Powerful gun batteries used to automatically shoot down organic bursts and other types of aerial munitions. In Nasheen, these are generally mounted on the tops of minarets. Larger cities tend to be bigger targets, and thus tend to have more antiburst guns.
Guns operate automatically when they detect and neutralize large swarms of virulent organics. This means they have also been known to fire on bug swarms. The explosive mixture usually contains tiny insects tailored to attack and devour the offending burst residue after the initial explosion, ensuring that as little contagion as possible reaches the surface below. This is not always effective, however, and trace amounts of chemical contagion often trickle down into the cities following a raid.
From GOD'S WAR:
At night, the sky above Amtullah lit up with the occasional violet or green burst, remnants of a border barrage that managed to get through the anti-burst guns. The sound of sirens sent him to bed most nights, as regular as evening prayer.