Preheat the air fryer to 400°F (200°C) for 5 minutes.
In a large bowl, whisk together the all-purpose flour, cornstarch, baking powder, and salt.
Slowly pour the ice-cold sparkling water into the dry ingredients while gently mixing with chopsticks or a fork until just combined; the batter should be lumpy and cold.
Pat the vegetables dry with paper towels to remove excess moisture.
Dip each vegetable piece into the tempura batter, allowing excess batter to drip off.
Lightly brush the air fryer basket with vegetable oil to prevent sticking.
Place the battered vegetables in a single layer in the air fryer basket, making sure they do not touch or overlap. You may need to cook in batches.
Lightly brush the tops of the battered vegetables with vegetable oil.
Air fry for 7 to 8 minutes, then carefully flip the vegetables, brush with a little more oil, and air fry for an additional 7 to 8 minutes or until golden and crispy.
Remove the tempura vegetables from the air fryer and place them on a paper towel-lined plate to absorb any excess oil.
Serve immediately with dipping sauce of choice.