Ozone is a secondary pollutant formed under the effect of solar ultraviolet acting on precursors (NOx, NMVOC, CO). The impact of pollution on ecosystems arises when concentrations exceed certain thresholds. The high ozone concentrations can affect key physiological processes of plants (photosynthesis, respiration) and reduce the production plant. AOT40 (Accumulated Over Threshold of 40 ppb) is an indicator of exposure of plants to the ozone. It represents the accumulated concentrations exceeding the threshold of 40 ppb (parts per billion), or 80 μg/m3.h during the period of plant growth and during daylight hours. The daylight hours are defined as the hours when global radiation received by the Earth are at least 50 Watt/m2. For crops and natural vegetation that have sensitivities to ozone equivalent, the calculation is carried out between May 1 and July 31. For forests, the AOT40 is determined for the period 1 April to 30 September.
The goal of protection of vegetation and crops (6 000 μg/m3.h three months) has been exceeded in most rural sites in 2005. Especially in rural sites in southern and eastern countries that we meet the highest standards. Conversely, western countries experiencing levels generally below the goal of protection of vegetation and crops. The target value (18 000 μg/m3.h) was exceeded mainly in the south due to a combination of factors favorable to ozone production: strong sunshine, traffic and industries supporting the production of precursors.
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