National Park Service 

U.S. Department of the Interior 

Natural Resource Stewardship and Science

Federal Land Managers’ Interagency Guidance for 

Nitrogen and Sulfur Deposition Analyses 

November 2011 

Natural Resource Report NPS/NRSS/ARD/NRR—2011/ 465 

 
ON THE COVER 

Atmospheric nitrogen deposition causes enrichment effects that may lead to simplified plant communities dominated 

by weedy species at the expense of native wildflowers, which are a source of food and shelter for some wildlife, 

including the Bay Checkerspot butterfly in California. 

Credit: U.S. Fish and Wildlife Service. 

ON THIS PAGE 

Some plant species are very sensitive to the acidifying effects of atmospheric nitrogen and sulfur deposition. The 

decline of native sugar maple trees in some areas of the eastern U.S. has been linked to high levels of acid deposition. 

Credit: National Park Service 
Federal Land Managers’ Interagency Guidance for 

Nitrogen and Sulfur Deposition Analyses 

November 2011 

Natural Resource Report NPS/NRSS/ARD/NRR—2011/ 465

U.S. Forest Service 

Air Quality Program 

1400 Independence Ave, SW 

Washington, DC 20250 

National Park Service 

Natural Resource Stewardship & Science 

Air Resources Division 

PO Box 25287 

Denver, Colorado 80225 

U.S. Fish and Wildlife Service 

National Wildlife Refuge System 

Air Quality Branch 

7333 W. Jefferson Ave., Suite 375 

Lakewood, CO 80235 

 

November 2011 

U.S. Department of the Interior 

National Park Service 

Natural Resource Stewardship and Science 

Denver, Colorado 
ii Interagency Guidance for Nitrogen and Sulfur (2011) 

The National Park Service, Natural Resource Stewardship and Science office in Denver, Colorado publishes 

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This report is available from the Air Resources Division of the NPS 

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Management Web site (http://www.nature.nps.gov/publications/nrpm/) on the Internet. 

Please cite this publication as: 

U.S. Forest Service, National Park Service, and U.S. Fish and Wildlife Service. 2011. Federal land managers’ 

interagency guidance for nitrogen and sulfur deposition analyses: November 2011. Natural Resource Report 

NPS/NRSS/ARD/NRR—2011/465. National Park Service, Denver, Colorado. 

NPS 909/111538, November 2011 
National Park Service iii

Contents 

Page 

1. Background ................................................................................................................................................................. 1

2. Deposition Analysis Thresholds (DATs) .................................................................................................................. 3

3. Refined Analysis ......................................................................................................................................................... 5

3.1. Are AQRVs in the affected area sensitive to deposition? ..................................................................................... 5

3.2. Are AQRVs in the affected area currently impacted by deposition? ..................................................................... 5

3.3. Have critical loads or target loads been developed for the affected area? ........................................................... 5

3.4. Does current deposition exceed the critical load(s) or target load(s) in the area? If so, by how much? Is current 

deposition approaching the critical load or target load? ............................................................................................... 6

4. Adverse Impact Determination .................................................................................................................................. 7

References ...................................................................................................................................................................... 9

Figures 

Page 

Figure 1. FLM Assessment of Potential Deposition Effects from New Emissions Sources.. ........................................... 2 

 

National Park Service 1

Federal lands such as Bridger-Teton National Forest have 

resources sensitive to nitrogen and sulfur deposition. 

Credit: U.S. Forest Service. 

1. Background 

Increased nitrogen (N) or sulfur (S) deposition may result 

from emissions from new or modified facilities subject to 

New Source Review (NSR), activities subject to 

requirements under the National Environmental Policy Act 

(NEPA), and other actions. This increased deposition may 

have a negative impact on air quality related values (AQRVs) 

sensitive to N or S deposition, including lakes, streams, 

estuaries, coastal waters, soils, vegetation, and wildlife. The 

Federal Land Managers (FLMs), including the National 

Park Service (NPS), the U.S. Forest Service (USFS), and the 

U.S. Fish and Wildlife Service (FWS), have developed 

guidance for evaluating the impact of additional nitrogen 

(N) or sulfur (S) deposition on lands under their 

management for NSR permit applicants and NEPA action 

lead agencies and project proponents. This guidance is 

provided in the Federal Land Managers’ Air Quality Related 

Values Workgroup Report (FLAG 2010),1

 which also 

includes guidance for evaluating visibility and ozone 

impacts from new or modified emissions sources. In FLAG, 

the FLMs anticipated that over time, more information 

would become available on ecosystem responses to N and S 

deposition and that further guidance would be needed to 

use this information in evaluating impacts of new emission 

sources. This current document summarizes existing and 

emerging deposition analysis tools and explains when and 

how these tools should be applied. It reflects the FLMs’ 

commitment to continue to develop consistent, predictable 

review processes for NSR permits and Environmental 

Assessments and/or Environmental Impact Statements 

under NEPA. The information and procedures outlined in 

this document are generally applicable to both Class I and 

Class II areas for evaluating the effect of increased N or S 

deposition. 

This current guidance describes a minor change in the FLM 

assessment process given in Figure 1. Previously, in step 2 

the FLM evaluated whether the source’s contribution to 

deposition in the FLM area was predicted to be less than the 

Deposition Analysis Thresholds (DATs) or Concern 

Thresholds established by the FLM. The DATs were used 

by the NPS and FWS; one DAT value was established for the 

eastern U.S., while another value was established for the 

western U.S. Concern Thresholds were used by the USFS 

and these values varied across USFS areas. Step 2 has been 

simplified; all FLMs have now adopted the DATs, 

eliminating the need to consider Concern Thresholds in this 

step. In addition to describing this change, the current 

guidance provides more detail on incorporating critical 

loads for deposition into the refined analysis (step 3). 

Figure 1 illustrates the recommended decision process that 

the FLM will use to determine whether a deposition impact 

analysis is warranted and, if so, to determine if predicted 

 

1

 U.S. Forest Service, National Park Service, and U.S. Fish and Wildlife 

Service. 2010. Federal land managers’ air quality related values work 

group (FLAG): phase I report—revised (2010). Natural Resource 

Report NPS/NRPC/NRR—2010/232. National Park Service, Denver, 

Colorado.

impacts are potentially adverse. As part of the process, the 

applicant2

 provides the FLM with information on the types 

and amounts of emissions and modeled deposition levels from 

the proposed source. The FLM asks the following questions: 

1. Are a source’s impacts negligible? Agencies will 

consider a source locating greater than 50 km from an 

FLM area to have negligible impacts with respect to 

AQRVs if its total SO2, NOx, PM10, and H2SO4 annual 

emissions (in tons per year, based on 24-hour maximum 

allowable emissions), divided by the distance (in km) 

from the FLM area (Q/D) is 10 or less (Q/D ≤ 10).3

 If so, 

the Agencies would not request any further AQRV 

analyses. If Q/D > 10, the FLM proceeds to the next step. 

2. Is the source’s predicted contribution to deposition (in 

kilograms per hectare per year - kg/ha/yr) in the FLM 

area less than the Deposition Analysis Threshold 

(DATs) established by the FLM? If the answer is yes, the 

source’s impacts are presumed to be negligible. If the 

answer is no, the FLM proceeds to the next step. 

3. Does the refined/contextual analysis alleviate concerns, 

e.g., indicate that the source’s impacts would not cause 

or contribute to harm to an AQRV, or that the source 

would provide some ancillary environmental benefit, as 

described in FLAG 2010?4

 If the answer is yes, the 

impacts are presumed to be not adverse. If the answer is 

no, the impact is potentially adverse and the FLM 

proceeds to the next step. 

4. Are there mitigation strategies that could alleviate the 

potential adverse impact? FLAG 2010 should be 

consulted for a more detailed discussion of the adverse 

impact determination process.5

 

The following sections describe in more detail the DATs 

and the refined analysis.

 

2

 The terms “permit applicant” or “applicant” refer to any proponent of 

a project that will result in new emissions, including NSR, NEPA, and 

other projects. 

3

 The Q/D procedure is explained in more detail in Section 3.2 of the FLAG 

2010 guidance: http://www.nature.nps.gov/air/permits/flag/index.cfm. 4 Ibid., Section 4.3.

5

 Ibid., Section 2.2.4; Section 4.3. 
2 Federal Land Managers’ Interagency Guidance for Nitrogen and Sulfur (2011) 

Figure 1. FLM Assessment of Potential Deposition Effects from New Emissions Sources (Revised from FLAG 2010). 

*Q/D test only applies to sources located greater than 50 km from an FLM area. 
National Park Service 3

Alpine meadow plant communities are very sensitive to the 

enrichment effects of nitrogen deposition. 

Credit: U.S. Forest Service. 

2. Deposition Analysis Thresholds (DATs) 

A DAT is defined as the additional amount of N or S 

deposition within an FLM area, below which estimated 

impacts from a proposed new or modified source are 

considered negligible. In other words, if the new or 

modified source has a predicted N or S deposition impact 

below the respective DAT, the FLM will consider that 

impact to be negligible, and no further analysis would be 

required for that pollutant. In cases where a source’s impact 

equals or exceeds the DAT, the FLM will make a project 

specific assessment of whether the projected increase in 

deposition would likely result in an “adverse impact” on 

resources considering existing AQRV conditions, the 

magnitude of the expected increase, and other factors. 

The DATs are based on “naturally occurring deposition” 

that park and wilderness ecosystems may have experienced 

prior to anthropogenic influences and are scaled to enable 

assessment of the impacts of individual sources of air 

pollution. Scaling is done by use of a variability factor and a 

cumulative factor applied to natural background deposition. 

The variability factor of 0.5 is based on the FLM’s decision 

that all combined anthropogenic sources could contribute 

up to 50% of natural background deposition without 

triggering concerns regarding resource impacts. The FLMs 

believe this is appropriate because the natural background 

estimates selected are very conservative (0.50 

kilograms/hectare/year (kg/ha/yr) for the eastern U.S. and 

0.25 kg/ha/yr for the western U.S. for both N and S) and the 

range of natural variability in deposition could be plus or 

minus 50%. The cumulative factor of 0.04 (1/25) is based on 

the assumption that, over time, one might expect 25 or 

fewer new sources to increase deposition in an FLM area. 

These assumptions ensure that cumulative deposition from 

all new sources would not exceed 50% of natural 

background. The DAT is then calculated as: 

DAT = Natural Background Deposition * 

 Variability Factor * Cumulative Factor 

Using this equation, the DATs for both N and S in eastern 

and western FLM areas are 0.010 and 0.005 kg/ha/yr, 

respectively. Guidance on DATs can be found at: 

http://www.nature.nps.gov/air/Permits/flag/index.cfm. The 

DATs are used by the NPS, FWS, and USFS. 

The FLMs reserve the right to modify the DAT based upon 

new scientific information or in situations in which the 

assumptions are violated. For example, as noted above, the 

cumulative factor inherent in the DAT is based upon the 

assumption that 25 or fewer new sources will add to 

deposition in an FLM area over time. However, situations 

may occur in which that assumption is not valid, because the 

number of existing and proposed new projects near an FLM 

area exceeds 25. In such situations, the FLM reserves the 

right to modify the DAT according to new information. 

If predicted deposition impacts from new or modified 

sources or projects are below the DATs, the impacts are 

considered negligible. If the predicted impacts are equal to 

or exceed the DATs, the source or project proponent 

should consult with the FLM regarding the need for a 

refined analysis to determine the severity of the impacts, 

described below. The refined analysis is based on all 

available information, including information from the 

FLMs’ websites: 

FWS: http://www.fws.gov/refuges/airquality/

NPS: http://www.nature.nps.gov/air/Permits/aris/index.cfm

USDA FS: http://www.fs.fed.us/air/index.htm

 
4 Federal Land Managers’ Interagency Guidance for Nitrogen and Sulfur (2011) 

 
National Park Service 5

3. Refined Analysis 

If the source’s contribution to deposition in an FLM area is 

predicted to equal or exceed the DAT, a refined analysis 

may be conducted. In the refined analysis, the FLM 

evaluates several questions: 

• Are AQRVs in the affected area sensitive to deposition? 

• Are AQRVs in the affected area currently impacted by 

deposition? 

• Have critical loads or target loads been developed for 

AQRVs in the area? 

• Does current deposition exceed the critical load(s) or 

target load(s) in the area? If so, by how much? Is current 

deposition approaching the critical load or target load? 

3.1. Are AQRVs in the affected area 

sensitive to deposition? 

AQRV information is available for many parts of the country 

and for various ecosystems, ranging from alpine 

environments to coastal estuaries. In general, poorly 

buffered soils, lakes, and streams, as well as certain 

vegetation types, are sensitive to acidification from N and S 

deposition. Acid-sensitive areas are often located at 

relatively high elevation, on steep slopes having shallow, 

base-poor soils. Some types of vegetation are more sensitive 

to acidification effects than others. Some lichens are 

especially sensitive, with documented effects occurring in 

the deposition range of only a few kilograms of S or N per 

hectare per year. Among the vascular plants, red spruce and 

sugar maple trees are known to be very sensitive. In aquatic 

ecosystems, acidification can alter water chemistry, causing 

harm to certain species of fish, aquatic invertebrates, and 

other biota, allowing more tolerant species to thrive, 

changing the diversity of these ecosystems. 

In addition to contributing to acidification, N deposition 

causes enrichment effects that may allow fast-growing, 

weedy plant species to thrive at the expense of native species 

that generally are adapted to low N conditions. As a 

consequence, diversity changes and the ecosystem may 

become simplified, dominated by species like annual grasses 

that can take advantage of added N. 

Existing research, monitoring and other inventory data in 

various ecosystems can give important information 

regarding a particular AQRV’s sensitivity to deposition. 

FLMs will reference research on the AQRV in question 

based on the most current and representative data. If 

sensitive AQRVs are located in the area, the FLM will 

carefully evaluate potential impacts from significant 

increases in deposition. Not all FLM areas will have sitespecific data and so the FLM may use data from another, 

similar area to evaluate sensitivity. 

3.2. Are AQRVs in the affected area 

currently impacted by deposition? 

Research and monitoring data may be available for the 

affected area, providing information on whether AQRVs are 

currently impacted by acidification or N enrichment. For 

example, lake and stream monitoring data showing 

decreased pH or acid-neutralizing capacity (ANC) may 

indicate effects from N and S acidic deposition. Absence of 

oligotrophic lichens in a forest may indicate effects from N 

enrichment. If available information indicates that AQRVs 

are currently impacted by deposition, the FLM is more 

likely to consider significant increases in deposition to 

constitute a potential adverse impact. 

3.3. Have critical loads or target loads been 

developed for the affected area? 

The term critical load is used to describe the threshold of air 

pollution deposition that causes harm to sensitive resources 

in an ecosystem. A critical load is technically defined as “the 

quantitative estimate of an exposure to one or more 

pollutants below which significant harmful effects on 

specified sensitive elements of the environment are not 

expected to occur according to present knowledge.” Critical 

loads are typically expressed in terms of kilograms per 

hectare per year (kg/ha/yr) of wet or total (wet + dry) 

deposition. Critical loads can be developed for a variety of 

AQRV and ecosystem responses, including shifts in 

microscopic aquatic species, increases in invasive grass 

species, changes in soil chemistry affecting tree growth, and 

lake and stream acidification. When critical loads are 

exceeded, the environmental effects can extend over 

significant distances. For example, excess nitrogen can 

change soil and surface water chemistry, which in turn can 

cause eutrophication of downstream estuaries. 

Critical loads are calculated using data from ecosystem 

modeling, observations, or experiments. Observations and 

dose-response experiments can identify the loading at 

which a specific response occurs, but information is often 

limited to a specific area. Modeled critical loads are also 

based on site-specific data but can, in some cases, be 

extrapolated over larger areas with more limited data. 

Critical load models typically are steady-state models, that 

is, they are used to calculate the deposition loading that 

could be sustained by a specific AQRV or ecosystem over 

the long-term without harm. Dynamic models, on the other 

hand, incorporate a time element to determine the loading 

that would result in a given AQRV or ecosystem condition 

at a particular point in time. For example, a dynamic model 

could be used to estimate the deposition reductions needed 

for recovery of an acidified stream in 20 years versus 100 

years. Loadings determined by dynamic modeling are 

termed “target loads,” and may consider policy or 

management goals.

 
6 Federal Land Managers’ Interagency Guidance for Nitrogen and Sulfur (2011) 

High elevation ecosystems are at a particular risk from nitrogen and sulfur deposition due to greater 

amounts of snow and rain, short growing seasons, steep slopes, and shallow base-poor soils that 

have limited capacity to buffer or absorb sulfur and nitrogen. 

Credit: National Park Service. 

U.S. scientists, air regulators, and natural resource managers 

are currently developing critical loads and target loads for 

areas across the United States and collaborating with 

scientists developing critical loads and target loads in 

Europe and Canada.6

 Critical and target loads are not yet 

widely available. However, if an AQRV is experiencing harm 

such as decreased pH or ANC, by definition the critical load 

for that AQRV is exceeded, even if it has not yet been 

quantified. The FLM would evaluate additional deposition 

to the area as a potential adverse impact, as described for the 

previous question. 

3.4. Does current deposition exceed the 

critical load(s) or target load(s) in the area? 

If so, by how much? Is current deposition 

approaching the critical load or target load? 

Critical loads and target loads provide valuable tools for 

quantifying effects to AQRVs. As noted above, critical loads 

define the amount of loading of N or S that an AQRV can 

tolerate over time without harm, while a target load might 

be used as an interim management goal during recovery of 

an impacted ecosystem. If critical loads are available for 

AQRVs in an area, the FLM would determine whether 

current deposition exceeds those loads. By definition, an 

exceedance of a critical load would indicate that AQRVs are 

 

6 Links to critical loads information are available at 

http://nadp.sws.uiuc.edu/clad/links.aspx.

experiencing deposition in excess of what can be sustained 

without harm, and additional deposition would be evaluated 

as a potential adverse impact. The FLM would evaluate the 

magnitude of the exceedance; a larger exceedance 

constitutes a greater threat to resources. For example, in 

acid-sensitive areas of the Southeast, including Shenandoah 

NP and several USFS wildernesses, current deposition is 

about twice the critical load estimated to sustain healthy 

stream biota (Sullivan et al. 2010). If a critical load is not 

exceeded, but current deposition is approaching the critical 

load, the FLM would evaluate the likelihood that an 

exceedance will occur in the future, either as a result of 

increased deposition from a specific source or project, or 

other growth in the region that would increase deposition. 

Similarly, if a target load has been established for AQRVs in 

an area, the FLM would determine whether current 

deposition exceeds that target. An exceedance of a target 

load would indicate that management goals for recovery and 

protection of AQRVs would not be achieved. For example, 

target loads to achieve recovery of acid-neutralizing 

capacity by the year 2100 in the most sensitive streams in 

Shenandoah NP are estimated to be in the range of less than 

0 to 6 kg/ha/yr (Sullivan et al. 2008). Current deposition 

exceeds these target loads, slowing or preventing recovery 

of streams and associated fish communities. Any significant 

increase in deposition would be evaluated as a potential 

adverse impact. 
National Park Service 7

Rare plant species found in some wetlands are at risk from nitrogen enrichment that may increase the establishment of non-native 

species, leading to the loss of rare species. 

Credit: U.S. Fish and Wildlife Service. 

4. Adverse Impact Determination 

An exceedance of the DAT in conjunction with documented 

deposition-caused impacts to an AQRV or an exceedance of 

a critical or target load does not automatically trigger an 

adverse impact determination. Rather, each proposed 

source or project is evaluated on a case-by-case basis. The 

FLM would consider all information on critical or target 

loads, current deposition, and exceedances, in conjunction 

with information on the condition and sensitivity of AQRVs 

in the affected area when evaluating a proposed new source 

or project. The FLM will review this information from the 

refined analysis and consider additional factors as described 

in Chapter 4 of FLAG 2010. These factors include air quality 

trends in the area, the expected useful life of the source, the 

stringency of the emissions limits, and any ancillary 

environmental benefits to AQRVs proposed by the source 

(e.g., from reductions in toxic contaminants). A source or 

project may choose to alleviate potential adverse impact 

concerns by obtaining emission offsets or reducing emission 

rates (FLAG 2010). All these things will be taken into 

consideration when the FLM determines whether 

deposition from the source or project constitutes a potential 

adverse impact to the affected FLM area.

 
8 Federal Land Managers’ Interagency Guidance for Nitrogen and Sulfur (2011) 

 
National Park Service 9

References 

[FLAG 2010] U.S. Forest Service, National Park Service, and 

U.S. Fish and Wildlife Service. 2010. Federal land 

managers’ air quality related values work group (FLAG): 

phase I report—revised (2010). Natural Resource Report 

NPS/NRPC/NRR—2010/232. National Park Service, 

Denver, Colorado. Available for download at 

http://www.nature.nps.gov/air/permits/flag/index.cfm. 

Guidance on nitrogen and sulfur deposition analysis 

thresholds deposition analysis thresholds guidance 

(2002). Available for download at 

http://www.nature.nps.gov/air/Pubs/pdf/flag/nsDATGui

dance.pdf. 

Sullivan, T.J., Cosby, B.J., McDonnell, T.C. 2010. Aquatic 

critical loads and exceedances in acid-sensitive portions 

of Virginia and West Virginia. Report to National Park 

Service, US EPA, USDA Forest Service, and USGS. April 

2010. 

Sullivan, T.J., Cosby, B.J., Webb, J.R., Dennis, R.L., Bulger, 

A.J., Deviney Jr., F.A. 2008. Streamwater acid-base 

chemistry and critical loads of atmospheric sulfur 

deposition in Shenandoah National Park, Virginia. 

Environmental Monitoring & Assessment 137:85–99. 

 

 

The Department of the Interior protects and manages the nation’s natural resources and cultural heritage; provides scientific 

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affiliated Island Communities. 

NPS 909/111538, November 2011 
National Park Service 

U.S. Department of the Interior 

U.S. Forest Service 

Air Quality Program 

www.fs.fed.us/air 

National Park Service 

Natural Resource Stewardship and Science 

www.nature.nps.gov/air 

U.S. Fish and Wildlife Service 

National Wildlife Refuge System 

www.fws.gov/refuges/airquality 

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