I thought some data provided in Grant Petty’s excellent book would be valuable as it helps explain some important points about the role of CO2 in the atmosphere.
Just recently I was kindly given access to the HITRAN database by Dr. Laurence S. Rothman but I’ve spent a bit too much time in the last few days trying to get MATLAB to read it (still a novice at MATLAB). I do plan to provide a followup article with some calculations of my own on the HITRAN data. Who knows how long that will take.
HITRAN is an acronym for high-resolution transmission molecular absorption database. It is a treasure trove of spectroscopic data.
First, here’s a couple of extracts from A First Course in Atmospheric Radiation by Grant Petty. (I briefly reviewed this book in Find Stuff Out and Book Reviews):
What is “zenith transmittance”?
It is the proportion of radiation that is transmitted (not absorbed or scattered) through the atmosphere from the surface to the top of atmosphere. It doesn’t include any re-radiation by the atmosphere – an important element of atmospheric radiation (see, for example, Part Three).
What is “zenith transmittance of CO2”?
The above effect when only CO2 is taken into account.
This graph is therefore calculated not measured. The only way to measure it would be to take out all the water vapor while the satellite was taking the measurement, which is tricky. It would also be important to stop the atmosphere re-radiating which is even more challenging.
Of course, the calculations are based on the measured parameters of CO2.
Now, the value of this graph is in demonstrating that for much of the CO2 absorption band (e.g. around 600 cm-1 and 750 cm-1) the transmission through the entire atmosphere is not zero and not 1. Therefore, more CO2 will have an effect on the transmittance of the atmosphere.
Here’s the value through a 1m path around the better known peak absorption of CO2 at wavenumbers around 667cm-1 = wavelengths around 15μm:
Fascinating stuff. As you can see 95% of radiation at 15μm is absorbed in just 1 meter of atmosphere at the surface of the earth (1000 mb).
Amazing, considering that CO2 is only 370ppm or thereabouts.
You can see the effect of what is called “pressure broadening” of the individual lines with the 1000mb (surface pressure) vs the 100mb (about 16km altitude) value.
This is something we will return to in a later article.
Many people write about the strong absorption of CO2 at 667 cm-1 / 15μm without commenting on the absorption at 600 cm-1 or 750 cm-1.
The less strong absorption around the “wings” of the band is the reason for this graph below of the wavelength dependent impact of a doubling of CO2 (from pre-industrial levels) on the “radiative forcing”:
From Radiative forcing by well-mixed greenhouse gases: Estimates from climate models in the IPCC AR4, W.D. Collins et al, Journal of Geophysical Research (2006). Note that the vertical axis units are incorrect.
Early comments on CO2 in the HITRAN database
There are almost 315,000 individual absorption lines for CO2 recorded in the database. The database has 2.7M absorption lines in total for 39 molecules.
Between 665 – 669 cm-1 there are over 2,000 lines.
Between 647 – 687 cm-1 there are over 16,000 lines.
Between 500 – 800 cm-1 (12.5 μm – 20μm) there are almost 63,000 lines.
Over 248,000 lines for CO2 are above 800 cm-1 , i.e., between 0-12.5 μm.
The main reference paper for HITRAN 2008 (see below) says:
The present atmospheric version of CDSD (Carbon Dioxide Spectroscopic Databank) consists of 419,610 lines .. covering a wavenumber range of 5–12784 cm-1.
– so I’m not sure whether not all of them made it into the HITRAN database, or whether I have missed something important.
Now that I’ve read the database into some Matlab arrays I will have a tinker around and see if I can come up with the same kind of calculations as people like Grant Petty.
If I can’t, I will immediately announce that climate science has made a huge mistake, write up my results and become the next internet-celebrated debunker of atmospheric physics.
Or, I’ll try and figure out why I got a different result from all the people who know so much more than me..
Other articles in the series:
Part One – a bit of a re-introduction to the subject.
Part Two – introducing a simple model, with molecules pH2O and pCO2 to demonstrate some basic effects in the atmosphere. This part – absorption only.
Part Three – the simple model extended to emission and absorption, showing what a difference an emitting atmosphere makes. Also very easy to see that the “IPCC logarithmic graph” is not at odds with the Beer-Lambert law.
Part Four – the effect of changing lapse rates (atmospheric temperature profile) and of overlapping the pH2O and pCO2 bands. Why surface radiation is not a mirror image of top of atmosphere radiation.
Part Five – a bit of a wrap up so far as well as an explanation of how the stratospheric temperature profile can affect “saturation”
Part Six – The Equations – the equations of radiative transfer including the plane parallel assumption and it’s nothing to do with blackbodies
Part Seven – changing the shape of the pCO2 band to see how it affects “saturation” – the wings of the band pick up the slack, in a manner of speaking
Part Eight – interesting actual absorption values of CO2 in the atmosphere from Grant Petty’s book
Part Nine – calculations of CO2 transmittance vs wavelength in the atmosphere using the 300,000 absorption lines from the HITRAN database
Part Ten – spectral measurements of radiation from the surface looking up, and from 20km up looking down, in a variety of locations, along with explanations of the characteristics
Part Eleven – Heating Rates – the heating and cooling effect of different “greenhouse” gases at different heights in the atmosphere
Part Twelve – The Curve of Growth – how absorptance increases as path length (or mass of molecules in the path) increases, and how much effect is from the “far wings” of the individual CO2 lines compared with the weaker CO2 lines
And Also –
Theory and Experiment – Atmospheric Radiation – real values of total flux and spectra compared with the theory.
The reference describing the HITRAN database:
The HITRAN 2008 molecular spectroscopic database, L.S. Rothman et al, Journal of Quantitative Spectroscopy & Radiative Transfer 110 (2009) 533–572