啊

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.

Don't confuse this stand-alone interjection with the toneless sentence-final particle 啊. Position tells you which: at the START of a sentence and tone-marked → interjection; at the END and toneless → particle. Compare 啊，是你 ('oh, it's you') with 是你啊 ('it IS you').