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Critical Loads

The first critical loads defined and calculated were for acidity (CCE Technical Report 1991). In the negotiations of the 1994 Oslo Protocol a so-called sulphur fraction had been used to derive a critical deposition of sulphur from the acidity critical load (CCE Status Report 1993)

In the negotiations leading to the 1999 Gothenburg Protocol, nitrogen (N) became the focus and thus new critical loads had to be defined. This lead to an extension of the simple critical load of sulphur (S) to the so-called critical load function of S and N which is characterized by three numbers (CCE Status Report 1995): The maximum critical load of S, CLmaxS, which equals the net input of base cations minus a critical leaching of acid neutralization capacity, linking the soil solution chemistry to the biological impact. As long as the deposition of N stays below the so-called minimum critical load of N, CLminN, all deposited N is consumed by sinks of N (immobilization in the soil and net uptake by plants). Finally, the maximum critical load of N, CLmaxN, not only takes into account these N sinks, but also considers deposition-dependent N processes (such as denitrification). Therefore, no unique acidity critical load can be defined, but the combinations of N and S deposition not causing "harmful effects" lie on the critical load function of the ecosystem, defined by these three quantities. An example of such a trapezoid-shaped function is depicted in the figure below. For details see Chapter 5 of the Mapping Manual.

Fig Critloads 436

Excess N deposition contributes not only to acidification, but can also lead to the eutrophication of soils and surface waters. Thus a critical load of nutrient N, CLnutN, has been defined, which accounts for the N sinks and allows an acceptable leaching of N.

It is the four critical load quantities described above which Parties to the LRTAP Convention were asked to submit to the CCE and which were used to prepare European maps and data bases. To map critical loads, e.g. on the same grid on which N and S depositions are given, the many critical load values within a grid cell are assembled into a cumulative distribution function (CDF), taking into account the areas of the ecosystems. From this CDF, percentiles (or other statistical quantities) can be calculated and mapped (for details see Chapter 8 of the Mapping Manual).

last update 5 Jun 2013