Convention on Long-range Transboundary Air Pollution
Working Group on Effects (WGE)

ICP Modelling and Mapping
Critical loads and levels approach

For the purposes of ICP Modelling and Mapping, and as a basis for the commonly agreed methodology, the following definitions have been adopted:

Critical levels: concentrations of pollutants in the atmosphere above which direct adverse effects on receptors, such as human beings, plants, ecosystems or materials, may occur according to present knowledge. In order to provide guidance to users of this definition, the following clarification constitutes part of the definition:

• Concentrations: includes high or low concentrations of air pollutants which may occur over short periods of time (< 24 h) or continuously over longer periods (> 24 h).
• Pollutants: means, in particular: SO₂ , NO_X , O₃ , NH_X. These pollutants may occur in combinations, either simultaneously or sequentially.
• Direct effects: refers to the impact of pollutants directly, such as occurs with SO₂ and plant leaves, rather than indirectly via the sulphur pathway through soils. Effects may be chronic or acute and must be distinguishable from any natural variation. Since there are different scientific viewpoints on what is to be considered an adverse effect on ecosystem function and structure, several types may be defined, including:- the onset of physiological and/or biochemical changes which are expected to be of ecological relevance; - the onset of decline of growth, vitality or quality (above ground, roots) in individual species (crops, trees, natural species); - the onset of significant changes in ecosystem structure and/or function (productivity decline, population changes, genetic diversity); - changes in material properties shortening the lifetime of the object.
• Receptors: refers to living organisms or materials which are affected, and includes interrelated collections of living organisms - i.e. ecosystems. A receptor may or may not be the most sensitive component in a given region.

Critical load: 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. For the user of this definition the following clarifications should constitute part of the definition:

• Exposure: means deposition experienced on an area basis e.g. eq m^-2 yr^-1; kg ha^-1 yr^-1.
• Pollutants: includes sulphur and nitrogen (NO_Y and NH_X) compounds.
• Significant harmful effects: could be the consequence of short term or long term deposition. Several types of harmful effects may be defined including: - chemical changes in soils and waters which might cause direct or indirect effects on organisms - changes in individual organisms, in populations and ecosystems.
• Sensitive elements: can be part or whole of an ecosystem or of ecosystem development processes.
• Acid Neutralising Capacity (ANC): Ability of a solution to neutralise inputs of strong acid to a preselected equivalence. An important chemical parameter used in critical loads calculations.
• Biological Indicator: selected organism(s) or population sensitive to chemical effects resulting from changes in sulphur and nitrogen deposition.
• Critical chemical value: The highest value of a critical chemical parameter or combination of parameters (e.g., pH, Al/Ca ratio) that does not cause a significant harmful response in a biological indicator.

General definitions:

• Functional Subregions or Sensitivity Subregions: Geographic areas of single or combined environmental characteristics (e.g., soil groups, elevation classes, ecoregions) that govern or characterise receptor response and provide a means of stratifying a country or region.
• Landscape: A territory with common origin, common history of development, formed under conditions of homogeneous geographical foundation, single dominating type of relief, identical climate, with characteristic combination of soils, plant societies and comprising the minor geosystems of the local level.
• Receptor: An ecosystem or part of an ecosystem of interest that is potentially impacted by pollutant concentrations or atmospheric deposition (e.g., organisms, plant communities, terrestrial ecosystems, groundwater, surface waters, monuments, materials).
• Primary Maps: The minimum set of maps that are essential for international evaluation of critical levels/loads.
• Secondary Maps: Supplementary maps providing supporting information to evaluate critical loads for individual countries, e.g., as documentation for primary maps or for defining national objectives.

These definitions are part of the agreed methodology of ICP Modelling and Mapping laid down in Annex I of the "Manual on methodologies and criteria for mapping critical levels/loads and geographical areas where they are exceeded", UBA Texte 71/96, ISSN 0722-186X, Berlin 1996. Further definitions for specific application are provided in the appropriate parts of the Manual.

The Modelling and mapping manual was updated in 2004. The updated manual is available either from the German Environmental Agency (Umweltbundesamt) web page (Modelling and mapping manual, as published in print in December 2004) or from ICP Modelling and Mapping web page (Modelling and mapping manual, possible updates after printing).