Mesosphere carbon dioxide
I have been curious about what happens to carbon dioxide at very high altitudes. Does it freeze?
I stumbled across an interesting tidbit about the mesosphere, the atmospheric layer above the stratosphere:
In the lower atmosphere CO2 acts as a so called greenhouse gas by absorbing infrared radiation radiated by the earth's surface. In the rarefied mesosphere CO2 actually cools the atmosphere by radiating heat into space. (The summer mesopause is getting colder, possibly because of the cooling effect of increased anthropogenic carbon dioxide and methane emissions.)
A temperature minimum of -90°C and lower is reached at ~85 km - the mesopause. Near here is the realm of the 'night shining' or noctilucent clouds.
Beyond the mesopause temperatures rise again because of reduced radiative cooling combined with heating by absorption of short wavelength,<180 nm, UV radiation by O2, O atoms and N2.
I had mistakenly believed that carbon dioxide froze at -40 F, but it actually freezes at -109.3 F or -78.5 C. It only gets that cold near the mesopause, about 47-60 miles up, and the air is super-thin there, 1/100K to 1/1M of sea level. So, in theory, you could get some carbon dioxide crystals up there, but by definition not very many.
It never gets cold enough in the Arctic to freeze carbon dioxide (at least on the surface), but it has gotten below -110 F in Eastern Antarctica (down to -128 F.) The normal temperature chart for Antarctica shows -70 F as the normal coldest there. That is at the surface. I have no idea what the normal lapse rate is, but it seems possible that it could get cold enough up at 2 to 5 miles to freeze carbon dioxide out of the air. But, except for rare occasions, falling towards the surface would reach the sublimation temperature and it would revert to a gas.
In short, very little carbon dioxide would normally freeze in even at a few extreme locations.