Critical load

In the study of air pollution, a critical load is defined as ”A quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge”. (Nilsson and Grennfelt 1988)

Critical loads and the similar concept of critical levels have been used extensively within the 1979 UN-ECE Convention on Long-Range Transboundary Air Pollution. As an example the 1999 to the LRTAP convention takes into account acidification (of surface waters and soils), eutrophication of soils and ground-level ozone and the emissions of sulfur dioxide, ammonia, nitrogen oxide and non-methane volatile organic compounds (NMVOCs). For acidification and eutrophication the critical loads concept was used, whereas for ground-level ozone the critical levels were used instead.

To calculate a critical load, the target ecosystem must first be defined and in that ecosystem (e.g. a forest) a sensitive "element" must be identified (e.g. forest growth rate). The next step is to link the status of that element to some chemical criterion (e.g. the base cation to aluminium ratio, Bc/Al) and a critical limit (e.g. Bc/Al=1) which should not be violated. Finally, a mathematical model (e.g. the Simple Mass Balance model, SMB) needs to be created so that the deposition levels that result in the chemical criterion reaching exactly the critical limit can be calculated. That deposition level is called the critical load and the difference between the current deposition level and the critical load is called exceedance.

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