Unlike stratospheric ozone (protective), tropospheric ozone (O₃) is a harmful pollutant. It forms from photochemical reactions between NOx (from vehicles and industry) and VOCs (volatile organic compounds) under intense solar radiation, peaking on sunny summer afternoons.
In Spain
Spain frequently exceeds EU ozone thresholds (120 μg/m³ 8-hour average) during heat waves, especially in peri-urban and rural areas downwind of large cities. Paradoxically, concentrations are higher outside cities because urban NO destroys some O₃. It irritates airways and damages vegetation.