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Sunday, January 10, 2010

A Primer on Infrared Spectroscopy and Global Warming


This post and the posts linked to it through section headers together form a primer on infrared spectroscopy and how it relates to global warming.  The purpose of the primer is not be to convince skeptics that global warming is real, but rather to explain some of the terms and issues being discussed in climate science.  My goal is not to write a super technical explanation of infrared spectroscopy.  That's been done so many times that it is hardly worth doing again.

Rather, my intent is to write something that clearly describes infrared spectroscopy and relates it to global warming that tries to explain some fairly technical concepts in reasonably plain language.  As such there is an inevitable loss of fidelity about some of the fine points of infrared spectroscopy.  Anyone interested in such detail can follow some of the sources that I will provide.  At some point one has to compromise between accessibility and technical accuracy.  I hope that the choices made in this primer are helpful to some people trying to understand this topic.  This post is an outline of the topics addressed in the linked entries.

What is Infrared Radiation (IR)?

Figure source

This first post starts with the basics.  It discusses the electromagnetic spectrum and where infrared radiation fits into it.  It discusses waves and their measures.  It introduces the idea of electric dipole radiation.  It discusses the units of radiation wavelength, frequency. It ends by discussing photons and energy quantization.

Infrared Radiation, Black-bodies, and temperature

This post starts the process of looking at the interaction between infrared radiation and matter and discusses black-bodies and the relationship between temperature and infrared radiation. It discusses the relationship between temperature and wavelength for an ideal black-body.  It discusses the fact that black-bodies both absorb and emit radiation and the relationship between what they absorb and what they emit.  It also discusses the relationship between real bodies and ideal black-bodies.

Figure source

Molecules and Radiation I: Molecular Structure

This post starts the discussion of why gas phase molecules absorb and emit infrared radiation.  It introduces some of the considerations for molecules to interact with radiation and analyzes the degrees of freedom of a molecule.  It introduces the Born-Oppenheimer approximation that allows for the separation of electronic and nuclear degrees of freedom.

Molecules and Radiation II: Molecular Vibration, Rotation, and Translation

This post looks at the nuclear degrees of freedom of a molecule and some of their implications for IR spectroscopy. It also looks at the number of vibrational modes in a molecule and starts to discuss the nature of those modes.

Figure Source

Molecules and Radiation III: Vibration, Dipoles, and Ro-Vibrational Spectra

Figure Source

This post looks at why certain molecules such as HCl and carbon dioxide absorb IR, whereas other molecules such as nitrogen do not. It discusses the quantum harmonic oscillator, zero-point energy, energy quantization, selection rules, and the breakdown of the harmonic approximation.  It discusses HCl and its IR spectrum.

The Infrared Spectra of Molecules of Interest

This post shows how to use and access the National Institute of Standards and Technology (NIST) database of infrared spectra.  It  examines the infrared spectra of CO2, N2O, H2O, O3,  and CH4

This post examines the question of how much radiation is absorbed by gas phase molecules in a laboratory setting, how much radiation is transmitted through a gas cell in such a setting, and what it means to saturate the absorption in this context. It introduces the idea that the atmosphere is very different than a laboratory gas cell.
A Note On Saturation of the Carbon Dioxide 15-micron Band

This post looks more deeply at the single-layer model and the saturation of the 15-micron band of carbon dioxide.

A Two-Layer Model

The previous post discusses the issue of saturation in the 14-micron band of carbon dioxide in a single-layer model.   The post before that discusses Beer's Law, and is a necessary prerequisite to understanding this post.  This post looks at what happens when the source of infrared radiation is relatively close in temperature to the the molecules absorbing the radiation, introduces a two-layer model, the concept of brightness temperature, and starts the discussion of radiative transfer.

A Three-Layer Model

This post expands the notion of a two-layer model to a three-layer model.  It continues the introduction to radiative transfer and discusses temperature and density effects on the transmittance.

Structure of the Atmosphere

From the previous  post it should be apparent that one needs to include an understanding of the structure of the atmosphere to understand radiative transfer through the atmosphere. This post provide a summary of the structure of the atmosphere.  It is intended to be a quick introduction, rather than a detailed treatise.  Some of the sources listed go into more detail for the interested reader.This overview of the structure of the atmosphere is needed to understand radiative transfer.

A Multi-Layer Model of Carbon Dioxide

I have put together a simple multi-layer model of  carbon dioxide in the troposphere.  It is based upon the same principles as the two-layer model and the three-layer model.  It accounts for the temperature and pressure profiles from previous post and it is part of a primer on infrared spectroscopy and global warming.  Just like those other models there are still caveats; this model is intended to be illustrative of concepts and therefore it is conceptually simple.

Radiative Transfer

This post introduces the reader to the concept of radiative transfer and radiative transfer models.


At this  point, I hope, that the reader is in a position to understand some of the basics of global warming with respect to the absorption of infrared radiation by carbon dioxide and other gases. I will be able to refer back to these posts in future posts discussing the topic.


Chuck Schamel said...

WOW! I'm impressed! I've been looking for a primer on the topic of climate change that references the physics of the situation.

I'm a mechanical engineer, so I'm looking for the kind of scientific logic and support that it looks like you'll be providing.

I'll certainly be watching for the details.

Chuck Schamel

Chuck Schamel said...

If I may request, though, a slight change in your outline.

I don't know how many people are in my situation, but I'm familiar with most of the concepts.

I know what blackbody radiation is and how spectral transmittance of CO2 relates to the greenhouse effect. I've even done a some reading on the Beer-Lambert Law, so I have some of the basics of how CO2 concentration in the atmosphere correlates.

What I'm struggling to put together is some of the data associated with these concepts.

I'd REALLY APPRECIATE a post (hopefully sooner rather than at the end of the series) that assembles the data (especially blackbody radiation spectrum and spectral CO2 transmittance) and does the conversions (or at least explains how to do the conversions) so that everything is in consistent units (frequency, wavelength, and wavenumber converted and at the same scale on the graphs).

I guess, under ideal circumstances, this would be combined with some calculations of the percentage of IR energy, in the frequency range appropriate for 300 K that actually gets transmitted back into space for different CO2 concentrations.

Thanks again!!

Rich said...

My current plan is to step through the IR spectroscopy, and then take a step back and look at the big picture. I think that most readers probably do not have your familiarity with these topics.

It sounds like you are interested in some of the details of radiative transfer calculations, and I do plan to provide some of that. Note that the calculations really require a computer to do, but it is possible to understand what the programs are doing.

A post on unit conversions is a good idea, although I try to discuss units in the places that they are introduced.