China Unicom Number in Xian Airport

Hi,
I'll arrive around 11:15 PM. I need to buy China Unicom sim card. Can I buy a sim card at the airport? Does China Unicom has a store at the airport? Does it open after 11 PM? or can I buy it at one of the convenient stores? I really need internet connection after arriving as I'll feel secure to travel with it.
Wan