Critical Loads - Implementation Strategy

A critical load (CL) is the level of atmospheric deposition below which no detrimental ecological effects occur over the long term, based on current scientific knowledge. 

The Critical Loads section of the Air Quality Portal for Land Management Planning hosts information for the following critical loads data sets: 1) nationwide CLs of acidity for surface waters, 2) nationwide terrestrial CLs of acidity for forested ecosystems, and 3) nationwide empirical CLs of nutrient nitrogen for several ecosystem receptors and responses (fungi, lichens, herbaceous vegetation, forests, nitrate leaching).  Information for Alaska and Puerto Rico are not available for the terrestrial CLs of acidity for forested ecosystems.  The critical loads included in this strategy are thresholds for sulfur and nitrogen deposition.  The Forest Service hopes to eventually incorporate critical levels for mercury and ozone deposition into this assessment process. 

The portal also hosts information on atmospheric deposition.  Nitrogen and sulfur deposition occur as wet deposition (rain and snow), dry deposition (gases and particles), and occult deposition (cloud and fog).  Deposition information is measured and modeled from a variety of sources.  The CL exceedances provided on the portal are based on the most recent three-year average estimates produced by the NADP Total Deposition Science Committee (TDEP) v2014.01 because these estimates incorporate both measured and modeled information.  We are continuously working to refine nitrogen and sulfur deposition estimates, and will therefore accept exceedance calculations using more current regional deposition data sets, if available and appropriate.  All deposition datasets must reflect total deposition (wet and dry) for CL exceedance calculation, because CLs are set as thresholds for total deposition, not wet or dry deposition on their own. 

The CLs and deposition data are used together to determine areas of CL exceedance.  Examining CL exceedance is a “risk assessment” to evaluate the risk of sulfur and/or nitrogen deposition causing harm to the resource/ecosystem of concern.  Exceedance is calculated as:

    Exceedance = Deposition – Critical Load

If deposition equals or exceeds the critical load, the pollutant is likely causing harm to the ecosystem, or will cause harm in the future.  If pollutant exposure is less than the CL, adverse ecological effects are not anticipated, and recovery is expected over time if an ecosystem has been damaged by past exposure.  A CL exceedance is a measure of pollutant exposure above the CL.  This means the pollutant exposure is higher than, or “exceeds,” the CL and the ecosystem continues to be exposed to damaging levels of pollutants.  In other words, deposition must be below the CL in order to protect the ecosystem.  Exceedance calculations based on the 2010-2012 Total Deposition Science Committee (TDEP) deposition data are provided on the portal for the three nationwide CL assessments described above.

The following strategy outlines the process, or steps, for incorporating the concept of critical load exceedance into the air quality assessments for Forest Plan Revisions.  If a forest manages any wilderness with Class I area designation, critical loads may be established for air quality related value (AQRV) protection through this process and these may differ from CLs on the remainder of the forest.  Refer to the Air Quality Portal for Land Management Planning for the supporting documentation and details necessary to complete the assessment. 

Remember: Planning Rule Directives also require the assessment of additional air quality resources not discussed in this strategy (e.g., ozone and visibility).  We hope to eventually provide guidance for these assessments on the portal.

Proceed to Step 1 of the Critical Load Implementation Strategy: Initial critical load exceedance screening.