First,what you are talking about is "啊“but not"阿”.And they pronounces similarly. Second,the character啊can be used either at the begin or the end.like"啊我看到了,小姐你白痴啊"they are all right. Remember,this character啊 DOES NOT have any other meanings.it just functions as an interjection(usually put at the begin)，or Auxiliary verb(usually put at the end) first,It can describe your feelings.for example,when people are shocked,or amazed,you may hear them say啊,and here it is interjection.second,it is usually put at the end ,not for showing your strong feeling as above,but just as part of a spoken sentence.and here it is Aux. verb.

i agree with Frank white, it is indeed 啊， and it doesn't have any speacial meanings , it is kinda expression, not LOL(this is laugh), it is more like Chinese manner to speak, usually happens on who you are familiar with, i use it quite frequntly.. can be girly with different voice tone. for example if i send a message : 你在干吗---(you would feel my face my look like-_- this) if i type: 你在干吗啊（can be a smile or more friendly face） difficult to explain, but good question, just don't bother with that

white.frank3解释的不错，我再补充一些： "啊"放在前面有表惊讶，痛苦，赞叹的语气，还有表疑问，反问的语气。 "啊"放在后面市语尾助词，无义。 为什么汉语会有语尾助词？因为汉语几乎不管怎么说都很平，不像印欧语系利用高低起伏来表示说话者的情绪，所以会用语尾助词来表达表示说话者的情绪，希望你明白。

Like what you said,阿is totally different from啊,and will never appear to show your feelings,and 阿will be used ONLY with certain characters to form new words like 阿宝，阿爸etc.