exclamatory particle
啊 is an exclamatory particle used in speech to express emotion or emphasis. Its meaning is tied to vocal expression rather than a concrete object or action.
啊 combines 口 (mouth) with 阿, which likely contributes the sound. The mouth radical indicates a spoken exclamation.
The mouth radical 口 on the left suggests a sound coming from the mouth. The right side 阿 looks like a person leaning back to call out: together they make an exclamatory particle.
For ア, imagine opening your mouth wide and saying 'Ah!' in surprise: Ah -> ア, and the mouth radical 口 shows the sound is spoken.