MadSci Network: Chemistry |
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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