What is a Critical Load?
Air pollution emitted from a variety of sources is deposited from the air into ecosystems. These pollutants may cause ecological changes, such as long-term acidification of soils or surface waters, soil nutrient imbalances affecting plant growth, and loss of biodiversity. The term critical load is used to describe the threshold of air pollution deposition below which harmful effects to sensitive resources in an ecosystem do not occur. Critical loads are based on scientific information about expected ecosystem responses to a given level of atmospheric deposition. For ecosystems that have already been damaged by air pollution, critical loads help determine how much improvement in air quality would be needed for ecosystem recovery to occur. In areas where critical loads have not been exceeded, critical loads can identify levels of air quality needed to maintain and protect ecosystems into the future.
U.S. scientists, air regulators, and natural resource managers have developed critical loads for areas across the United States through collaboration with scientists developing critical loads in Europe and Canada. Critical loads can be used to assess ecosystem health, inform the public about natural resources at risk, evaluate the effectiveness of emission reduction strategies, and guide a wide range of management decisions.
Critical Loads Assessment for Land Management Planning
The Forest Service is incorporating critical loads into the air quality assessments performed for Forest Plan revision. The Air Program has developed this section of the Air Quality Portal to guide users through an assessment of critical loads of air pollution. The following components outline the assessment process and provide the information necessary for successful completion:
- Background: Briefing papers and other communications materials are provided to improve both internal and external communication about the effects of atmospheric deposition on ecosystems.
- CL Implementation Strategy: A decision tree is provided to outline the process for incorporating the concept of critical load exceedance into the air quality assessments for Forest Plan Revisions.
- Target Load Strategy: A decision tree is provided to outline the process for developing target loads on a National Forest, when CL exceedances have been determined to be significant and reliable.
- Monitoring Strategy: A decision tree is provided to outline guidance specific to the resource and deposition data collection needs for improving CL estimates.
- Data: Spatial GIS data is provided for nationwide critical load calculations, CL exceedance calculations, deposition estimates, and relevant land boundaries. This data should be used along with the strategy document to conduct an assessment of critical load exceedances for the Forest Plan air quality assessment.
- Protocols: Documents outlining guidance and protocols for calculating critical loads, critical load exceedances, and target loads, along with other aspects of the critical loads assessment are provided for those seeking detailed instructions.
- Management Strategy: This document outlines some of the management options available to reduce critical load exceedances and mitigate the effect of air pollution on national forests.
- Glossary: Definitions for technical terms used in the Critical Loads section of the Air Quality Portal are provided on this page.
- Training: Training has been developed to assist users in performing the critical loads assessment.
- Frequently Asked Questions: Answers to common questions about the air quality assessment process for Land Management Planning have been posted for future reference. Additional questions should be directed to the Deposition Focused Air Resource Management Team.
Other sections of this portal incorporate remaining aspects of air quality assessment.