Critical Loads of Atmospheric Deposition Science Committee (CLAD)

Critical Loads of Atmospheric Deposition (CLAD) is a Science Committee of the National Atmospheric Deposition Program (NADP).

The objectives of CLAD are to:

  • identify risk to aquatic and terrestrial ecosystems from air pollution.
  • advance the development and application of CLs of atmospheric deposition to promote ecosystem health in the United States.

A critical load identifies how much air pollution an ecosystem can tolerate without harmful ecological effects (Porter et al., 2005).  The critical load concept was first described at a meeting held by the United Nations Convention on Long-Range Transboundary Air Pollution (LRTAP) in the 1980s by scientists documenting the effects of acid rain on Scandinavian lakes (Nilsson and Grennfelt, 1988).

CLAD is primarily concerned with critical loads for North America. Critical loads have been adopted by many agencies in the United States (EPA, USFS, NPS, FWS) for evaluating air pollution effects on ecosystems. CLAD also collaborates with other colleagues who develop critical loads for Canada (ECCC) and Europe (UN ICP CCE and M&M).

Critical loads can protect a single species, a community of species, a particular location such as a water body, or a broad land cover category or ecosystem type (i.e., forests, alpine lakes).  Critical loads can be national in scale, particular to a specific watershed, or vary with abiotic conditions. 

Need the numbers or data?  The CLAD database includes CLs that can be applied in United States.  The CLAD database is updated annually.

Need some background information?  The Forest Service video series is an introduction to the critical load datasets. 

Want to get involved?  CLAD meets twice yearly, at the spring and fall NADP meetings, and welcomes all interested to participate. CLAD is an international organization of volunteers at all career stages from government agencies, academic institutions, and businesses. Join the NADP-CLAD Google Group (see Contacts Box to right) for occasional emails on topics of interest. 

What’s new? Working groups allow members to continue to develop the science and application of critical loads.  Current projects include:

  • incorporation of uncertainty into critical loads,
  • development of critical levels of ozone, and

For more information about critical loads and CLAD, refer to the follow information and summary documents:

CLAD Documents
Meeting Minutes and Presentations
CLAD Annual Reports

The Annual Report documents CLAD activities and products that occurred during the year spanning October 1st – September 31st. These include: summary of CLAD accomplishments; progress of the CLAD scientific Working Groups; new CLAD products; summary of Fall and Spring CLAD meetings; summary of the United Nations Economic Commission for Europe (UNECE) International Cooperative Programme (ICP) Modelling and Mapping (M&M) Working Group on Effects (WGE) Coordination Centre for Effects (CCE) annual meeting; and critical load--related publications added to the CLAD website during the year.

Critical Load Maps

The Critical Load Map Summary is a collection of critical load maps with supporting critical load information for the United States. These maps were developed by CLAD members using publicly-available critical load data that are included in the NADP CLAD National Critical Load Database v3.1 (NCLD).

Scientific Working Groups

Scientific working groups (WGs) have been a component of CLAD since 2011. The objectives of the WGs are to increase our understanding and ability to estimate and represent critical loads of deposition in the United States. To date, there are five active CLAD WGs:

  • WG-1: Adding new data and critical loads to the CLAD NCLD
    • produce, adopt, and practice a standardized method for review and incorporation of new published data and critical loads into the NCLD
    • Contact: Jason Lynch
  • WG-2: Characterizing uncertainty in critical load estimates
  • WG-3: Synthesizing multiple critical loads
    • develop a repeatable and defensible method for representing multiple critical loads in the same geographical area
    • Contact: Michael Bell and Linda Pardo
  • WG-4: Deposition uncertainty
    • develop an understanding of the range of uncertainty that is associated with measured versus modeled deposition and how it relates to critical loads
    • Contact: Michael Bell
  • WG-5: Critical Load Communications and Outreach (CLOC)
    • To develop an outreach and communication strategy to communicate the concept and use of critical loads to stakeholder groups
    • Contact: Linda Geiser
  • WG-6: Coordinate research efforts of ozone effects on ecosystems and develop ozone Critical Levels for North America



Critical Load Web Resources

The below are websites/tools related to critical load resources. These resources are not products of the NADP and CLAD, but instead developed by government agencies in the US and Europe and other organizations. Any opinions, findings, conclusions, or recommendations as part of these resources do not necessarily reflect the views of CLAD, NADP, and/or respective members’ affiliations.


US Forest Service Critical Loads Video Series

An ecosystem is a community of living and nonliving things. The plants, animals, air, water, soil, and sunlight all work together to create balanced, healthy ecosystems. These ecosystems are essential life-support systems for our own well-being and livelihoods. They provide us with vital goods and services, such as fish, timber, and clean water. To maintain a healthy ecosystem, land managers must be able to predict when and where environmental threats occur on the landscape. Air pollution is one of these environmental threats. When pollutants containing nitrogen and sulfur are deposited to the environment as components of rain, snow, clouds, fog, gases, and fine particles, they can cause ecosystem harm. Scientists use the concept of critical loads to identify when an ecosystem, or a part of an ecosystem, begins to experience harm from this deposition. This video series looks at how air pollution, specifically excess nitrogen and sulfur deposition, affects species and ecosystems. It also highlights how land managers can use critical loads to assess ecosystem health and guide resource management decisions. The series includes seven videos. A playlist is available here Critical Loads Video Series Playlist and individual links below.

Geiser, L.H. and Outka-Perkins, L., editors. 2021. Air Pollution Effects on Ecosystems: Critical Loads Video Series. Script text authors: Bill Jackson, Jason Lynch, Mike Bell, Emmi-Felker Quinn, Chris Clark, Robert Sabo, Jill McMurray, Claire O’Dea. Editing and filming by Lisa O-P and Ian Grob, animated graphics by Circuit Media, production by USDA. Videos are finished and were posted by the US Forest Service Chief’s Office of Communications to YouTube and to the FS National Technology and Development Program’s on-line library.

Critical Load Web Videos

The below are links to critical load videos. These resources are not products of the NADP and CLAD, but instead developed by government agencies, Universities, and other organizations. Any opinions, findings, conclusions, or recommendations as part of these products do not necessarily reflect the views of CLAD, NADP, and/or respective members’ affiliations.

Critical Load Videos from Washington State University

Nitrogen Deposition Effects on Alpine Lakes at Mount Rainier

Nitrogen Deposition Effects on Alpine Plants at Mount Rainier