Reply To: Do CO2 molecules stop rising at a certain point in our atmosphere?

To answer your first part of the question, the CO₂ barely rises in our atmosphere. The second part of the question, not likely.

To a certain extent in our atmosphere called the turbopause layer, all gases exist as a homogenous mix of gases due to factors like turbulence from winds, thermal convection, and molecular diffusion. This region (homosphere) extends up to 100kms in our atmosphere to the turbopause. And within this region, the turbulence of the air dominates any stratification of gases.

Take helium for example. It exists in our atmosphere in trace amounts–as a homogenous mix along with oxygen, nitrogen, argon, carbon dioxide, and other molecules in trace amounts in the homosphere. Whenever you introduce helium at the surface of the Earth (not as helium balloons) the molecules would tend to rise, but collisions with the other molecules, and stronger turbulent currents, convection, all work stronger to keep helium close to the Earth.

But beyond the turbopause, the gases actually stratify based on their density. Hydrogen and helium, being the lightest would rise up. And for a gas molecule to leave the Earth’s atmosphere, it should require high kinetic energy and a longer mean free path (a huge space with no collisions happening) to overcome the escape velocity of the Earth. With the energy from the thermosphere and a large mean free path in the upper regions of the atmosphere, helium sometimes escapes Earth’s atmosphere.

Now back to carbon dioxide (and methane – even more potent than CO₂). They are heavy. Compared to helium, a carbon dioxide molecule is approximately 14 times heavier than helium and methane 4 times.

As you see, methane would likely rise up than carbon dioxide. And when methane is oxidized in the troposphere, carbon dioxide is released along with water vapour. And for the CO₂ molecule to blast off into space, it would require 14 times the energy of helium. And if the CO₂ gains kinetic energy (14 times the energy required for helium), it would blast off into space. Or the solar storms would strip them away.