Four toned readings, all sentence-initial cries:
啊 (1st tone) = surprise or sudden realization ('Ah! Oh!');
啊 (rising) = querying or asking for repetition ('Eh? What?');
啊 (3rd tone) = startled disbelief ('What?! My!');
啊 (4th tone) = recognition or grudging agreement ('Ah, I see / uh-huh'). The character is identical — pitch contour does all the emotional work.