Mr Brown (I assume that is your name), my name is Abraham. That used to be my nick here but I left and then tried to come back and management forced me to change it. If you go back through my posts in this latest discussion, I think you will find that I've been generally courteous; certainly more courteous than I have treated other people in this forum. I realize not everyone on my side of this argument has made the same effort (aside to RT: hint hint).
You know (as does everyone that has read my statement above) that I never said molecules have no mass and never said they weren't present in the atmosphere. I never said air wasn't under pressure in the atmosphere or that I could put an unlimited amount of air in a given volume.
What I said and what I explained by clear example is that changing the pressure and density of a gas does not affect the ppm ratios of it's constituent gases. As you saw in the Wikipedia text, the actual meaning of ppm in this usage is, effectively "molecules per million molecules" or "moles per million moles" (if you happen to know what moles are). Because you have the same entity in the numerator and denominator, the two cancel and you are left, as Wikipedia notes, with a massless quantity. Massless quantities are common in science and engineering. For example, if I wanted to speak of the error in a car's speedometer that was dependent on the actual velocity, I could end up with the units "mph error per actual mph", thus "mph per mph". Algebraically, the two cancel and you are left with a massless quantity. This is handy because the value of that ratio is independent of what units I use to measure the error. It would have the same value if measured in "kilometers per second per kilometers per second" Carbon dioxide's ppm ratio is simply the proportion of the atmosphere's mixture for which CO2 accounts. That proportion is not changed by altering the pressure or density (or temperature) of the air.
Now, let's say I start with, say, one million molecules of air, 400 of which will be carbon dioxide, and put them in a sealed container. Now I add, for example, 1,000 more molecules of CO2. We now have a total of 1,001,000 molecules in our container, 1,400 of which are carbon dioxide. Pulling out our calculator and doing the math we find that we now have 1,400 * (1,000,000 / 1,001,000) = just slightly in excess of 1,398.6 ppm carbon dioxide in our mixture. This may be where your misunderstanding lies. However, recall that in the Earth's atmosphere, to create a CO2 molecule, we will consume one O2 molecule (the carbon coming from the coal or oil). Thus the molecule count is unchanged by combustion and even this minor affect does not take place.
The point of all this is that adding carbon dioxide to the atmosphere does not simply replace one greenhouse gas with another. It ADDS to the amount of greenhouse gas molecules in the atmosphere which increases the number of such molecules any photon of light will run into wending it's merry way into and finally out of the Earth's atmosphere. Adding CO2 or any other greenhouse gas (ex water vapor or methane) will increase the planet's equilibrium temperature - will cause the planet to warm.
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