# What sort of gas is thick and does not disperse easily in an urban environment?

I am interested in writing a survival-themed interactive fiction story (a sort of "what would you do" scenario game). Obviously, the zombie apocalypse might come to mind as the setting, but this has been done to death (pun intended), and I would like to avoid those tropes.

My alternative scenario is this: a dense, persistent gas/smog descends on a city. Visibility is extremely limited, and the gas does not blow away with the wind; it sits in the streets and open buildings of the city, and remains for the most part at the same density/thickness (i.e. so visibility remains constant, within some degree).

The important part is that the gas is problematic enough that the government is unable to reclaim the city; so it becomes a sort of Pripyat, or otherwise a 'besieged' city, where no one can leave easily. Hence limited visibility is an important obstacle.

The other effect of the gas is that when it was first deployed, it was lethal. A week later or so it is no longer immediately lethal, but is still potent; it is corrosive, and does hideous damage to exposed skin; it does damage to mucous membranes, upon contact; i.e. it will kill you if you are not protected. However, people need to be able to survive with amateur protection, such as scarves wrapped around their face/mouth, since gas masks won't be everywhere (though a month or so of this sort of exposure will kill you off).

I'm not sure whether the gas will suddenly be released onto the city, implying residents will need to survive, and will not be able to easily leave the city (nor will help be easily reached), or whether it is a known issue and the city was evacuated months before, and instead it becomes a hub of illegal activity, S.T.A.L.K.E.R like.

Basically; what sort of realistic 'gas' substance would I want to use here? Most importantly: What sort of gas/smog would persist in an urban environment and not get blown away/diffused by the wind? Secondly: what sort of gas of this type would have the health effects as listed above (mustard gas would seem obvious, but that doesn't meet criterion 1).

• Did you research the London Fog of the 1950's, essentially coal smoke held down by a temperature inversion and mixed with a real fog? The NOx smogs of southern California in the 1970's come close, and the industrial smogs in China are very close to what you describe. Feb 18 '17 at 13:49
• @Thucydides: But smogs aren't persistent, they are continually regenerated. E.g. southern California smog would blow away in a good wind, but would be back as soon as the wind died down. And of course if you put emission controls on cars & such, you reduce the problem. Feb 18 '17 at 18:43
• So, Scarecrow's gas from "Batman: Arkham Knight", without the "go crazy" feature? Nov 16 '18 at 12:49

The not being blown away by wind thing is unrealistic. Wind can get really powerful and horrific when it decides to. Remember tornadoes and hurricanes? And you think such winds would not be able to blow away some petty smog?

Tungsten Hexafluoride (WF$_6$) is the gas which comes to mind. It is extremely heavy, very corrosive and extremely deadly if a huge cloud tends to wrap a city. Upon reaction with water/fog, it forms deadly Hydrogen Fluoride (HF) which is worse than hydrochloric acid. The problem with tungsten hexafluoride is that it is not manufactured on large scale and it is not formed naturally.

You could simply use chlorine gas too. It is much heavier than air (70 grams per mole versus ~30 grams per mole). Besides being corrosive, it is also brightly colored and a city-wide cloud of chlorine would easily impair the vision of all inhabitants. However it is very lethal and a 2 minutes exposure to dense chlorine fumes is enough to seriously damage the eyes, create deadly amounts of hydrochloric acid in the lungs and burn the skin.

You might want to consider an airborne organism rather than a non-living gas. Billions of tiny creatures filling the city streets like an airborne algae bloom. Cohesion among the organisms and extremely fast reproduction could explain why they can't just be cleaned off or blown away. Mutation could explain why symptoms of exposure to it changed over time.

Throw in a little bioluminescense and your entire city could be trapped in an eternal twilight. Great atmosphere for terror (pun intended).

I had a setting like this in one of my short stories, and the Vile (which is what residents called the living smog) was an artificial organism designed to feed off of air pollution. It was released over San Francisco as a test case, but quickly mutated and became a bigger problem than the original smog. City residents had to wear full biological containment suits whenever they went outside and those suits had to be replaced regularly because of the Vile's acidic secretions.

# Make it hard to disperse because of the area's geography

Making the gas hard to disperse by wind could be due to geography of the area rather than the innate properties of the gas. All the gas would have to do is to be heavier than or equally as heavy as air in that case. Similar to question in Deadly, Heavier than Air Gas, but you would have to make sure that you pick one of the opaque gases. Some good candidates I can see (but haven't checked throughly, sorry) are nitrogen dioxide, anhydrous ammonia or bromine. They may or may not meet your needs, as they may be too aggressive/mild or too slow acting for your needs.

One limitation of heavier-than-air gases I picked is that it would be pretty hard to see them in nature, either no natural occurrence at all or rare ingredients would be needed. Why not have a local refinery leak for example?

# Consider using two gases

Why not have two gases leak one after another to meet your first criterion: first, a really potent gas leaks out. It's instantly lethal but it's either lighter-than-air and floats away, or decomposes/reacts away chemically. Then, the weakened gas tanks of the refinery burst again, this time with the permanent gas. It's not instantly lethal, but still dangerous.

# Example

The city is located right in the middle of a "bowl" area, surrounded by hills/mountains or located within a crater. A terrorist attack bursts one of the gigantic Gas A tanks, which kills a lot of people. Gas A slowly disperses over the period of two days, but before it's gone, manages to corrode away one of the gigantic tanks of Gas B. The one corroded the worst explodes, triggering a chain reaction which bursts the remaining ones. The city is forever shrouded in the heavier-than-air gas cover which doesn't dissipate.

• Your post does not answer the question. OP was not asking the characteristics of the gas(es) he should use, but which gas fits these characteristics and be useful for his storyline. Feb 18 '17 at 6:16