(b) Measuring Methods: The area of soil where critical loads are exceeded
determined by the sum of all ecosystem areas in grid cells where exceedances occur.
Levels of acidification that exceed critical loads are calculated by considering both
sulphur and nitrogen deposition. The data are directly derived from official national
sources.
(c) Limitations of the Indicator: The values of the exceedance of critical loads are
highly dependent on the size of the grid cells used for the calculations. In particular,
the area of a protected ecosystem can vary considerably depending on the spatial
resolution of the grid system. This means that the accuracy of the method depends on
the grid size (currently 50x50 km for deposition modelling). More research is needed
to increase the robustness of the calculations.
In many cases the critical loads are determined only for the acidity of sulphur. The
total acidity of sulphur and nitrogen needs to be determined so that a coherent
agreement can be reached regarding abatement policies. Numerous methods are
available for obtaining critical loads. To obtain values for the critical loads, an
ecosystem has to be chosen and then a suitable indicator species must be selected to
represent the ecosystem. A chemical limit is subsequently defined as the
concentration at which the indicator species will die. In forests the indicators are trees,
and in fresh waters they are fish.
(d) Alternative Definitions/Indicators: The concept behind critical loads is based on
a dose-response relationship where the threshold of harmful response (within the
ecosystem) is triggered by a certain load of pollutant — the critical load. However, it
is not always easy to apply the concept without careful consideration of the nature of
the affected ecosystem and the threshold effects of harmful pollutants. For critical
loads to be used, ‘target loads’ can be set for different areas in order to try to halt the
acidification processes. Target loads have been defined as ‘the permitted pollutant
load determined by political agreement’. Therefore, target loads can be either higher
or lower than the scientifically determined critical load values. For example, the target
load may be lower so as to give a safety margin, or the target load may be higher for
economic reasons. There are also increasing possibilities for mapping critical loads
for individual ecosystems (e.g. there has been much recent attention given to the
application of acidification modelling to forest ecosystems).
