Rim four glasses with chamoy and Tajín, then fill each with ice.
In a large cocktail shaker (preferably a 28 oz/18 oz tin-on-tin shaker), combine the mezcal, lime juice, and Pomegranate Spiced Syrup.
Fill the shaker with ice and shake vigorously for 15–20 seconds, until well chilled.
Double strain the mixture evenly between the four prepared glasses.
Top each glass with 1 oz of grapefruit soda and gently stir to combine.
Garnish with a grapefruit wedge dusted with Tajín and a sprinkle of fresh pomegranate arils.