MadSci Network: Chemistry
Query:

Re: Is the reason carbon dioxide does not settle-out is due to air currents?

Date: Sun Sep 20 17:19:08 1998
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Chemistry
ID: 906056130.Ch
Message:

Carbon dioxide does not settle out from air because of air currents. A 
strong wind would go at 36 km/hr, which is 10 m/s; a very gentle breeze 
would go at 3.6 km/hr, which is 1 m/s; the rate at which heavier carbon 
dioxide molecules would settle out of air (in theory -- it is not easy to 
observe) is around 1 cm/hr.

Air currents sweep everything with them. Even gentle air currents will 
thoroughly mix the heaviest molecules into the air, and also very much 
heavier things like pollen grains and dust particles. Did you know that all 
of the salt that forms at Lake Eyre in Central Australia, or the Great Salt 
Lake in Utah, was carried there from the oceans on gentle winds? Splashing 
waves produce tiny droplets of sea water (spray) near the surface of the 
sea. These are carried higher and far away by wind currents. In the first 
little while the water evaporates, leaving tiny solid particles. Many of 
these are eventually deposited over land, either dry or in rain. They are 
deposited several weeks later, and thousands of kilometres away from where 
they formed. Stronger air currents can, of course, pick up pieces of 
roofing, or vehicles, or even carry Dorothy off to the land of Oz!

By far the main factor in producing air currents is sunlight. The sunlight 
mostly passes through the lower atmosphere (some of it is screened out by 
the upper atmosphere, around 50 km high and higher, but that is far above 
where the weather happens), and reaches the ground. It warms up the ground 
but not the air. So the air next to the ground gets warmed up by the ground 
while the air up higher stays cooler. That makes the whole set-up unstable, 
and we get convection cells forming, where the hot air can rise in one 
place, and cooler air moves down in another. It is easy to see a similar 
sort of effect when you put a pot of water on a gas ring.

The Earth's rotation also contributes to air currents. It creates coriolis 
forces, that cause the air to move in whirlpools rather than straight. It 
also causes a succession of warmer days and cooler nights, that produce 
surface temperature changes that help to determine the actual form of the 
weather systems.

Finally, even if the air were completely and perfectly still, the carbon 
dioxide would not form a pool on the surface. There is a "dynamic 
equilibrium" set up between gravitation -- the tendency for the denser 
material to go to the bottom -- and diffusion -- the tendency for a 
material not to concentrate in one place, but to spread itself out. The 
atmosphere we have contains roughly 78% nitrogen, 21% oxygen, 0.93% argon, 
and 0.036% carbon dioxide. Its composition does not vary until you get 
above 80 km in height. If the air were perfectly still, its composition 
would be

Ground level: 75% nitrogen, 23% oxygen, 1.3% argon, 0.055% carbon dioxide
10 km high: 79% nitrogen, 20% oxygen, 0.75% argon, 0.026% carbon dioxide
20 km high: 82.5% nitrogen, 17% oxygen, 0.43% argon, 0.012% carbon dioxide

So even in these circumstances, the heavier gases like carbon dioxide would 
have higher concentrations lower down, but could not form a lethal pool.

While gases stay unmixed, you can do tricks with them based on their 
density, like pouring carbon dioxide to smother a flame, or pouring 
hydrogen upside down. But once they get mixed, they do not unmix.



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