Chlorine dioxide does a job.  ...

Chlorine dioxide does a job.  It is volatile because it is a gas dissolved in water and is driven off by the fans of cooling towers so it is possible to have a clean system and a fouled pack.  Chorine reacts with ammonia to form chloramines which are also volatile.  If you have it available, I would use bromine which reacts with ammonia and forms bromamine, which is much less volatile and does control algae with much lower chloride.