From: John Beckett 

"Geo."  wrote in message
news::
> You are talking total concentration of about .0003 molecules, that's less
> than 1 molecule in the room you are in right now.

One molecule of CO2 in a room!? ONE!?

The number of molecules of CO2 in a small room is:
  300 thousand million million million

At sea level, dry air has properties:
  28.97 g/mole (mass in grams of 6.022e23 molecules of air)
  1.2 kg/m^3 (density)

In one cubic meter of dry air at sea level:
  mass = 1.2 kg = 1200 grams
  1200 / 28.97 = 41.4 moles
  41.4 * 6.022e23 = 2.49e25 molecules

That's 2.49 times 10 to the power 25 molecules in one cubic meter.

CO2 is 380 ppm, so number of CO2 molecules per cubic meter of air:
  2.49e25 * 380 / 1e6 = 9.48e21
[perhaps should use 350 ppm by mass, not 380 ppm by volume]

Therefore a small room of 32 cubic meters contains
  9.48e21 * 32 = 303e21 molecules of CO2

I'm laughing too much to be sure of these calculations, but if you consider
a cube of air containing one CO2 molecule, you find:
  The length of each side of the cube is 47 nm (nanometers)!

If you're smart enough to understand the above, you will see why it's quite
plausible that a photon of infrared radiation trying to pass through the
air in a room would hit a CO2 molecule.

John

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