Capture the indicated number of gang leaders alive to unlock the corresponding Emergency mission:

Emergency Downtown: Capture 1 gang leader alive.
Emergency South Side Lot: Capture 2 gang leaders alive.
Emergency Dockyard: Capture 4 gang leaders alive.
Emergency Sewers: Capture 6 gang leader alive.
Emergency Thompson St.: Capture 8 gang leader alive.
Emergency Angel Heights: Capture 10 gang leader alive.