# Tabulated absorption spectra of greenhouse gases?

Are there any free sources of tabulated absorption spectra of greenhouse gases from UV/Vis to far infrared (say 70 micrometers)?

This link shows the net absorption over the whole atmosphere (at least as far as I understand it), but I would be interested in the absorption by an infinitesimally small volume/distance. It would suffice to have the spectra for just one temperature, like $$\pu{273K}$$ or so.

On the NIST site I have found some information, for example IR spectrum of $$\ce{H2O}$$ vapor, but apart from the limited spectral window the spectrum is pretty useless for making a radiation simulation because concentration info was missing (as said in the disclaimer).

Is there still hope of getting accurate spectra suitable for understanding molecule specific greenhouse effects?

• – Poutnik May 27 at 4:48

Remote sensing of the simple (few atoms) greenhouse gases works by comparing the measurement spectra with calculated spectra. These calculations are very precise and probably the reason why you do not find so many measured reference spectra.

The reason for calculation is that the atmosphere has pressure, concentration and temperature gradients which even depend on path angle(s). So sensing by comparison with lab-measured spectra requires many spectra under different conditions and sophisticated interpolation schemes (Which is nevertheless done for the more complex greenhouse gases or the HFCs)

One important database for greenhouse gases is HITRAN. Since the simulation code is not so trivial to implement I would suggest you to use a website which does the calculation for you. e.g.:

There you enter your gas parameters (pressure, temperature, concentration, path-length) and so on, and it will calculate the spectrum in (almost) any wavelength range for you. Of course the accuracy may vary with wavelength range, there you would need to check the accompanying literature. Nevertheless, the strong absorption bands of the greenhouse gases are well researched and the data is of high accuracy. E.g. for the 4.3 µm absorption band of $$CO_2$$ we probably have the best researched spectral data of all gases, with accuracy of probably better than 1% if not 0.1 %.

For the more complex molecules, where a line-by-line simulation is not available, you find high-quality measured spectra on the HITRAN website (-> "Absorption cross sections")

• So when you say "simulation code is not so trivial" you probably don't mean the part of calculating vibration spectra in GAMESS for example, do you? I don't do spectroscopy on a professional basis, so bear with me. But I remember from my physics studies back then, that there are several line broadening effects (pressure, doppler, impact), some more important than others. Is this what you mean is difficult, calculating line width from single molecule spectra? – oliver May 27 at 10:03
• @oliver Yes there are multiple broadening effects. The coefficients for this are also tabulated in the database, but putting everything correctly together with the correct temperature corrections requires some physical understanding. It is not impossible of course as there is documentation on the HITRAN website, but it is incomplete with gaps needed to be filled in on your own. If you want to understand how simulation is really done, you need to download some source code (e.g. HAPI found at the HITRAN website) that performs this simulation and look at it. – Andreas H. May 27 at 10:41
• So I would say the difficulty is really putting everything correctly together. – Andreas H. May 27 at 10:42
• Looks/sounds promising and about as close to my goal as possible. So if I understood you correctly, I could just fill in the query masks on the HITRAN website, get the ready absorption spectra (for example for H2O and CO2) and then solving the radiation transport equation (either by writing my own code for that, or finding an open source code that does it) to understand total absorption by the atmosphere (given the actual/current concentrations of H2O and CO2), right? – oliver May 27 at 10:51
• Cool! Thanks for the link. Though I often prefer to write my own code because it helps with understanding the physics, it is always a question of spare time of course. ;-) – oliver May 27 at 10:58

Contributing to the first half of the answer: The MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest (public access here) allows you to search by chemical name, sum formula, CAS registry number, InChi key or author.

The output is a plot, annotated with the literature reference, e.g.

It does not cover IR data to cover the second half of your question. However, why not asking them if they in turn would know about a complementary database and letting the users of ChemSE know about their answer?

(Otherwise, if affilated with a research institution with a department of chemistry, knowing a chemical identifier like the CAS RN could help you to retrieve spectral data in Elsevier's Reaxys [rooted in Beilstein and Gmelin database], too.)