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Critical Loads Mapper Tool

What is the Critical Loads Mapper?

Atmospheric deposition of nitrogen (N) and sulfur (S) is a major stressor to natural ecosystems, often leading to acidification and eutrophication of both terrestrial and aquatic ecosystems. The amount of deposition below which harmful effects do not occur according to present knowledge is known as the critical load. Although the U.S. has seen large improvements in air quality and reductions in deposition since the enactment of the Clean Air Act Amendments of 1990, there are still many regions where the critical load for a variety of ecological end points is exceeded.

The Critical Loads Mapper tool (CL Mapper) is a joint project supported by the Environmental Protection Agency (EPA), USDA Forest Service (USFS), and the National Park Service (NPS) to make information more accessible on effects from atmospheric nitrogen (N) and sulfur (S) deposition.

How does the tool work?

The tool is essentially an interactive “front end” to the National Critical Loads Database, hosted by the National Atmospheric Deposition Program (NADP), and various deposition datasets developed by NADP, EPA, and other research groups. It allows the user to add layers of three basic types (i.e. deposition, critical loads, or exceedances to:

  • Examine these layers across the contiguous U.S. and through time,
  • Download data, and
  • Share data and images with colleagues.

Users may also download summary reports that cover all Class I Areas, and all other federal land units managed by the USFS, NPS, and USF&WS.

What are the benefits of using the Critical Loads Mapper?

The CL Mapper is an interactive mapping tool that enables decision makers, researchers, and the public to easily access information for the contiguous U.S. on:

  • Atmospheric deposition of N and S (estimates through time are provided for several different air quality models),
  • Critical loads for terrestrial and aquatic ecosystems (from the National Atmospheric Deposition Program’s National Critical Loads Database), and
  • Critical load exceedances (defined as the deposition minus the critical load)

Areas where atmospheric deposition levels are higher than critical load (i.e. positive exceedances) indicate potential vulnerability to atmospheric deposition. The CL Mapper helps users better understand local and regional vulnerability to atmospheric pollution, and how pollution levels and ecosystem effects may have changed through time.

Who should use the Critical Load Mapper?

The Critical Loads Mapper can be used by anyone interested in the effects of atmospheric deposition of nitrogen and sulfur on ecosystems. Specifically, the Critical Loads Mapper should be used by federal and state air quality managers and regulators, and their contractors, involved with Forest Planning, Wilderness Characterization, and permitting under the National Environmental Protection Act and for the Prevention of Significant Deterioration. In addition to these specific uses, the Critical Loads Mapper should be used by researchers and the public to better understand the impacts from atmospheric deposition of nitrogen and sulfur on local and regional ecosystems.

How can I access the Critical Loads Mapper?

The Critical Loads Mapper is open source and available at:

Critical Loads Mapper

Where can I get more information?

For more information on atmospheric deposition, critical loads, critical load exceedances, and how these concepts are used to support decision making, visit:

USFS CL Portal

EPA’s Integrated Science Assessment of the secondary standards of NOx and SOx.

Related Publications

  • Clark, C. M., et al. (2018). "Atmospheric deposition and exceedances of critical loads from 1800− 2025 for the conterminous United States." Ecological Applications 28(4): 978-1002.
  • Lynch, J.A., et al. (2017). “Detailed Documentation of the National Critical Load Database (NCLD) for U.S. Critical Loads of Sulfur and Nitrogen, version 3.0”, National Atmospheric Deposition Program, Illinois State Water Survey, Champaign, IL.
  • Pardo, L. H., et al. (2011). "Effects of nitrogen deposition and empirical nitrogen critical loads for ecoregions of the United States." Ecological Applications 21(8): 3049-3082.    

EPA Technical Contacts:

Christopher Clark –

Jason Lynch –