AOT40 5000ppb over 6 months
AOT40 is the Accumulated concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. The units are therefore ppb multiplied by hours. For forests, the AOT40 is summed for the daylight hours over a six month period. Daylight hours are defined as when solar radiation exceeds 50 W m-2. The daylight hours are when plant stomata are normally open. Most research has been done on beech, and there is little on birch or the other species typical of birch woodland.
Flux-based critical levels, based on biomass reduction, are also available for local and regional assessment but are not yet incorporated into APIS. See ICP Vegetation Manual 2010.