Pour a thin layer of smoked salt in a saucer. Moisten the rim of a glass with a lime wedge. Gently roll the rim of the glass in the salt, turning to coat.
Fill the glass with ice. Add grapefruit juice, mezcal, lime juice and simple syrup, stirring gently to combine. Top with club soda. Garnish with grapefruit wedges and, if desired, a rosemary sprig.