AP Environmental Science (APES) Atmospheric Pollution Practice Test 2026 - Free APES Atmospheric Pollution Practice Questions and Study Guide

Ozone formation in urban air hinges on sunlight driving photochemical reactions and the presence of both NOx and VOCs. Sunlight photolyzes NO2 to produce an oxygen atom, which quickly combines with O2 to form O3. However, ozone can be destroyed again when NO reacts with ozone, returning to NO2. VOCs supply radicals (like RO2 and HO2) that convert NO back to NO2 without destroying ozone, sustaining the cycle and allowing ozone to accumulate. Without sunlight the photochemical steps stall; without NOx there’s no NO2 to start the process; without VOCs the radical chain is limited and ozone production is much weaker. Therefore, significant urban ozone formation requires NOx, VOCs, and sunlight.