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There is not as much ozone in the troposphere as in the stratosphere. Ozone molecules are mainly found in two layers of our atmosphere.

This is a good thing, because elevated levels ozone can be harmful for health and vegetation. Moreover, because it can absorb infrared radiation, it can influence the climate by contributing to the well-known greenhouse effect.

It is widely recognized that ozone levels increase in polluted areas like in Western Europe. We have also good reasons to believe that tropospheric ozone increases in most areas including the most remote regions, far away from the polluting sources.

Nevertheless, ozone is not directly emitted by human activities.

Causes of increased tropospheric ozone

A small fraction of the tropospheric ozone at our latitudes comes from the stratosphere, especially in winter and spring. The rest is produced by chemical reactions in the troposphere itself. The main ingredients of this production are:

  • sunlight
  • nitrogen oxides (NO and NO2)
  • hydrocarbons
  • carbon monoxide (CO)

Most of theses gases have a relatively short stay in the troposphere, between a few hours and several weeks. Therefore, the production of ozone mainly occurs in the vicinity of the pollution sources.

Tropospheric ozone lifetime and depletion mechanism

The photochemical lifetime of ozone itself varies between a few days (at low altitudes) and several months (in the higher troposphere).

An important loss mechanism for ozone is photolysis, followed by the reaction of the product atom with water vapor. As a consequence of this destruction, hydroxylradicals (OH) are created. These radicals play a fundamental role in tropospheric chemistry as the main oxidant for a large number of particles. This complex chemistry involves hundreds of chemical species and reactions.

Other processes are important as well, like:

  • the washout of soluble species by precipitation
  • the deposition of gases -among which ozone - on soils
  • water and vegetation
  • the chemical reactions in clouds or at the surface of aerosols

Many among these processes are not well understood and quantified.