Her nickname can be read as A-chan, Achan or Aachan. The official website registered states that it's A-chan. But since her nickname in the Japanese registry indicates アーチャン, you can literally translate it also as Aachan, since アー can be translated as "Aa", as the Chouonpu (ーChouonpu?) kana (in the subclass hiragana) elongates a vowel before it.
I would appreciate if you stop changing that one (Kaze no Rasen) with something just copied from Wiki48! I made that specifically so that it would be different from them, cuz someone will get angry again. I researched that part for a whole day, and the one I put there had particularly classified/categorized the difference in the members, because Nogizaka46 is separated by generations instead of Teams, so that they can be more distinguishable. :'(