Lifetimes of various gases in the atmosphere don’t have much to do with their segregation in various places. Yesterday, I could see Albuquerque’s brown cloud. It’s brown because of nitrogen oxides, which are gases, and particulates, which are solids. The nitrogen oxides are visible and more concentrated in that brown cloud because they haven’t mixed into the rest of the atmosphere or been rained out. Mixing doesn’t have much to do with lifetime, but being rained out does. Gases may be soluble in water and get rained out onto the ground, where they may react with vegetation and soil. That’s what happens with nitrogen oxides, and they have short lifetimes. Methane doesn’t have a long lifetime because it reacts with gases in the atmosphere: oxidation, just like burning, but slower. Carbon dioxide is water-soluble, so it may get rained out, but it’s not as reactive as the nitrogen oxides, so it may just go back into the atmosphere. So it has a long lifetime in the atmosphere.
The “different layers in the atmosphere” – that metaphor again. Gases mix rapidly and completely. That’s the primary fact. But we have the “ozone layer.” We have statements like “Most of the atmospheric ozone is contained in the ozone layer.” Which is true. All this gives us a picture of a clearly-defined shell of mostly ozone around the earth. But
Ozone concentrations are greatest between about 20 and 40 km, where they range from about 2 to 8 parts per million. If all of the ozone were compressed to the pressure of the air at sea level, it would be only a few millimeters thick.Emphasis mine. And further, the article goes on to say that the “boundaries” of the ozone layer fluctuate with the seasons and from one place to another. Those “boundaries” are defined as ozone concentrations. Here's some more about the ozone layer. There isn't nearly as much of this kind of explanation on the Web as I'd like to see.
If you boil a bit less than a gallon of water for pasta and put about a tablespoon of salt in it, you have 500 parts of salt per million. That tablespoon of salt would be few parts per million in a smallish swimming pool. So you’re talking about really small differences in composition between those “different layers.” The rest of the gases in the ozone layer are the usual earthly suspects, nitrogen, oxygen, and argon.
The differences resulting from the masses of the gases (could we make a song out of that?) are much smaller than the other processes in the atmosphere. Albuquerque’s brown cloud is gone today because a new air mass has moved in. It’s pushed the brown cloud out, but it’s also mixed the particulates, nitrogen oxides, and whatnot into the rest of the atmosphere, giving us the adage that
Dilution is the solution to pollution.Those air masses keep moving around, so there’s not much chance for molecules to settle out. In any case, carbon dioxide (44 mass units) is heavier than oxygen (32 mass units), so it should be less prevalent at altitude if such settling were possible. Blood concentrations aren’t simply related to atmospheric concentrations.
Phila’s comments require a separate post. Perhaps today, perhaps the weekend.