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I am working on a scenario that begins with an impact event. I have multiple questions, but I'll start with the catalyst for this world.

Is there a type of impact event that would not be a global killer, but basically poison the air, forcing humanity to live in shelters for a few years, while allowing vegetation to (at least some degree) survive?

The World: The world I am creating forces humanity into shelters, where they can resurface in a few years in protective gear, leaving behind a, still vegetated, but drought-like surface. I'd like a realistic scenario to allow for this, ideally stemming from an impact event.

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    $\begingroup$ Your title says "humanity would not [survive]" but your questions says they survive in shelters. Which is it? Also, define vegetation. It is pretty easy to imagine an impact that extinguishes land-based life, but not sea-life; or one that exterminates all multi-cellular life but leaves bacteria and algae. $\endgroup$ – kingledion May 10 '17 at 1:07
  • $\begingroup$ Thanks for the comment. I made a few edits to my question above based on your clarifcations. Basically, I would like the air to be poisonous to humanity but still allow for significant surface vegetation. $\endgroup$ – socalemc May 10 '17 at 1:32
  • $\begingroup$ You don't even need an impact, just something like the Permian-Triassic extinction. $\endgroup$ – jamesqf May 10 '17 at 3:43
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    $\begingroup$ Plants are a lot hardier than people. In particular, the seeds of many plants can lie dormant for years, decades, centuries, or, in favourable conditions, even millenia before germinating. $\endgroup$ – Michael Vehrs Jul 7 '17 at 9:49
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Global Lake Nyos event.

enter image description here from http://www.neatorama.com/2007/05/21/the-strangest-disaster-of-the-20th-century/

From Wikipedia

A pocket of magma lies beneath the lake and leaks carbon dioxide (CO2) into the water, changing it into carbonic acid. Nyos is one of only three known exploding lakes to be saturated with carbon dioxide in this way, the others being Lake Monoun, also in Cameroon, and Lake Kivu in Democratic Republic of Congo.

In 1986, possibly as the result of a landslide, Lake Nyos suddenly emitted a large cloud of CO2, which suffocated 1,746 people[2] and 3,500 livestock in nearby towns and villages.[3][4] Though not completely unprecedented, it was the first known large-scale asphyxiation caused by a natural event.

The premise for your event: it turns out the climate change deniers were right. Rising CO2 was not man made but due to a gradual, colossal outgassing of CO2 at one site at the bottom of the ocean. Like Lake Nyos, immense amounts of CO2 accumulate. When the meteor hits that area, the water outgasses and a Nyos-like "cloud of doom" rolls over the earth. Animals suffocate. Plants chortle merrily, replete in delicious CO2 and loads of nitrogenous fertilizer. Not only do they survive, they thrive.

Some humans make use of tunnels and other CO2 free air to survive and ride out the trouble.

I am struggling some to figure out how to make this last years. The Lake Nyos event lasted a couple of hours tops. CO2 can definitely kill you and you could throw in a fair proportion of CO which kills you better. But a continued outgassing from some secret source so as to keep the surface uninhabitable strains my credulity.

You could have the CO2 arrive from space - a giant frozen dry ice meteor. A lot. Most of it would sublimate away on entering the atmosphere, creating the "cloud of doom" - a really big meteor but not a really big impact. The rest would turn to gas from the heat of the impact. You could have enough CO2 arrive that it would take a long time to disperse.

CO2 is also good because it is not cyanide or something immediately deadly. It is more interesting and offers more story opportunities to have the atmosphere be bad but not outright lethal - people who had to venture outside would feel sick and hyperventilate as their CO2 level rose and pH fell. The plants would not just survive - many would thrive. Also, there is plenty written about what other things would happen with high CO2 - icecaps melt, poison ivy takes over etc. All of that stuff would happen in fast forward. Humans would emerge into a green, hot, wet, itchy world.

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    $\begingroup$ Will, thank you for this. Great information. This is very much the type of world I am trying to create. I hadn't even considered going this route but it really opens up the possibilities for this world. $\endgroup$ – socalemc May 10 '17 at 1:36
  • $\begingroup$ Tunnels would be the worst option for this kind of event, since CO2 is heavier than nitrogen and tends to sink down the holes. Normal gas masks such those firefighters use would be the best option, and humans would be living on the surface, albeit in a sort of Mass Effect's quarian custome. Otherwise I think that's the closest thing the OP has going to get. $\endgroup$ – Rekesoft May 10 '17 at 7:14
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    $\begingroup$ Mind that if you add too much CO2 you will turn Earth into a giant greenhouse, steaming all the plants and ending up in a Venus scenario. $\endgroup$ – L.Dutch♦ May 10 '17 at 7:28
  • $\begingroup$ How much CO2 can you add to make it toxic for humans but still keep vegetation alive? It doesn't have to be an extremely long period, but a few years would be nice if it were realistic. $\endgroup$ – socalemc May 10 '17 at 22:45
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The meteor could hit the yellow stone national park volcano, trigger a huge ash cloud. The ensuing fire and loss of sunlight (plants dont like darkness for extended periods of time, or so I am being told) could be enough to create on other continents a poisonous atmosphere.
Fires, created via looting, and burning oil reserves will increase the ashcloud density and size as well as the C02 concentration.

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    $\begingroup$ The OP asked "vegetation can survive but human not". You are not answering this. $\endgroup$ – L.Dutch♦ Jul 7 '17 at 10:06
  • $\begingroup$ Is increase in c02 levels to toxic levels not the answer? Will thought so as well. $\endgroup$ – Git Jul 7 '17 at 10:08
  • $\begingroup$ This answer is similar to Will's, but it's still focusing on a different variation. I think this answer is fine as it is. @L.Dutch The OP is asking for an event that forces humanity to live in shelters for a few years. It's about them not being able to survive on the surface, but it's fine if they survive. $\endgroup$ – Secespitus Jul 7 '17 at 10:28
  • $\begingroup$ @Secespitus, but the loss of sunlight, as also pointed out in the answer, does not help the vegetation survival. that's why I feel the answer is missing the point. $\endgroup$ – L.Dutch♦ Jul 7 '17 at 10:46
  • $\begingroup$ @L.Dutch Not all vegetation is supposed to survive. See "allowing vegetation to (at least some degree) survive", so if there are a few plants that survive not-so-much sunlight it's okay, but I can see your point. $\endgroup$ – Secespitus Jul 7 '17 at 10:50

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