Effect of multiple tons of dry ice being dropped bomb-like into lake near a city

I have a bomberplane capable of carying a nuke. A large nuke. Let us say that I don't like the idea of wiping a city from the face of the earth. Instead, I decide to mass-produce solid CO2, and drop a lump (say, 2 cubic meters) of it into a lake near a large city.

If you put a small amount of dry ice into a bowl or bucket of water, you get some very dense, heavy fog that fills the container and overflows, behaving much like a liquid. I would like to know how it (as in the fog) behaves if it is made by multiple cubic meters of solid CO2. Does it fill the basin and overflow, does the fog settle and the CO2 float away, or does it behave like normal fog, causing some mild inconvenience, but leaving no trace after a week?

2 Answers

Dry ice sublimates absorbing $573.02$ $kJ/kg$ and has a density of $1870$ $kg/m^3$.

You have produced $2$ $m^3$, they weight $3740$ $kg$, and absorb $2.1$ $MJ$ while sublimating.

Assuming your lake is at an average temperature of $15$ $C$, the mass of dry ice will be able to lower to $0$ $C$ the temperature of about $60$ $kg$ of water, barely $60$ liters. Since you talk about lake, and not pond, your mass of water will be way bigger.

The net result of your dry ice bombing will be a somehow cooler water and a lot of fog.

About the dry ice, producers advise to transport it in non sealed containers (so remember to open the venting valve of your bomber), and since it sublimates at a rate of 10% every 24 hours, to transport it immediatly before usage (I think this is your case).

Once the dry ice splash on water it will initally break down in smaller parts, rising some waves on the lake surface. Then, since the solid is denser than water, it will sink to the bottom of the lake. During the sinking it will sublimate thanks to the heat transferred from water.

From the surface this will be visible as a big boiling, as the gas will start flowing out of the liquid. Together with the gas, a dense fog will start to cover the surface of the lake.

For a reference, see this video.

The main concern it would not be about the visibility, but about the effect of gas on living beings. CO2 displaces oxygen from air, in this case from the ground level, therefore any animal out of water risk to suffucate due to the presence of CO2 at higher concentrations.

Something similar has happened in the past at lake Nyos:

In 1986, possibly as the result of a landslide, Lake Nyos suddenly emitted a large cloud of CO2, which suffocated 1746 people and 3500 livestock in nearby towns and villages

Provided there is a decent ventilation (which around a lake should be almost granted) the local excess of CO2 will be dispersed within hours, and its level will fall below deadly concentrantion accordingly.

• Pretty sure their focus is on what happens to the CO2, not the water. – Salda007 Apr 7 '17 at 6:53
• Actually, how the fog behaves.(i.e. does is form a 1 cm thick layer covering the whone lake, or does it make the city imposible to navigate) – Mark Gardner Apr 7 '17 at 6:59
• @MarkGardner if it looks like people are not sure what you're asking, it might be a good idea to edit your question and highlight your main point. – Mołot Apr 7 '17 at 7:53
• Sounds like it would definitely make the city impossible to navigate because everyone would be dead. – Erik Apr 7 '17 at 9:54
• Ok, but let's assume Mark Gardner's military staff is much better informed on the capabilities of their bombers than him ;-) What would happens with a real life payload, let's say 55000 kilos of dry ice? – motoDrizzt Apr 7 '17 at 17:38

How much gas will be produced:

Density of dry ice: 1562 kg/m^3

so 2 m^3 of dry ice equals 3124 kg of solid CO2

CO2 gas density: 1.977 kg/m^3

3124/1.977= 1580 m^3 of CO2

That is a cube of around 11.5m on a side which sounds pretty impressive.

Looking up various things related to CO2 poisoning
https://en.wikipedia.org/wiki/Hypercapnia
https://en.wikipedia.org/wiki/Inert_gas_asphyxiation

indicates that the displacement of oxygen and CO2 concentrations greater than 7% are very bad to be around quickly leading to unconsciousness and death. So obviously going to be a bad thing in the immediate vicinity if this gas is released all at once, but it wouldn't be released all at once, it is only going to be released as the cube of dry ice sublimates to gas, which is going to cause atmospheric mixing, quickly dispersing the gas as it is released, reducing any danger from accumulated CO2.

Accidental spills happen somewhat frequently during transport of much larger quantities than you are talking about dropping: http://www.bostonherald.com/news/local_coverage/2016/06/overturned_truck_spills_dry_ice http://www.wmcactionnews5.com/story/33131413/18-wheeler-runs-off-the-road-spills-dry-ice http://articles.mcall.com/2005-07-19/news/3626364_1_tractor-trailer-dry-ice-department-and-trash

No one dies from these spills as the gas is quickly dispersed into the air as it sublimes from the solid dry ice.

To be problematic you are going to either have to drop a lot more dry ice, or you should release it into an enclosed/unventilated space where it will not dissipate as rapidly.

• A cube 11.5 m on a side sounds pretty impressive if it's let off in a building, but not so much when on an open air lake. For maximum damage, assume the CO2 all clings to the ground at max 2 m depth (just enough to suffocate tall people). To reach the deadly a 7% CO2 concentration at the 2 m depth, that 1580 m^3 of pure CO2 can only cover 11286 m^2, or about 1.5 regulation soccer pitches (2 American football fields). -- As the linked real-life accidents prove, you need much more CO2 in a large, open-air setting to be dangerous to an entire city. – R.M. Apr 7 '17 at 16:34