阿斯 rarely appears alone — it's the standard syllable pair for foreign 'as-' / 'aas' sounds, where
阿 (ā) carries the open vowel and
斯 (sī) supplies the sibilant. Note
阿 is the same prefix used in
阿姨 (āyí, auntie) and many other transliterations like
阿拉伯 (Arab).