Saturday, April 19, 2008

Basic CO2 Physics


Next, lets talk about a scientific process called Atomic Absorption Spectrometry. It is a method by which we can measure precisely which wavelengths of radiation a particular gas is capable of absorbing.

In our highly simplified drawing above, a radiation source is beamed through a glass container containing a gas sample. As the radiation passes through, a portion of it is absorbed at particular narrow bandwidths (often more than one ) so the end result are some "missing" sections of the whole spectrum coming from the source, which show up as dark lines. They're missing because they were absorbed by the sample in the chamber. They are called absorption lines, or absorption spectra, and when analyzed by a knowledgeable person, can tell one what the gas or gas mixture is in the sample chamber based on a catalog of known spectra. It's a wonderful tool for analyzing unknown gas samples.

Let's look at a real result, below - the absorption spectrum for pure carbon dioxide plus an amount of water vapor equal to that in our current atmosphere as the sample and infrared radiation from a black body spectrum as the source. This is part of the so-called "greenhouse effect"

As we can see above, carbon dioxide absorbs infrared radiation (IR) in only three narrow bands of frequencies, which correspond to wavelengths of 2.7, 4.3 and 15 micrometers (┬Ám), respectively. The percentage absorption of all three lines combined can be very generously estimated at about 8% of the whole IR spectrum, which means that 92% of the "heat" passes right through without being absorbed by CO2. In reality, the two smaller peaks don't account for much, since they lie in an energy range that is much smaller than the where the 15 micron peak sits - so 4% or 5% might be closer to reality. If the entire atmosphere were composed of nothing but CO2, i.e., was pure CO2 and nothing else, it would still only be able to absorb no more than 8% of the heat radiating from the earth.

Taken in entirety from James A. Peden.