Carbon dioxide, CO2

Carbon dioxide has a perfectly linear molecule due to the C=O double bonds.

Carbon dioxide molecule and C=O bond distance, from Wikipedia. The unit pm (picometer) is 10-12 m.

CO2 is an uncolored and non-smelling gas at room temperature which, directly freezes into solid state below -78°C (“dry ice”) at normal pressure. The concentration of CO2 in atmosphere is around 0.039 %, or 390 ppm (parts per million). The gas is active in the large carbon cycle since plants use CO2 for photosynthesis and living beings evaporate CO2 when they breath.


CO2 is regarded as non-toxic at low concentrations. At higher levels it may displace oxygen in a closed space and cause a risk for suffocation. Also it directly influences on human performance and capacity when the levels are >1 % (10 000 ppm). Carbon dioxide is a major greenhouse gas and due to its steadily increase mainly caused by fossile fuel combustion, it is believed to bring on global heating. All greenhouse gases are compared to each other using the unit CO2 equivalents expressed as the Global Warming Potential (GWP), which is the heat rise a certain amount of CO2 will cause during a period of 100 years. CO2 has by convention a GWP of 1. More powerful greenhouse gases are for example methane or dinitrogen oxide, which have a GWP of 25 and 298, respectively. The GWP value for a gas molecule depends on how stable it is, how strongly it absorbs IR radiation and where in the spectrum the absorption peaks are placed.


IR spectrum of CO2

The IR spectrum of carbon dioxide has a strong absorption band consisting of many overlapping peaks. This band, caused by unsymmetrical C=O stretching, is placed at 2300 cm-1 corresponding to a wavelength of 4.3 μm.

CO2 molecules in the atmosphere absorb emitted heat radiation from the earth at this wavelength and in that way decrease cooling especially during clear nights. The mechanism of the increasing amount of carbon dioxide and other more powerful greenhouse gases is that they block IR radiation from exiting from earth back to space.

Theoretical IR spectrum of CO2 calculated with the program Spectracalc found at: