AOT40 exposure index
A series of experiments has shown that the impacts of ozone are often better related to accumulated exposure above a threshold concentration rather than the mean growing season concentration. An index of accumulated exposure index above a threshold concentration of x ppb (AOTx) has thus been used as the principal measure of ozone pollution for assessing effects on vegetation. AOTx is calculated as the summed product of the concentration above the threshold concentration and time (T), with values expressed in ppb h or ppm h:
∑ ([O3]-x) T
A value of 40 ppb (AOT40) has been employed in impact assessment research in Europe, while a higher value of 60 ppb (AOT60) has been used in the USA. There is evidence that ozone may affect vegetation at concentrations well below 40 ppb, and a lower threshold of 30 ppb (AOT30) has been proposed as more appropriate.
For forest trees, exposure is generally limited to the period when the stomata will be open and so AOT40 is calculated for daylight hours during the growing season (April to September). Different exposure periods may be used to allow for differences in phenological development – for example, in the Mediterranean region, plants are often active during the winter months, with limited conductance during the summer as a result of water limitation.
Although the AOT40 index gives a good fit with experimental data in many, care needs to be exercised in applying it to assess the effects of ozone across the range of growth conditions experienced in Europe. In northern Europe, particularly Scandinavia but also much of Britain, ambient concentrations are generally lower than in central and southern Europe, and often close to 40 ppb. Exposure according to the AOT40 index is thus negligible, suggesting that ozone pollution is not an issue in the region. However, reduced growth rates have been reported as a result of ambient ozone levels in Finland, for example. It has been proposed that long daylength and lack of drought conditions that are typical for the region lead to significant chronic exposure to ozone, rather than the acute exposure episodes that are more common in central and southern Europe. In contrast, the AOT40 index is likely to overestimate physiological exposure to ozone (ie exposure within the stomatal cavity) in southern Europe as a result of frequent and widespread soil moisture deficits limiting stomatal opening.
Where concentrations are well above 40 ppb and particularly for acute pollution episodes, the AOT40 index provides highly credible assessments of the impact of ozone pollution. However, in unpolluted rural regions, where ambient concentrations are generally close to 40 ppb, a small measurement error, or spatial differences in concentration can have a disproportionately large effect on AOT40. For example, in a study in the Lake District, the relatively small difference in ozone concentration between the top of an oak canopy, and a typical 2 m high sampling point in an exposed location raised the cumulative exposure index from 6.5 to 22 ppm h.
Physiologically effective ozone dose
An alternative approach has been proposed in which the functional, or physiologically effective, ‘dose’ of ozone to which the plant is exposed is calculated, as compared to external exposure implicit in the AOT40 index. In this approach, ozone dose is calculated as the product of concentration and a measure of the ease by which the ozone molecules gain entry to the leaf – the stomatal resistance. The most important factors are the canopy aerodynamic, leaf boundary layer and stomatal resistances. Aerodynamic resistance is modelled as a function of vegetation height and windspeed, and leaf boundary layer resistance as a function of leaf dimensions and windspeed.
A number of models are available for modelling stomatal resistance as a function of climatic variables, including light, temperature, leaf-air vapour pressure deficit and atmospheric CO2 concentration. The model most commonly used for estimating ozone uptake is a ‘multiplicative’ model, with each environmental variable acting independently. Measurements of stomatal resistance to water vapour have been widely reported in the literature, although relatively few data-sets are available for parameterising the model, and as a result, it can be applied to few tree species with certainty.
Plants have the ability to withstand a certain level of ozone pollution as a result of detoxification processes within the leaf itself. Certain are involved in these detoxification processes, and as a result different species have the ability to withstand different doses of ozone. This level, below which there is no harm to the plant is known as the threshold dose, and the physiologically effective ozone dose is accumulated for fluxes above this threshold value.