Critical Loads - Glossary

Acidifying deposition: Deposition of substances from the atmosphere as rain, snow, fog, or dry particles that have the potential to acidify the receptor medium, such as soil or surface waters.  Emissions of sulfur and nitrogen oxides and ammonia are the most common sources of acidifying air pollutants.

Acid Neutralizing Capacity (ANC): A measure of the ability of a solution to neutralize inputs of strong acids, commonly applied to surface water or soil solution.

Air Quality Related Values (AQRV): A resource, as identified by the federal land manager (FLM) for one or more Federal areas, which may be adversely affected by a change in air quality. The resource may include visibility or a specific scenic, cultural, physical, biological, ecological, or recreational resource identified by the FLM for a particular area.  "These values include visibility and those scenic, cultural, biological, and recreation resources of an area that are affected by air quality" (43 Fed. Reg. 15016).

Air regulator: State and federal officials charged with implementing the Clean Air Act and protecting air quality.

Atmospheric deposition: The transfer of air pollutants from the atmosphere to the Earth’s surface.  Atmospheric deposition occurs as wet (e.g., rainfall, fog, or snow) and dry deposition (e.g., gaseous or particulate deposition).

Base cations: Base cations are elements or ions with a positive charge (cations) that can neutralize acids.  Calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+) are base cations; these base cations also serve as nutrients for forest ecosystems.

Base saturation (BS): Base saturation is a way of measuring the base cations are available to plants. Base saturation is given as the percentage of potential cation exchange sites that actually have exchangeable base cations on them.  It is expressed as a percentage of the total cation exchange capacity. The higher the amount of exchangeable base cations in soil, the more acidity can be neutralized.

Bioaccumulation: The increase in concentration of a contaminant in an individual organism relative to the surrounding environment or medium (e.g., water, sediment).

Biomagnification: The increase in concentration of a contaminant from lower trophic levels to higher trophic levels in the food chain.

Critical load (CL): The 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.  Critical loads and target loads can be calculated for different inputs: acidity (N and S combined), S, N, or N nutrient (to address detrimental effects caused by excess N and N saturation).

  • The critical load for acidity is the total deposition of acidity inputs to the ecosystem (S and N combined) below which no harmful ecological effects occur; it is sometimes referred to as critical load S+N.
  • The critical load for S is the total deposition of S to the ecosystem below which no harmful ecological effects occur; when no critical load for N is calculated, the critical load for S is equal to the critical load for acidity.
  • The critical load for N nutrient is the deposition of N to an ecosystem below which no harmful effects of N saturation occur.
  • The critical load for N is the total deposition of N to the ecosystem below which no harmful ecological effects occur. The critical load for N is either simply the critical load for N effects from acidification which is calculated by subtracting the S deposition from the critical load for acidity.  Preferably, a critical load for N nutrient is calculated and the critical load for N used will be the lower of the two.

Critical threshold: A critical threshold is a chemical characteristic (usually easily measurable) that is related to the ultimate biological or ecosystem effect of concern.  For example, in aquatic systems, a given ANC (critical threshold) may be related to fish mortality (the biological effect of interest).

Denitrification: A microbial process which converts nitrogen to a gaseous form which can then be exported from the ecosystem.

Deposition scenario: An estimated history and future projection (temporal pattern) of deposition that is used in dynamic modeling.

Dynamic model: Dynamic models are computer models that incorporate internal feedbacks—such as accumulation of N in the system, or exchange of base cations between soil and soil solution from year to year—and allow for the prediction of time to damage and time to recovery.  Dynamic models are data intensive and can therefore be expensive to run, but provide useful information for setting target loads.  Dynamic models are therefore appropriate in areas with high data availability.

Ecological threshold: The dose of a pollutant at which a measurable change occurs in the response of some component of an ecosystem (e.g., NO3 leaching at nitrogen deposition of 8 kilograms per hectare per year (kg/ha/yr)).

Ecosystem services: Benefits to society from a multitude of resources and processes that are supplied by natural ecosystems (e.g., clean drinking water).

Empirical critical load: Empirical approaches are based on observations of response of ecosystem or ecosystem component (e.g., foliage, lichens, soil) to a given, observed deposition level.  Empirical critical loads can be calculated for the site where the data were obtained; generally, they are applied to similar sites where such data are not available.

Exceedance: The exceedance of the critical load is the actual deposition minus the critical load.

Exceedance class: Exceedance proportions binned into classes allows for examination of the relative magnitude of the exceedance.

Exceedance proportion: The relative magnitude of critical load exceedance, determined by dividing the amount of deposition by the critical load.

Exceedance value: The amount of deposition by which the critical load is in exceedance, determined by subtracting the critical load from the deposition.  Possible units include kilograms per hectare per year (kg/ha/yr), milliequivalents per square meter per year (meq/m2/yr), and equivalents per hectare per year (eq/ha/yr).

FLAG: Federal Land Managers Air Quality Related Values Work Group.

Endpoint: The ultimate ecological, biological, or human condition or process to be protected from harm.  Two examples of endpoints are human health and forest sustainability.

Indicator: A measurable physical, chemical, or biological characteristic of a resource that may be adversely affected by a change in air quality (e.g., ANC).

Mass balance: The mass balance approach is a technique used to determine the status of an ecosystem by comparing the inputs to the system and the outputs from the system.  A mass balance can be calculated for any quantity of interest, for example, water, nitrogen, etc.

Nitrogen saturation: Syndrome of effects occurring in an ecosystem caused by an overload of nitrogen, usually from long term atmospheric nitrogen deposition.

Policy threshold: A quantitative value of desired ecological condition established by policy and selected based on a balancing of science and land management or policy goals.

Regionally refined deposition estimates: Deposition estimates for a specific geographic area that are informed by additional site-specific measurements such as precipitation, bulk deposition, and/or throughfall deposition.

Sensitive receptor: The indicator that is the most responsive to, or the most easily affected by a type of air pollution. The sensitive receptor might be a particular organism (salamander, lichen, tree species) or it might be an ecosystem compartment (soil, trees, etc.).

Simple mass balance model: Simple mass balance approaches are based on estimating the net loss or accumulation of nutrients based on inputs and outputs of the nutrient of concern (e.g., base cation, nitrogen).

Steady state: Steady state is a condition of an ecosystem where inputs are matched by output; there is no net change in a system at steady state.

Target load: The acceptable pollution load/level of deposition that is agreed upon by policy makers or land managers. The target load may be higher or lower than the critical load based on considerations of economic cost of emissions reductions, timeframe, and other matters.  The target load is set below the critical load to provide a reasonable margin of safety, but could be higher than the critical load at least temporarily.

Tipping point: The point at which an ecosystem shifts to a new state or condition in a rapid, often irreversible, transformation.


Pardo, LH.  2010.  Approaches for estimating critical loads of N and S deposition for forest ecosystems on U.S. federal lands.   Gen. Tech. Rep. NRS-71. Newtown Square, PA.  U.S. Department of Agriculture, Forest Service, Northern Research Station. 25 p.

Fenn, ME, KF Lambert, T Blett, DA Burns, LH Pardo, GM Lovett, RA Haeuber, DC Evers, CT Driscoll, DS Jeffries.  2011.  Setting limits: Using air pollution thresholds to protect and restore U.S. Ecosystems. Report Number 14. J.S. Baron, Editor-in-Chief. Ecological Society of America. Washington, DC. 21 p.

Pardo, LH. Critical Loads Website. US Forest Service, Northern Research Station.