Alright I've got a bit of a theory here:

The "facts":
Whatever the source of the gas is (either the remains of large numbers of zombie corpses or some sort of other natural deposit) it does seem to be contained underground.
Ink and his army are also located underground.
The gas seems to have a strange quality whereby it can both change humans into zombies but also drives zombies away in large concentrations (since no zombies were seen at ground zero while the haze was there.)

So here's my idea:
What if the haze is not only the cause of the outbreak (possibly) but also the key to defeating Ink. If a large concentration of gas were to be released to flood Ink's tunnels while the zombies are trapped inside (seal up the entrances that Michael, Vic, and Saul are ferreting out) it could kill them. And this could explain why Michael is going back over his journal, he's preparing himself to make a sacrifice by releasing the gas that will trap him as well.

OK you can all take off your