carrying molecule in red blood cells, can
   carry both oxygen and carbon dioxide, although in quite different ways.
   The decreased binding to oxygen in the blood due to increased carbon
   dioxide levels is known as the Haldane Effect, and is important in the
   transport of carbon dioxide from the tissues to the lungs. Conversely,
   a rise in the partial pressure of CO[2] or a lower pH will cause
   offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

   According to a study by the USDA, an average person's respiration
   generates approximately 450 liters (roughly 900 grams) of carbon
   dioxide per day.

   CO[2] is carried in blood in three different ways. Most of it (about
   80%–90%) is converted to bicarbonate ions HCO[3]^− by the enzyme
   carbonic anhydrase in the red blood cells. 5%–10% is dissolved in the
   plasma and 5%–10% is bound to hemoglobin as carbamino compounds. The
   exact percentages vary depending whether it is arterial or venous
   blood.

   The CO[2] bound to hemoglobin does not bind to the same site as oxygen;
   rather it combines with the N-terminal groups on the four globin
   chains. However, because of allosteric effects on the hemoglobin
   molecule, the binding of CO[2] does decrease the amount of oxygen that
   is bound for a given partial pressure of oxygen.

   Carbon dioxide may be one of the mediators of local autoregulation of
   blood supply. If it is high, the capillaries expand to allow a greater
   blood flow to that tissue.

   Bicarbonate ions are crucial for regulating blood pH. As breathing rate
   influences the level of CO[2] in blood, too slow or shallow breathing
   causes respiratory acidosis, while too rapid breathing,
   hyperventilation, leads to respiratory alkalosis.

   It is interesting to note that although it is oxygen that the body
   requires for metabolism, it is not low oxygen levels that stimulate
   breathing, but is instead higher carbon dioxide levels. As a result,
   breathing low-pressure air or a gas mixture with no oxygen at all
   (e.g., pure nitrogen) leads to loss of consciousness without subjective
   breathing problems. This is especially perilous for high-altitude
   fighter pilots, and is also the reason why the instructions in
   commercial airplanes for case of loss of cabin pressure stress that one
   should apply the oxygen mask to oneself before helping others—otherwise
   one risks going unconscious without being aware of the imminent peril.

   Plants remove carbon dioxide from the atmosphere by photosynthesis,
   which uses light energy to produce organic plant materials by combining
   carbon dioxide and water. This releases free oxygen gas. Sometimes
   carbon dioxide gas is pumped into greenhouses to promote plant growth.
   Plants can potentially grow up to twice as fast in conditions where
   extra CO[2] is available, although there is no additional benefit at
   concentrations beyond 1200 ppm. Plants also emit CO[2] during
   respiration, but on balance they are net sinks of CO[2].

   Carbon dioxide is a surrogate for indoor pollutants that may cause
   occupants to grow drowsy, get headaches, or function at lower activity
   levels. To eliminate most Indoor Air Quality complaints, total indoor
   carbon dioxide must be reduced to below 600 ppm. NIOSH considers that
   indoor air concentrations of carbon dioxide that exceed 1000 ppm are a
   marker suggesting inadequate ventilation (1,000 ppm equals 0.1%).
   ASHRAE recommends that CO[2] levels not exceed 1000 ppm inside a space.
   OSHA limits carbon dioxide concentration in the workplace to 0.5% for
   prolonged periods. The U.S. National Institute for Occupational Safety
   and Health limits brief exposures (up to ten minutes) to 3% and
   considers concentrations exceeding 4% as " immediately dangerous to
   life and health." People who breathe 5% carbon dioxide for more than
   half an hour show signs of acute hypercapnia, while breathing 7%–10%
   carbon dioxide can produce unconsciousness in only a few minutes.
   Carbon dioxide, either as a gas or as dry ice, should be handled only
   in well-ventilated areas.

Concentrations of CO[2] in atmosphere

   Atmospheric CO2 concentrations measured at Mauna Loa Observatory.
   Enlarge
   Atmospheric CO[2] concentrations measured at Mauna Loa Observatory.

   As of 2006, the earth's atmosphere is about 0.038% by volume (381 µL/L
   or ppmv) or 0.057% by weight CO[2]. This represents about 2.97 × 10^12
   tonnes of CO[2]. Because of the greater land area, and therefore
   greater plant life, in the northern hemisphere as compared to the
   southern hemisphere, there is an annual fluctuation of about 5 µL/L,
   peaking in May and reaching a minimum in October at the end of the
   northern hemisphere growing season, when the quantity of biomass on the
   planet is greatest.

   The latest data, as of March 2006, shows CO[2] levels now stand at 381
   parts per million (ppm) — 100ppm above the pre-industrial average.

   Despite its small concentration, CO[2] is a very important component of
   Earth's atmosphere, because it absorbs infrared radiation at
   wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and
   14.99 µm (bending vibrational mode) and enhances the greenhouse effect.
   The three vibrational modes of carbon dioxide: (a) symmetric, (b)
   asymmetric stretching; (c) bending. In (a), there is no change in
   dipole moment, thus interaction with photons is impossible, while in
   (b) and (c) there is optical activity.
   Enlarge
   The three vibrational modes of carbon dioxide: (a) symmetric, (b)
   asymmetric stretching; (c) bending. In (a), there is no change in
   dipole moment, thus interaction with photons is impossible, while in
   (b) and (c) there is optical activity.

   The initial carbon dioxide in the atmosphere of the young Earth was
   produced by volcanic activity; this was essential for a warm and stable
   climate conducive to life. Volcanic activity now releases about 130 to
   230 teragrams (145 million to 255 million short tons) of carbon dioxide
   each year. Volcanic releases are about 1% of the amount which is
   released by human activities.
   Global fossil carbon emissions 1800–2000.
   Enlarge
   Global fossil carbon emissions 1800–2000.

   Since the start of the Industrial Revolution, the atmospheric CO[2]
   concentration has increased by approximately 110 µL/L or about 40%,
   most of it released since 1945. Monthly measurements taken at Mauna Loa
   since 1958 show an increase from 316 µL/L in that year to 376 µL/L in
   2003, an overall increase of 60 µL/L during the 44-year history of the
   measurements. Burning fossil fuels such as coal and petroleum is the
   leading cause of increased man-made CO[2]; deforestation is the second
   major cause. Around 24,000 million tonnes of CO[2] are released per
   year worldwide, equivalent to about 6500 million tonnes of carbon. (See
   List of countries by carbon dioxide emissions.)
   Smoke and ozone pollution from Indonesian fires, 1997.
   Enlarge
   Smoke and ozone pollution from Indonesian fires, 1997.

   In 1997, Indonesian peat fires may have released 13%–40% as much carbon
   as fossil fuel burning does . Various techniques have been proposed for
   removing excess carbon dioxide from the atmosphere in carbon dioxide
   sinks. Not all the emitted CO[2] remains in the atmosphere; some is
   absorbed in the oceans or biosphere. The ratio of the emitted CO[2] to
   the increase in atmospheric CO[2] is known as the airborne fraction
   (Keeling et al., 1995); this varies for short-term averages but is
   typically 57% over longer (5 year) periods.

   The Global Warming Theory (GWT) predicts that increased amounts of
   CO[2]in the atmosphere tend to enhance the greenhouse effect and thus
   contribute to global warming. The effect of combustion-produced carbon
   dioxide on climate is called the Callendar effect.

Variation in the past

   CO2 concentrations over the last 400,000 years
   Enlarge
   CO[2] concentrations over the last 400,000 years

   The most direct method for measuring atmospheric carbon dioxide
   concentrations for periods before direct sampling is to measure bubbles
   of air ( fluid or gas inclusions) trapped in the Antarctic or Greenland
   ice caps. The most widely accepted of such studies come from a variety
   of Antarctic cores and indicate that atmospheric CO[2] levels were
   about 260–280µL/L immediately before industrial emissions began and did
   not vary much from this level during the preceding 10,000 years.

   The longest ice core record comes from East Antarctica, where ice has
   been sampled to an age of 800,000 years before the present . During
   this time, the atmospheric carbon dioxide concentration has varied
   between 180–210 µL/L during ice ages, increasing to 280–300 µL/L during
   warmer interglacials . The data can be accessed at
   http://www.ncdc.noaa.gov/paleo/icecore/antarctica/vostok/vostok_data.ht
   ml.

   Some studies have disputed the claim of stable CO[2] levels during the
   present interglacial (the last 10 kyr). Based on an analysis of fossil
   leaves, Wagner et al. argued that CO[2] levels during the period 7–10
   kyr ago were significantly higher (~300 µL/L) and contained substantial
   variations that may be correlated to climate variations. Others have
   disputed such claims, suggesting they are more likely to reflect
   calibration problems than actual changes in CO[2]. Relevant to this
   dispute is the observation that Greenland ice cores often report higher
   and more variable CO[2] values than similar measurements in Antarctica.
   However, the groups responsible for such measurements (e.g., Smith et
   al.) believe the variations in Greenland cores result from in situ
   decomposition of calcium carbonate dust found in the ice. When dust
   levels in Greenland cores are low, as they nearly always are in
   Antarctic cores, the researchers report good agreement between
   Antarctic and Greenland CO[2] measurements.
   Changes in carbon dioxide during the Phanerozoic (the last 542 million
   years). The recent period is located on the left-hand side of the plot,
   and it appears that much of the last 550 million years has experienced
   carbon dioxide concentrations significantly higher than the present
   day.
   Enlarge
   Changes in carbon dioxide during the Phanerozoic (the last 542 million
   years). The recent period is located on the left-hand side of the plot,
   and it appears that much of the last 550 million years has experienced
   carbon dioxide concentrations significantly higher than the present
   day.

   On longer timescales, various proxy measurements have been used to
   attempt to determine atmospheric carbon dioxide levels millions of
   years in the past. These include boron and carbon isotope ratios in
   certain types of marine sediments, and the number of stomata observed
   on fossil plant leaves. While these measurements give much less precise
   estimates of carbon dioxide concentration than ice cores, there is
   evidence for very high CO[2] concentrations (>3,000 µL/L) between 600
   and 400 Myr BP and between 200 and 150 Myr BP. On long timescales,
   atmospheric CO[2] content is determined by the balance among
   geochemical processes including organic carbon burial in sediments,
   silicate rock weathering, and vulcanism. The net effect of slight
   imbalances in the carbon cycle over tens to hundreds of millions of
   years has been to reduce atmospheric CO[2]. The rates of these
   processes are extremely slow; hence they are of limited relevance to
   the atmospheric CO[2] response to emissions over the next hundred
   years. In more recent times, atmospheric CO[2] concentration continued
   to fall after about 60 Myr BP, and there is geochemical evidence that
   concentrations were <300 µL/L by about 20 Myr BP. Low CO[2]
   concentrations may have been the stimulus that favored the evolution of
   C4 plants, which increased greatly in abundance between 7 and 5 Myr BP.
   Although contemporary CO[2] concentrations were exceeded during earlier
   geological epochs, present carbon dioxide levels are likely higher now
   than at any time during the past 20 million years and at the same time
   lower than at any time in history if we look at time scales longer than
   50 million years. NOAA research estimates that 97% of atmospheric CO2
   created each year is from natural sources and approximately 3% is from
   human activities.

Capturing/Extracting CO[2]

   Methods of CO[2] extraction/separation include:
    1. Aqueous solutions
          + Amine extraction
          + High pH solutions

                For example, Carbon Dioxide reacts with dissolved CaO, to
                form Calcite (CaCO[3])

    2. Adsorption
          + Molecular Sieve
          + Activated Carbon www.netl.doe.gov (pdf file)
          + Metal-organic frameworks(MOF's)
    3. Solid reactants
          + Serpentine, Olivine, Quicklime
    4. Membrane gas separation
    5. Regenerative Carbon Dioxide Removal System (RCRS)

                The RCRS on the space shuttle Orbiter uses a two-bed
                system that provides continuous removal of CO[2] without
                expendable products. Regenerable systems allow a shuttle
                mission a longer stay in space without having to replenish
                its sorbent canisters. Older lithium hydroxide
                (LiOH)-based systems, which are non-regenerable, are being
                replaced by regenerable metal-oxide-based systems. A
                metal-oxide-based system primarily consists of a metal
                oxide sorbent canister and a regenerator assembly. This
                system works by removing carbon dioxide using a sorbent
                material and then regenerating the sorbent material. The
                metal-oxide sorbent is regenerated by pumping air heated
                to around 400 °F at 7.5 scfm through its canister for 10
                hours.

Oceans

   Air-sea exchange of CO2
   Enlarge
   Air-sea exchange of CO[2]

   The Earth's oceans contain a huge amount of carbon dioxide in the form
   of bicarbonate and carbonate ions—much more than the amount in the
   atmosphere. The bicarbonate is produced in reactions between rock,
   water, and carbon dioxide. One example is the dissolution of calcium
   carbonate:

          CaCO[3] + CO[2] + H[2]O ⇌ Ca^2+ + 2 HCO[3]^-

   Reactions like this tend to buffer changes in atmospheric CO[2].
   Reactions between carbon dioxide and non-carbonate rocks also add
   bicarbonate to the seas, which can later undergo the reverse of the
   above reaction to form carbonate rocks, releasing half of the
   bicarbonate as CO[2]. Over hundreds of millions of years this has
   produced huge quantities of carbonate rocks. If all the carbonate rocks
   in the earth's crust were to be converted back into carbon dioxide, the
   resulting carbon dioxide would weigh 40 times as much as the rest of
   the atmosphere.

   The vast majority of CO[2] added to the atmosphere will eventually be
   absorbed by the oceans and become bicarbonate ion, but the process
   takes on the order of a hundred years because most seawater rarely
   comes near the surface.

History

   Carbon dioxide was one of the first gases to be described as a
   substance distinct from air. In the seventeenth century, the Flemish
   chemist Jan Baptist van Helmont observed that when he burned charcoal
   in a closed vessel, the mass of the resulting ash was much less than
   that of the original charcoal. His interpretation was that the rest of
   the charcoal had been transmuted into an invisible substance he termed
   a "gas" or "wild spirit" (spiritus sylvestre).

   Carbon dioxide's properties were studied more thoroughly in the 1750s
   by the Scottish physician Joseph Black. He found that limestone (
   calcium carbonate) could be heated or treated with acids to yield a gas
   he termed "fixed air." He observed that the fixed air was denser than
   air and did not support either flame or animal life. He also found that
   it would, when bubbled through an aqueous solution of lime ( calcium
   hydroxide), precipitate calcium carbonate, and used this phenomenon to
   illustrate that carbon dioxide is produced by animal respiration and
   microbial fermentation. In 1772, Joseph Priestley used carbon dioxide
   produced from the action of sulfuric acid on limestone to prepare soda
   water, the first known instance of an artificially carbonated drink.

   Carbon dioxide was first liquefied (at elevated pressures) in 1823 by
   Humphry Davy and Michael Faraday. The earliest description of solid
   carbon dioxide was given by Charles Thilorier, who in 1834 opened a
   pressurized container of liquid carbon dioxide, only to find that the
   cooling produced by the rapid evaporation of the liquid yielded a
   "snow" of solid CO[2].

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