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Carbon Dioxide (CO₂)

The linear carbon dioxide molecule joins one carbon atom and two oxygen atoms using double bonds, O=C=O, and has no electrical dipole. Most common in gaseous form, CO2 is colorless and odorless at room temperature (20^oC) and is detected by chemical or radiation analysis.  CO2 liquifies at pressures exceeding 5.1 atm and condenses to form a solid dry ice at -78^oC.

Effects on Humans

The main hazard associated with CO2 is asphyxiation. Inhaled at high levels, the gas dissolves slightly in saliva to form a weak carbonic acid, H2CO3, which may irritate the nose and throat.

Applications

CO2 is a product of complete combustion. It is not flammable and can be used to smother a fire since it is 1.5 times denser than air; liquid CO2 is used in extinguishers for electrical fires. Chemical heating converts liquid CO2 into gas almost immediately, and it is used as a safety explosive in the coal-mining industry. In the oil industry, CO2 is dissolved into crude oil to decrease viscosity.  CO2 is increasingly used as a solvent in the pharmaceutical and chemical-processing industries.

Cylinders of compressed CO2 gas are used as a rapid inflation device for rafts, tires, airbags, life vests and other applications. CO2 is a stable propellant for paintball markers and airguns. Liquid and solid CO2 are common coolants in the food, beverage and transportation industries. Gaseous CO2 is used to carbonate water, soft drinks and, occasionally, beer and wine. CO2 is a levening agent in the baking industry.

CO2 plays a key role in plant photosynthesis and animal respiration. Greenhouses increase CO2 levels to foster plant growth and eliminate pests.

CO₂ and Climate Change

Changes in isotropic composition prove that increased CO2 levels are caused by anthropogenic activity, with 75% of increases attributable to the combustion of fossil fuels. The emission rate of CO2 increases 3.3 PgC/year (1990-99). Oceans have the capability to absorb 70-80% of foreseeable increases, but absorption rates are limited by ocean mixing. Stabilization at 450, 650 or 1000 ppm would require anthropogenic emission levels to drop below 1990 levels within about 20, 100 or 200 years respectively.

The spatial distribution of CO2 sources and sinks is poorly understood. Modeling results using latitudinal partitioning indicate that a land-based sink in the Northern hemisphere is required to explain current global distribution, but current information is insufficient to infer geographical distributions. Longitudinal variations are typically below 1 ppm on annual average, with seasonal variations reaching 15 ppm. High CO2 causes warming of the surface-troposphere region and cooling of the stratosphere by CO2 emission to space. The distribution of current measurement stations is uneven, and global observations from space are needed to constrain inversion models.

Detection

CO2 has strong spectral absorption at 15μm or 667 cm^-1. Absorption is partially saturated (opaque), but absorption band wings produce significant forcing associated with increases in concentration.

Computed at Earth standard temperature, pressure and surface composition:

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