What is a critical level or load?
A Critical Level is defined as the concentration of (or exposure to) a specific pollutant (or combination of pollutants) above which direct adverse effects on receptors may occur, according to current knowledge. At its most simple, a Critical Level can be set to protect the most sensitive component of an ecosystem. Critical Levels can also be applied to specific species, or to particular environmental conditions. Where a Critical Level is exceeded, this does not mean that harmful effects will occur, but that damage is unlikely at exposures below this value. Critical Levels (and Loads) form the basis of emissions control policies by providing information on where ecosystems are likely to be affected, and through model simulations, how best to achieve effective reductions in pollutant exposure. Because ozone is a long-range pollutant, these policies are more effective if applied at a European, rather than national or local level.
AOT40-based critical level
The availability of data-sets suitable for deriving Critical Levels for forest tree species is limited as a result of the short duration of many of the investigations into the effects of ozone on trees. Prior to 2003, the Critical Level (10 ppm h) was based on a limited range of experiments investigating the relationship between growth reduction and ozone exposure which suggested that a cumulative exposure during daylight hours from April to September of 10 ppm h resulted in a 10% loss of increment in these experiments. A new, lower value of 5 ppm h was adopted in 2003 based on the results of 21 experiments carried out at seven sites across Europe. A critical level of 10 ppm h is only exceeded over relatively small areas of southern England, while the new, lower critical cevel of 5 ppm h is exceeded over extensive areas of England and Wales, and also parts of east Scotland.
Flux-based critical level
As a result of some of the limitations of the AOT40 approach to assessments of ozone exposure, a critical level has also been adopted for use in the alternative, flux-based approach. The flux-based critical level of 4 mmol/m2/s accumulated during a single growing season over a threshold value of 1.6 nmol/m2/s has been set to protect sensitive species. For broadleaf trees, the critical level represents a 5% yield loss on an annual basis. A 5% yield loss on an annual basis is highly significant for long-lived tree species, and in addition to the direct effects on growth, increased susceptibility to biotic and abiotic agents may also result in increased mortality.
Experimental basis of critical levels
Both the new AOT40 and flux-based critical levels were based on pooled data-sets from a number of sites across Europe, including the open-top chamber site at Headley. Young trees were exposed to elevated ozone concentrations in controlled environment facilities for at least two growing seasons. Trees species were categorised as either conifer or broadleaf, and within those categories, as sensitive or resistant. In most cases, only ambient and one elevated ozone concentration were applied as treatments. Both treatments were used within the analysis by calculating dose and exposure for both treatments and extrapolating the relationship between dose and biomass production (assumed linear) to provide a hypothetical estimate of biomass in the absence of ozone. Percentage biomass reduction was then calculated for each treatment. Further details are given in the report of 2002 Gothenberg Workshop at which the analysis was presented.