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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 Gothenburg protocol 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.
In the early days, critical loads were often calculated as a single value, e.g. critical load of acidity. Today a two-dimensional critical load function is often calculated, with the x-axis as N-deposition and the y-axis as S-deposition. The critical loads concept is a steady-state concept and that it therefore includes no information whatsoever regarding how long it takes before effects are visible. A simplified illustration of dynamic aspects is the target load function, which is the load at which the chemical criterion recovers before a chosen year, the target year. Thus, for target years in the near future the target load function is lower than the critical load and for target years in the distant future the target load function approaches the critical load function.
Calculating critical load functions and target load functions include several simplifications and thus can be viewed as a risk concept: The higher the exceedance the higher the risk for adverse effects and there is a certain risk that zero exceedance will still lead to adverse effects.