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When is there too much ozone?

A number of conditions have to be  met before too much ozone occurs in the lower atmosphere (at ground level):

  • weather has to be sunny (lots of UV light). Clouds stop UV radiation emitted by the sun ( to a high degree).
  • it has to be sufficiently warm (>25 °C).
  • wind has to come from continental direction (E,SE,S) with low wind speed.
  • enough nitrogen oxides (NOx) and volatile organic compounds (VOC) have to be present in the atmosphere, and in correct ratios.

The (simplified) chemical reaction of ozone formation can be found here. It is important to state that a single day with sunny and warm weather does not lead to exceedances of the ozone threshold most of the time. Longer periods with such conditions are (most often) necessary.

An excess of ozone at ground level ('ozone smog') only occurs in Belgium during the months of May, June, July and August, or exceptionally during the end of April or the first half of September, in periods with a high pressure system located above continental Europe combined with eastern to southeastern wind currents towards Western Europe.

An overview of the number of 'ozone days' (i.e. days on which at least one exceedance of the EU threshold value (180 µg/m³) for informing the population was measured in Belgium) can be found here.

Under special meteorological conditions an intrusion of ozone can happen from the free troposphere or even the stratosphere towards the lower atmosphere. This can exeptionally (last occurred in Belgium on the 5th, 6th and 7th of May 1995) cause exceedances of the threshold value. This may also be called 'spring ozone'. Also, powerful storms in spring or autumn, with great downwards air currents can lead to increased ozone concentrations at ground level.