Phoenix

From Wiki 24

Phoenix was the location of a job that Dan Mounts helped Rick Allen out in. Dan brought it up during the course Day 1 2:00am-3:00am to remind Rick that he owed him with kidnapping Kim Bauer and Janet York.

George Mason's divorced wife, apparently, also lived there, at least until the events of Day 2.

Also the location of CTU Phoenix, an integral part of several operations.

Phoenix is in Arizona and is the capital and a large city. The second largest city in Arizona is Tucson.