ā

  1. interjection of surprise
  2. Ah!
  3. Oh!

á ㄚˊ

  1. interjection expressing doubt or requiring answer
  2. Eh?
  3. what?

ǎ ㄚˇ

  1. interjection of surprise or doubt
  2. Eh?
  3. My!
  4. what's up?

à ㄚˋ

  1. interjection or grunt of agreement
  2. uhm
  3. Ah, OK
  4. expression of recognition
  5. Oh, it's you!

a ˙ㄚ

  1. modal particle ending sentence, showing affirmation, approval, or consent

Where 啊 is used

Sources