The formation of ground-level ozone gets started early in the morning.
Vehicular traffic is by far their largest source.
Although the morning commute is over, continued traffic emmissions added to the earlier pollution begins to form ozone as the sun's heat and light intensifies. If there is little wind, the pollution disper- sion may be further restricted. By mid-afternoon, the sun has reached its apex and temperatures peak. The sun's radiation will cause a chemical reaction between the reactive hydro- carbons and nitrogen oxides to form ozone. Temperatures in the 90's or above are required for maximum ozone concentrations.