Sunday, February 7, 2010

CO2 Spectrum

After three weeks of searching, we found a carbon dioxide (CO2) spectrum on the web here at the NIST site. The spectrum was measured in 1964, and the original graph was digitally scanned. You will find the raw numbers here. The data gives transmission as a fraction for infrared light betwen 2.63 μm (3797.207 1/cm) and 21.79 μm (458.879 1/cm). The chemists measured transmission through a 10-cm chamber containing one third CO2 and two-thirds N2 (nitrogen) at 0.789 bar pressure (78.9% of sea-level atmospheric pressure).

We converted the transmission values into absorption length for pure CO2 at standard temperature and pressure (20°C and 101 kPa). The absorption length (or penetration depth) is the value L in the Beer-Lambert Law of transmission through an absorbing medium. We have A = 1 − ex/L, where A is the fraction absorbed and x is depth. You will find our plot of the spectrum in our Climate.xls spreadsheet, which is in

The flat-topped parts of the spectrum are where the original measurements are unable to detect any absorption by CO2 along the 10-cm path, and we have capped the spectrum at around 33 m, which corresponds to transmission of 0.999 in the original data.

We hope to use this spectrum to inform us of CO2's role in the greenhouse effect. We can use the atmospheric concentration of CO2 to determine the absorption length of atmospheric CO2. Before we do that, however, we see that pure CO2 has absorption length greater than 1 m between 5 μm and 13 μm. In our previous post we saw how the Earth's atmosphere is transparent between 8 μm and 13 μm. At 13 μm, the CO2 absorption length plunges by two orders of magnitude until it reaches a low of 10 cm at 14.92 μm. We are still looking for a good water-vapor spectrum, which will allow us to compare the role of CO2 to that of water-vapor in atmospheric absorption. But it looks likely that CO2 is responsible for atmospheric absorption in the 13-μm to 16-μm band.

UPDATE: You will find the text data for the above plot in the CO2 sheet of Climate.xls in For water-vapor spectrum see Continuum Absorption Length. For the spectrum of diluted CO2 in air, see CO2 Continuum.


  1. Could you provide the data in .txt format that would make things so easy!

  2. Download, open Climate.xls and find the CO2 sheet. There you will see two columns of numbers wavelength and absorption length. Yourss, Kevan

  3. Kevan, would this be any use?


  4. Hi Richard111. That's a nice graph, and certainly looks like the emission spectra of CO2 and water vapor at 1500 K. But I don't know the density of the layer (in kg/m2) nor the pressure (which plays a part in the water vapor spectrum). We get a better idea of the spectrum of both gases in our later post Earth's Atmosphere. There we present a graph showing the absorption spectrum for various 3-km thick layers of the Earth's atmosphere. Note that we present the absorption spectrum because it is closer to being a property of the gases rather than a property of their temperature. The emission spectrum is the product of the absorption spectrum and black-body emission spectrum (as we have discussed previously).