Real-time measurements of ozone, NOx, aerosols and radiation were performed very close to the launch area of high intensity pyrotechnic spectacles, the so-called Mascletàs, typical of celebrations in eastern Spain. During these events, a considerable number of spectators are potentially exposed to high concentrations of pollutants directly or indirectly produced by the ignition of thousands of firecrackers at ground-level. This work is focused on the mechanism of ozone formation throughout the Mascletàs. After the initial decrease in O3 levels (minimum concentration < 10 µg m–3) due to the reaction with NO emitted by fireworks explosions, peak concentrations of up to ~150 µg m–3 were recorded, clearly indicating that ozone was generated during these events. The results suggest that UV light produced by fireworks displays photolyzes O2 releasing O to form O3 as in the stratosphere.