I'm working on a sci-fi book, and a chapter I'm currently on is about a guerilla attack on a human city on the planet.

The outer atmosphere is mostly CO$\_2$ and methane. The aliens breathe methane (naturally) and fart O$\_2$. Air density is the same as Earth, as is the gravity.

Now the humans live in geodesic domed cities. Each dome is about 20-40 km in diameter and the domes are connected using tunnel systems. The air inside the domes is Earth standard with mostly Nitrogen and O$\_2$. The domes are reinforced against harsh weather and have robots doing maintenance work on them.

My questions are, how devastating would an incendiary mortar or missile attack be? 

Some considerations:

* Can city engineers shunt the inner atmosphere out in time to prevent a catastrophic collapse of the dome?
* Can the people stop the fire from spreading and reseal the damaged portion of the dome or will the inferno happen so fast there won't be any time for it?
* Would the whole city in that particular dome actually im/explode? If the answer is yes, how long till it happens from the initial attack?
* How much time do the citizens have in order to seal off their houses/buildings or get into underground shelters?