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I believe that carbon dioxide does not absorb light in the visible region, but is a spectrum available somewhere online that shows this as a fact? As in a straight horizontal line at 100% transmission?

I would be interested to see if there are any minimal areas of absorption anywhere at all. This spectrum must have been run and presumably discarded as worthless. It is not easy to find online amidst so much information concerning carbon dioxide's main absorption bands elsewhere.

Similar information for methane and water vapour would also be useful.

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Here are the absorption spectra of some atmospheric gases for wavelengths from $0.1\ \mu$m to $100.0\ \mu$m.

enter image description here
(image from "Cold Facts on Global Warming" by T.J.Nelson)

Especially you can see, the absorption of $CO_2$ is essentially zero in the wavelength range from $0.1\ \mu$m to $1.5\ \mu$m.

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  • $\begingroup$ Thanks for that. Slight variation on the question, what would a very deep layer of liquid CO2 look like? Water is faintly blue I believe due to limited adsorption at the far red end of the visible / near IR as we can see above, presumably CO2 would be totally clear and uncoloured regardless of depth? $\endgroup$
    – Slarty
    Commented Jul 7, 2021 at 14:46
  • $\begingroup$ What pressure and path length was used for the posted graphs? $\endgroup$ Commented Jul 7, 2021 at 18:13
  • $\begingroup$ @DavidWhite I'm not sure. But from reading the source article it seems to be the entire path through the whole atmosphere. $\endgroup$ Commented Jul 7, 2021 at 18:38
  • $\begingroup$ @Slarty, I was asking because a path length very much shorter than the "whole atmosphere" path length results in 100% absorbance. $\endgroup$ Commented Jul 7, 2021 at 21:48
  • $\begingroup$ @David White why? That would seem to be the heart of the matter. $\endgroup$
    – Slarty
    Commented Jul 7, 2021 at 22:59
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There is basically no important absorption from atmospheric CO$_2$ in the wavelength range you specify.

This picture, from Patat et al. (2011) identifies the various absorption components looking through the Earth's atmosphere in almost exactly the wavelength range you are interested in. The scale on the left applies to the red (observed) line and the Rayleigh scattering component. The right hand scale applies to the contributions from aerosols and ozone.

There are various molecular lines and bands identified, but these are due to water vapour and molecular oxygen.

The CO$_2$ in the atmosphere will of course make a tiny contribution to the Rayleigh scattering continuum that is roughly proportional to its concentration in the atmosphere compared with oxygen and nitrogen.

enter image description here

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