Wednesday, March 14, 2018

Dear Judge Alsup: The Spectroscopic basis

In a suit brought by cities in California against Exxon, Judge Alsup has asked of the parties a set of questions which some parties on the INTERNET are busy crowd sourcing the answers to.  Now Eli has never been one to avoid a pile on, so the Bunny thought he might essay an answer to two of the questions
2.  What is the molecular difference by which CO2 absorbs infrared radiation but oxygen and nitrogen do not? 
3.  What is the mechanism by which infrared radiation trapped by CO2 in the atmosphere is turned into heat and finds its way back to sea level?
Let Rabett Run start with question 2. Many of the answers start and end with what was learned in Modern Physics or Physical Chemistry.  Real Climate has settled on
Greenhouse gases are those that are able to absorb and emit radiation in the infrared, but this is highly dependent on the gases molecular structure. Diatomic molecules (like N2 or O2) have stretching modes (with the distance between the two molecules expanding and contracting), but these require a lot of energy (so they absorb only at higher energies. Vibrational modes in molecules with three or more atoms (H2O, CO2, O3, N2O, CH4, CFCs, HFCs…) include bending motions that are easier to excite and so will absorb and emit lower energy photons which coincide with the infrared radiation that the Earth emits. Thus it is these molecules that intercept the radiation that the Earth emits, delaying its escape to space.
This is approximately true, but not quite the whole story and much can be learned by going a bit deeper.  It is not that N2 or O2 cannot absorb or emit IR, but their absorption and emission is many orders of magnitude weaker than H2O, CO2 and other greenhouse gases found in the atmosphere.   How many orders of magnitude?  Well about ten.

A good place to start is the HITRAN data base maintained by the Harvard Smithsonian Center for Astrophysics.  HITRAN stands for High Resolution Transmission.  The database, just like the JANAF tables, is a fruit of the cold war started when the US Air Force was interested in learning more about the propagation of light in the atmosphere for such things as aiming missiles and such.  It is essentially a list of lines in the transmission spectra of various molecules under different conditions of  temperature, and pressure.  Using the database one can generate spectra of self-same molecules which are eerily accurate.  GATS among others provides a front end to calculate spectra using HITRAN, so let us start to explore.

The first question is does N2 or O2 absorb IR light. We know the vibrational frequency of these molecules, so we can look at what the database tells us how much light nitrogen would absorb in the atmosphere at a pressure of 1 mbar (1000th of atmospheric pressure. Be patient the reason for this choice will become clear in a few minutes), a temperature of 296 K and a path length of 1000 km.  Yes Eli knows that such a gas cell is not currently available, but with HITRAN we can accurately model this.

The alternating intensities of the lines are due to the symmetry of the nitrogen molecule but that is another story with which we need not concern ourselves at this time.  We can do the same for O2 

Turns out that the triplets seen in this spectrum are the source of the signal that the Microwave Sounding Units that measure tropospheric temperatures monitor.

But now we can do the same for CO2

The difference in path length for absorbing about the same amount of light by CO2 is 0.1 cm, or, if you wish 10-6 km.  So the difference in the absorption would be a factor of 10-9.  

But you say, the mixing ratio of CO2 in the atmosphere is 410 parts per million or 0.00041, and the concentration of N2 is 0.70 thus the number  of N2 molecules per CO2 molecule is just 1.75 x 104 while the N2 absorption is 10-9th of the COabsorption.  Put that together and the amount of IR absorbed by N2 is roughly 0.00002 of that absorbed by CO2.

Ms. Rabett is calling, so let Eli provide a bit of a teaser for Part II.  Here is the absorption of 400 ppm CO2 at atmospheric pressure across a 3 m cell.


Hank Roberts said...

Xref, including a puzzled query for Eli:


"...and a path length of 1000 km. Yes Eli knows that such a gas cell is not currently available"

Bunnydom should petition messrs Musk & Brin to launch a brace of RabbettSats orbiting ~1000 km apart, each bearing a pointer-tracker, a tunable laser, a spectrophotometric receiver , and the software needed to take advantage of the caustic refraction of the Earth's atmosphere to focus on each the beam of the other.

Come to thnk of it they should have used similar gear as helmet filler for the mannequin aboard the Tesla now entering solar orbit with enough batteries to run an IR laser til doomsday.

jgnfld said...

Oh dear! Is some silly bunny proposing that the greenhouse effect is real again???!!!

Bernard J. said...
This comment has been removed by the author.
jgnfld said...

I think you need to increase the gain on your sarcasm detector, J. Remember the plate thread?

Hank Roberts said...

Er, this one's making my head hurt. Eli?

EliRabett said...

No bunny uses thermopiles anymore, mostly things like HgCdTe etc. CO2 monitors use NDIR

Nobunny uses spectrometers to measure IR spectra anymore, we use Fourier Transform IR systems

Other than that. . . (Ms. Rabett is ministering to Eli's broken head)

Classic Rock Video DJ said...

Eli saves the world with this comment.

The good judge will never be the same.

Well, if he ever read this comment, that is.

Eli is a weally weally important wabbit.

The Sychophants and the Deniers would have nothing Useful to do with their self-important lives if Eli were not here.

All Hail Eli the Wabbit

Yours Kindly,
'And then there were none.'

Fernando Leanme said...

Interesting. The judge thinks he will dig into physics well enough to decide whether Exxon should have known that Co2 causes a slight forcing? What is he going to do next? Ask about the carbon cycle, the way climate models are put together, and how they pulled RCP8.5 out of their hats?

Classic Rock Video DJ said...

Wabbit, my comment needs explanation wif some wabbit level questions.

Why are wou welping the dewendents wawyers?

I whought wou widin't wike them.

And why would wanyone be wefewencing weally unweal climate wullshit?

Are wou weally weally wupid or womptin'?


Barton Paul Levenson said...

Henosis, you really shouldn't post when you're wasted.


There's plenty of room in the calendar for a Fudd season between geese & wabbits., albeit some of the denialati have long since moved past Elmer, and into the realm of Yosemite Sam.

Classic Rock Video DJ said...

This matters to me about this court case and everything said about it here and who allows it to be said.

Integrity is the qualification of being honest and having strong moral principles, or moral uprightness. It is generally a personal choice to hold oneself to consistent moral and ethical standards.

In ethics, integrity is regarded as the honesty and truthfulness or accuracy of one’s actions.

Integrity can stand in opposition to hypocrisy, in that judging with the standards of integrity involves regarding internal consistency as a virtue, and suggests that parties holding within themselves apparently conflicting values should account for the discrepancy or alter their beliefs. [see Cognitive Dissonance]

The word integrity evolved from the Latin adjective integer, meaning whole or complete.

In this context, integrity is the inner sense of “wholeness” deriving from qualities such as honesty and consistency of character.

As such, one may judge that others “have integrity” to the extent that they act according to the values, beliefs and principles they claim to hold.

Really Simple Stuff


Bernard J. said...

John Garland, apologies - I confused you with the person who baited many here a while back on the subject. Mea culpa.

CapitalistImperialistPig said...


Unlike the Real Climate paragraph, your much longer discussion says nothing about the why the N2 absorption is so much weaker. Your explanation might be fine for a spectroscopists, but offers nothing for the Judge or the educated public. Am I right in guessing that only a highly excited vibrational state of N2 can do the absorption, or is there some other explanation.

EliRabett said...

Part 2 is on the way

davidp said...

Equally, your long explanation doesn't say why N2 *does* absorb infra red, albeit weakly, when the short explanation says it doesn't.

EliRabett said...

Part II

The Jack of Spades just spit cider in your ear.