Two senses share the 1st tone: (1) a standalone exclamation of surprise —
呀!— sharper and more abrupt than
啊; (2) the creaking onomatopoeia of doors, hinges, oars, or anything wooden under stress, typically in the pattern
呀的一声 ("with a creak"). The toneless sentence-final particle
呀 is a separate reading and lives on its own page.