Ground-level ozone (O₃) and air quality
Find out how ozone forms near the ground, why it can rise away from roads and how to interpret its forecast.
How ozone forms
Ozone near the ground forms when nitrogen oxides and volatile organic compounds react in sunlight. The ingredients can come from transport, industry and other sources. Air movement can carry ozone and its precursors, so elevated concentrations can occur downwind, including in rural areas.
Ground-level ozone is the O₃ shown on city pages. The ozone layer higher in the atmosphere serves a different role by absorbing ultraviolet radiation.
What to look for in the timeline
Ozone can build during sunny periods. Read the hours shown in the chart alongside the source and local time. A city’s model forecast and a station measurement can differ, and a change in ozone need not track a change in roadside nitrogen dioxide.
The European AQI uses an hourly ozone concentration. Other indices and official standards can use a different averaging period, such as eight hours. Compare the period and method as well as the unit.
Worked example: an afternoon ozone rise
Suppose a model predicts 55 µg/m³ of ozone at 09:00 and 108 µg/m³ at 15:00 in the same grid cell on the same day. Those values fall in the Good and Moderate bands for ozone. The timeline describes the model’s estimate for those two hours. The overall index can be worse if another pollutant falls in a higher band.
To check whether a station recorded a similar rise, look for its ozone values at the same local times. The station samples its immediate surroundings while the forecast represents a wider area. Keep the hourly values separate from any eight-hour averages you encounter in other reports.