CO2 gas a molecular weight of 44 g/mol while air (mostly O2 & N2) has a MW closer to 29 g/mol. CO2 is ‘heavier’ than air and will tend to have a higher concentration near the ground.
In comparison H2O has a MW of 18 g/mol. As a vapor, water is much ‘lighter’ than air. Which is why clouds are generally high in the atmosphere; and also why a dropping barometric pressure indicates that the atmosphere has increasing concentration of water vapor.
CO2 is also soluble in water. Every time it rains most, if not all, of CO2 present in the region of rainfall is scrubbed from the atmosphere.
Just as there is a water cycle, there is also a CO2 cycle. CO2 concentration can vary greatly depending on where and when it is measured.
Claims of CO2 concentration levels both present and past should be viewed with skepticism, especially when underlying assumptions have not been explicitly stated.
It is my understanding that the lowest 10,000 feet or so of the atmosphere is well mixed by turbulence, there is little species differentiation. It’s like a snow globe, when left alone all the “snow” settles to the bottom, but when you shake it, it looks like a blizzard. Turbulence mixes it up. (I could misunderstand.)
Clouds are not water vapor, they are fog, patches of drizzle, tiny water droplets suspended by atmospheric turbulence. As the droplets fall, they evaporate and disappear. Clouds are recharged by condensation at the 0 C isotherm. As the tiny droplets fall and evaporate in the warmer, lower air, they are replaced by newly condensed ones. Clouds are the patches where condensing water is slowing falling towards earth. When the condensation is rapid enough it manifests as rain.
“CO2 is also soluble in water. Every time it rains most, if not all, of CO2 present in the region of rainfall is scrubbed from the atmosphere.”
Rain water is far more superior than tap/well water in my vegetable gardens.
This now explains the “why?” To me.