This fizzy sip's warm, sunny hue and tart edge is reminiscent of an Aperol spritz, minus the boouncese. If you don't have Angostura bitters on hand, any citrus bitter will make a fine substitute.
Add grenadine and grapefruit juice to an ice-filled goblet or wine glass. Top with soda and bitters. Stir gently. Garnish with a grapefruit wedge, if desired.