Fight my ignorance about carbon dioxide: why doesn't it settle to the ground?

Several years ago, I remember hearing about a big out-bubbling of carbon dioxide from a lake (in Aftrica?). This carbon dioxide, being heavier than air “pooled” in the low lying areas and killed a lot of animals and people.

Which got me thinking – always a dangerous proposition. :slight_smile:

If carbon dioxide is heavier than air (which it is), why doesn’t it all settle to the ground? What mechanism keeps it suspended or mixed in the atmosphere?

My first thought was “wind”, but there are lots of windless days. And why wouldn’t wind encourage the CO2 to settle out, kind of like shaking a bowl of popcorn (which allows the heavier unpopped kernels to fall to the bottom)?

Also, do the ratios of elements in the air significantly change as a function of altitude? E.g., are there significantly fewer “heavier than air” atoms in the upper atmosphere?

Help me fight my ignorance. Why does carbon dioxide stay mixed in the atmosphere?



Sounds like the plot of a hollywood blockbuster to me…

“Sir, global warming is at critical! Greenland is about to collide with New York!”
“Then we’re out of options…”
“Wait…carbon dioxide is heavier than air, right? What about if we shook the atmosphere? It would be just like shaking a bowl of popcorn…”
“Get me the president…”

The atmosphere is a chaotic system. CO[sub]2[/sub], is only a small percentage of the whole, and all of the processes that move air encourage diffusion of that small part into the whole. Also, CO[sub]2[/sub] is preferentially affected near the surface by being dissolved into water, and metabolized by plants. The total aggregate of such actions and the emissions of industrial and volcanic processes are a highly dynamic cycle. Although the mixing is the predominate effect, local imbalances in production, and depletion will give variations in concentration, but these are limited to small percentages of the CO[sub]2[/sub] content, which, as mentioned is a small percentage of the total atmosphere. The case of volcanic emission over short times is specific, and highly transitory. The tragedy you mention was one that occurred due to physical coincidence of vulcanism, seismic action, terrain, and weather. By the time others had entered the area those factors were changed, and no concentration existed.


Fine dust settles out, but nano-sized dust lifts off and rises. Smoke particles never settle. It’s because of bombardment from air molecules.

“Weaponized” bacteria don’t stick together. A visible pile of such bacteria will evaporate at room temperature, or “condense” back into a visible pile when chilled. (This threw some Anthrax researchers for a loop when the pile of bacteria powder in a sealed capsule would disappear if warmed in the hands, then reappear when chilled.)

Look up “suspended aerosols”

Lake Nyos, in Cameroon. The CO[sub]2[/sub] (1.6 million tonnes of it) didn’t just pool…it rolled downhill at 20-50 kph into villages downslope of the lake and killed 1700 people and 3500 livestock by suffocation. Additional injury was caused by sulfur and hydrogen that also was outgassed.

even without wind there is much movement of gases.

heat from the sun and earth causes gases to move. gas molecules bumping into one another keeps the pot stirred.

You might think about the atmosphere’s mixture of gases as similar to a large tank of water that has many salts and other materials dissolved in it. It might even help to think about the introduction of a drop of food coloring, which is made of tiny particles of material mixed with water. All those salts and materials are pretty evenly distributed in the tank and after a short amount of time, so would the food coloring be. In other words, even a complicated liquid will be very much mixed up and dispersed after a time, just because of the molecular motion. In a gas, the action is much more energetic and the molecules that are suspended get whacked around even more. Add the sun, which is the engine of all the atmospheric motion, and you’ll get a big bowl of mixed up stuff, with very little settling to the bottom (except in the library on the tops of books you don’t use).

It’s important to realize how fast air molecules are moving, too. You determine the average velocity of molecules in a gas by, first, convincing yourself the gas you’re interested in resembles an ideal gas, so you get to use the easy math, and, second, computing the root-mean-squared velocity, to get a reasonable picture of how fast the average particle (atom or molecule) is moving. Carbon dioxide at or anywhere near room temperature and pressure is an excellent approximation of an ideal gas, so we can apply the formula to determine the average velocity.

Doing the arithmetic with the standard *nix units program, the average velocity is just north of 327 miles per hour (a bit over 146 meters/second, which is very nearly 527 km/hr) at 68°F (37°C), which is just an arbitrary room temperature I pulled out of my hat.

Now, imagine a huge number of particles going that speed hitting every surface all the time. That is air pressure.

Doing precisely the same calculation for radon (same temperature, different molar mass), which does tend to collect in basements, we get an average velocity of almost 146 mph (just over 65 meters/second, almost 235 km/hr), which is something you could drive at if you spent more money on your car and various moving violation citations. So it’s still going at a reasonable clip, but compared to carbon dioxide it’s just puttering along because it’s so much heavier. Diatomic hydrogen gas, contrariwise, books along at just over 1529 mph (just under 684 meters/second, a bit over 2461 km/hr), which means those molecules are going fast enough to escape our atmosphere. Which they do.

Radon tends to collect in basements because it enters through the basement slab, and often there isn’t a whole lot of air circulation between the basement and the rest of the house. If you put a detector near the basement floor, and another near the basement ceiling, I would expect the same radon level to be measured on both, showing that radon diffuses upwards and tends toward an even vertical distribution in spite of its much larger molecular weight.

Escape velocity from earth is quite a bit higher than 1529 MPH. In fact, it’s more like 25,000 MPH. A hydrogen molecule traveling at the RMS velocity won’t make it out; only the fastest ones, the ones over on the right edge of the velocity histogram, will escape the earth.

Machine Elf: OK, that makes sense.

From the surface of the earth. If it’s already in the upper atmosphere, escape velocity is considerably less. :slight_smile:

There’s also this - all gases vary in density. And in a confined space, denser gases will, indeed, settle to the bottom. There are lots of classic classroom demonstrations of this. But, when the gases are free to move around, or if given enough time, they will all disperse and pretty evenly distribute themselves and mix together. That’s what happened at the lake where there was a huge bubble of CO2. It spread out from that area and smothered the life around it before it dispersed into the atmosphere.

That depends on your definition of “considerably”. If it’s 25,000 MPH from the earth’s surface, then from 62 miles up (the official “edge of space”), it’s 24,800 MPH.