In the air quality directive (2008/EC/50), the EU has set a target value and a long term objective value for ozone (O3) for the protection of vegetation. Target value: the AOT40 may not exceed 18000 (µg/m3) per hour in the period from 1 May to 31 July averaged over five years. Long term objective value: the AOT40 may not exceed 6000 (µg/m3) per hour in the period from 1 May to 31 July within a calendar year.
AOT40 (expressed in (μg/m3) ∙ hours) means the sum of the difference between hourly concentrations greater than
80 μg/m3 (= 40 parts per billion) and 80 μg/m3 over a given period using only the one-hour values measured
between 8.00 and 20.00 Central European Time (CET) each day. More information is provided in Annex VII of directive (2008/EC/50).
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