Why can a Chinese person have a nickname as 啊+surname?
Not 啊, is 阿. 

阿 + surname, or 阿 + given name can be a nickname. You can only attach one character to 阿.

e.g.  李白 ->  阿李, 阿白

e.g. 周树人 ->  阿周, 阿树, 阿人.  no 阿树人. 

阿 + last given name used more.