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2015 Critical Load Definition Summary______________________________________________________________________________

On the cover: XXXX

Use Condition and Citation; XXXXX

Contents

Background.................................................................................................................................................2

Critical Load Definitions..............................................................................................................................3

Critical Load

Empirical Critical Load

Target Load

Steady-State Critical Load

Supporting Definitions................................................................................................................................4

Chemical Threshold

Chemical Criterion

Biological Receptor

Biological Response

Biological Threshold

Deposition Load

Cited References.........................................................................................................................................5

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BackgroundCritical Loads of Atmospheric Deposition (CLAD) is a Science Committee of the National Atmospheric Deposition Program (NADP). The purpose of CLAD is to discuss current and emerging issues regarding the science and use of critical loads of atmospheric deposition for effects on ecosystems in the United States (U.S.). Members of CLAD identified the need for documentation of critical load terms and definitions. This document presents a summary of the critical load terms and definitions that were developed through a series of conference calls and webinars and have been adopted by CLAD. As critical load science continues to develop, these terms and definitions may be revised and updated.

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Critical Load Definitions

Critical Load: A quantitative estimate of 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 (UN ECE 1988, Nilsson and Grennfelt 1988).

Informal Definition: The threshold of deposition below which harmful ecological effects do not occur (Porter et al. 2005).

Empirical Critical Load: A critical load that is developed from observational spatial or temporal gradient studies or additions of pollutants to determine the deposition load at which chemical or biological changes occur in the environment (e.g., foliage, lichens, soil, aquatic chemistry/biota). Empirical critical loads are generally applied to sites or landscapes that are ecologically comparable to location(s) from which critical loads were determined (cf. Pardo 2010).

Target Load: The deposition load that is selected or determined to provide a level of protection or recovery of sensitive ecosystem components based on timeframe for resource protection, feasibility of emissions reductions, and/or other considerations. The target load may be set higher or lower than the critical load.

Steady-State Critical Load: Steady-state critical loads are derived from mathematical mass-balance models under assumed or modeled equilibrium conditions. The models used to derive steady-state critical loads vary in complexity with regard to process representation, however, a fundamental linkage among the various modeling approaches is the solution of elemental mass balances.

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Supporting Definitions

Biological Receptor: A biological element that is impacted by the conditions created by atmospheric deposition. Biological receptor is the same as biological indicator.

Biological Response: The biological response measure affected by atmospheric deposition that is used to predict the risk of damage to the biological receptor of interest.

Biological Threshold: The threshold or value of the biological response beyond which the biological receptor of interest is negatively impacted. The critical load of a system is the point where deposition levels result in the biological response being equal to the biological threshold. An example of a biological threshold is a change in species composition greater than 10% (McDonnell et al. 2014).

Chemical Criterion: The chemical measure affected by atmospheric deposition that is used to predict the risk of damage to the biological receptor of interest (Aherne et al. 2001). Chemical criterion is the same as chemical indicator.

Chemical Threshold: The threshold or value of the chemical criterion beyond which the biological receptor of interest is negatively impacted. The critical load of a system is the point where deposition levels result in the chemical criterion being equal to the chemical threshold. Chemical criterion is the same as critical limit. Some examples of chemical thresholds are 20% base saturation to protect forest growth and health, and ANC of 50 µeq/L to protect the majority of acid-sensitive aquatic biota. Chemical threshold is the same as critical limit.

Deposition Load: Mass per unit area per unit time

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Cited References

Aherne, J., E. P. Farrell, J. Hall, B. Reynolds, M. Hornung 2001. Using Multiple Chemical Criteria for Critical Loads of Acidity in Maritime Regions Water, Air and Soil Pollution: Focus: 1(1-2), pp 75-90, 10.1023/A:1011578311375.

Bobbink, R. and Hettelingh, J.P. 2010. Review and revision of empirical critical loads and dose- response relationships: Proceedings of an expert workshop, Noordwijkerhout, 23-25 June 2010. Coordination Centre for Effects.

CLRTAP, 2004. Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. UNECE Convention on Long-range Transboundary Air Pollution. http://icpmapping.org/Mapping_Manual

McDonnell, T. C., T. J. Sullivan, P. F. Hessburg, K. M. Reynolds, N. A. Povak, B. J. Cosby, W. Jackson, and R. B. Salter. 2014. Steady-state sulfur critical loads and exceedances for protection of aquatic ecosystems in the U.S. southern Appalachian Mountains. Journal of Environmental Management 146: 407–419. doi:10.1016/j.jenvman.2014.07.019

Pardo, L.H. 2010. Approaches for estimating critical loads of N and S deposition for forest ecosystems on U.S. federal lands. Gen. Tech. Rep. NRS-71. http://www.nrs.fs.fed.us/pubs/gtr/gtr_nrs71.pdfNewtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station. 25 p.

Porter, E., Blett, T.F., Potter, D.U., Huber, C., 2005. Protecting resources on federal lands: implications of critical loads for atmospheric deposition of nitrogen and sulfur. BioScience 55, 603-612

Sullivan, T. J., B. J. Cosby, C. T. Driscoll, T. C. McDonnell, A. T. Herlihy, and D. A. Burns. 2012. Target loads of atmospheric sulfur and nitrogen deposition for protection of acid sensitive aquatic resources in the Adirondack Mountains, New York. WATER RESOURCES RESEARCH, VOL. 48, W01547, doi:10.1029/2011WR011171.

UNECE. 2004. Manual on Methodologies and Criteria for Modelling and Mapping Critical Loads and Levels and Air Pollution Effects, Risks and Trends. UNECE Convention on Long-range Transboundary Air Pollution. http://icpmapping.org/Mapping_Manual

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