Critical Loads - Frequently Asked Questions

Question: What should I do if my Forest is located in an area with incomplete or unavailable national critical loads data (e.g., Alaska, Puerto Rico, arid SW)?

Response: The Air Quality Portal for Land Management Planning does not host critical load and critical load exceedance information for Alaska or Puerto Rico, due to an absence of modeled deposition estimates across the landscape.  The "National_Critical_Load_Exceedances.mxd" map has a "Monitoring Locations" layer that provides the locations of deposition monitors in Alaska and Puerto Rico.  Even if deposition has not been modeled for your area, measured data can be used to calculate critical load exceedances.  Review the attached guidance document, the published regional CLs for your region, and available sample specialist reports for neighboring forests to obtain relevant CL estimates for your forest.  Empirical CLs for nutrient nitrogen might be available by following the methodology outlined in the Empirical CLs for nutrient nitrogen user’s guide.  Finally, if the above suggestions have been unsuccessful, consider contacting your Air Program Regional Manager for assistance tracking down additional information.


Question: Are critical loads and target loads set through the forest planning process compatible with Federal Land Managers' Air Quality Related Values Work Group (FLAG) and/or FLM Interagency Guidance for Nitrogen and Sulfur Deposition Analyses documents?  Are we setting conflicting policy with the Chief’s signature on the FLAG/DAT process?  Or Does FLAG allow regional policy adjustments for deposition?

Response: The critical load implementation strategy located on the Air Quality Portal for Land Management Planning provides guidance for using critical loads, deposition, and exceedance information to help describe air pollution effects to natural resources.  The strategy also provides for the use of different types of data (from the national critical load database or more localized critical load efforts) in the planning process.  The strategy could be used to establish CLs specific to Class I areas, where none currently exist.  There is nothing in the strategy that would limit the Federal Land Manager’s ability to use critical loads that have been previously defined for Class I areas.  The strategy includes a reminder that Forests should determine whether CLs have been calculated and used in Class I areas, and incorporate this information into their planning documents.

In addition, there is nothing in the FLAG and Interagency Deposition Analyses documents that conflicts with the critical load strategy developed to support land management planning.  In fact, the 2010 FLAG guidance specifically includes a statement that “FLAG does not preclude more refined or regional analyses being performed under NEPA or other programs” (footnote on page ix of FLAG 2010).


Question: We have very limited measured deposition (wet and/or dry) in our area, and we are concerned that the modeled deposition estimates are underestimating the true exposure.  How should we account for this in our assessment of critical load exceedance?

Response: This is a problem for many arid areas where wet deposition is limited and dry deposition monitors may be infrequent or not representative of the ecosystem or vegetation of concern.  Without measurements of deposition it is difficult to validate modeling results.  Until the Forest has identified and implemented some type of dry deposition monitoring, use the most sensitive CL available to determine if critical load exceedance might be occurring.  This will likely lead to a “no exceedance” determination, along with a recommendation to track deposition trends until the next planning period.  However for arid areas we also recommend that the forest develop a monitoring plan.  Usually it is most cost effective to identify the sensitive resources and receptors and measure those for indications of negative effects.  More tips are available in the “Monitoring Strategy”.


Question: Some of the guidance documents use words like “small” or “large” that could be interpreted in multiple ways, without providing quantitative instructions.  How should I proceed?

Response: If directions are left open to interpretation, as in the examples listed above, you may use your professional judgment.  Uncertainty estimates have not yet been calculated for critical loads estimates; we are leaving room to qualitatively incorporate uncertainty until this information can be quantified.  Please feel free to direct questions to the Chair of the Deposition Focused Air Resource Management Team, listed on the Contacts Page.


Question: Are there multiple critical loads for forested ecosystems?

Response: Yes, the national critical load database contains a critical load of acidification as well as an empirical critical load for nutrient nitrogen specific to forests.  These critical loads describe different pollutant effects to forested ecosystems.


Question: I have deposition data that I think is more representative of the forest than what is modeled or posted by NADP.  Can this data be used for the Forest Plan revision air quality assessment?

Response: Some forests may have access to modeled deposition at a finer grid resolution, especially if they are cooperating with research scientists or regional air quality planning efforts.  Forests may also have additional measured deposition data (e.g., bulk deposition, snow pack measurements, throughfall sampling) associated with research or special monitoring programs.  Any forest may use different deposition data, as long as they justify the use of this data and provide documentation on the process used to create the deposition estimates.  The CL Implementation Strategy provides guidance on how to use alternate deposition estimates, together with critical loads data, to calculate critical loads exceedances for use in land management planning.



This page will be updated as more questions are received about the Air Quality Portal, and about the process of calculating and using critical loads and target loads for land management planning.  Additional questions should be directed to the Deposition Focused Air Resource Management Team (current Chair: Claire O’Dea,

For additional responses to frequently asked questions about critical loads, please visit: //