Guidance on Nitrogen and Sulfur Deposition Analysis Thresholds 

Background 

The National Park Service (NPS) and the U.S. Fish and Wildlife Service (FWS) have 

developed criteria for evaluating the contribution of additional nitrogen (N) or sulfur (S) 

to deposition within Class I areas. This document describes the equation and process by 

which Deposition Analysis Thresholds (DATs) have been developed for Class I areas. 

The NPS and FWS have developed this DAT equation in response to requests by 

permitting authorities and permit applicants to continue to develop consistent, predictable 

permit review processes, and to expedite the permit review process. In developing 

DATs, the NPS and FWS seek to further improve the process by providing a quantitative 

method with which to evaluate sulfur deposition in Class I areas. DATs for both sulfur 

and nitrogen have been developed and are presented here. 

The Clean Air Act Amendments of 1977 give Federal Land Managers (FLMs) an 

“affirmative responsibility” to protect air quality and air quality related values (AQRVs) 

within Class I areas. An AQRV is a resource that may be adversely affected by a change 

in air quality. The resource may include visibility or a specific scenic, cultural, physical, 

biological, ecological, or recreational resource identified by the FLM for a particular 

area. FLMs are responsible for reviewing air quality permit applications from proposed 

new or modified major sources near Class I areas, and determining the potential impacts, 

if any, that may result from source emissions. FLMs take into account the particular 

resources and AQRVs that would be affected; the frequency and magnitude of any 

potential impacts; and the direct, indirect, and cumulative effects of any potential 

impacts. In making these determinations, FLMs are mandated to err on the side of 

resource protection. 

Deposition-induced changes to AQRVs are of serious concern to FLMs and these 

thresholds are intended to distinguish where deposition increases may result in potentially 

adverse ecosystem stresses, as well as where the deposition increases are likely to have a 

negligible impact on AQRVs. 

Deposition Analysis Thresholds 

A DAT is the additional amount of N or S deposition within a Class I area, below which 

estimated impacts from a proposed new or modified source are considered insignificant. 

The DAT for a park or refuge will be compared with the amount of additional deposition 

resulting from a source, as modeled using CALPUFF or other appropriate models. The N 

DAT represents total N, including both wet and dry deposition. Total nitrogen includes 

NO, NO2, HNO3, NO3, NH3, and NH4. The S DAT represents total S deposition. Total 

N and total S were selected in order to be consistent with conventions used in deposition 

loading, to represent the total amount of N and S inputs received in an ecosystem and to 

be compatible with CALPUFF model outputs. 

The framework for calculating both the N and S DATs is: 

DAT = Natural Background Deposition * Variability Factor * Cumulative Factor 
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Using this framework, DATs for N and S have been calculated for the Eastern and 

Western regions of the United States, and are presented below. A discussion of each 

component used to develop this equation and DATs is also presented.

Determination of Background Nitrogen (BN) and Sulfur (BS) Deposition 

Natural background deposition was used to determine the DAT because aquatic and 

terrestrial ecosystems evolved under natural background deposition conditions. 

Therefore, some fraction of natural background deposition is likely within the range of 

natural variability for these ecosystems. 

The BN values were selected from a range of natural background deposition values 

published in peer-reviewed scientific literature, and from information provided by 

consultations with researchers (Dentener pers. comm.; Galloway et al. 1995; Galloway et 

al. 1996; NAPAP 1991; NADP 2000). The BS values were determined in the same 

manner (Bates, Lamb 1992; Bates et al. 1992; Dentener pers. comm.; Galloway et al.

1996; Galloway et al. 1982; Galloway, Whepdale 1980). From this range of deposition 

values, the values of 0.50 kg/ha/yr for the East and 0.25 kg/ha/yr for the West were 

selected for both N and S, as they fulfilled the requirements of being scientifically valid 

as well as being conservative. These values represent the low end of the regional range 

of values that are presented in estimates of regional natural background deposition. This 

conservatism is necessary in order to fulfill the mandate to err on the side of resource 

protection, and to protect air quality and AQRVs within Class I areas. A reference of all 

literature used to determine BN and BS is attached, as well as Supporting Literature 

references for all sources used in developing both DATs. 

Different BN and BS values were developed for the Eastern and Western United States. 

These separate values are based on the distinction between east and west natural 

deposition estimates made through global and national scale modeling analyses. The 

East DAT and West DAT are applicable to Class I areas located east and west of the 

Mississippi River, respectively. 

The NPS and FWS do not intend to devise methodology for assessing exact pre-industrial 

deposition throughout the United States. Currently it is not possible or necessary to 

determine natural deposition values for each Class I area. It is most appropriate to 

determine the BN and BS values on a large spatial scale, such as the Eastern and Western 

regions of the United States. This has the added advantage of allowing for a simpler 

application process for applicants. 

Use of a Variability Factor 

Once natural background deposition numbers are determined, FLMs have a responsibility 

to determine what fraction of this deposition could be added to existing natural and 

anthropogenic deposition amounts within an ecosystem and still be considered 

insignificant. The NPS and FWS selected very conservative natural background numbers 

from the range of values presented in scientific literature, and have determined that all 

combined anthropogenic sources could contribute up to 50% of this conservative natural 
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background value without triggering concerns regarding resource impacts. Rationale for 

this decision came from looking at the modeled historical deposition scenarios in the 

scientific literature, where the range of estimates for any given area are often + or – 50% 

or more between various studies. Furthermore, the range of natural variability associated 

with annual natural background deposition at any given site is unknown, but 50% above 

or below the historical mean is plausible during any given year due to fluctuations in 

climate, biotic productivity, bacterial decomposition, lightning occurrence, fire, volcanic 

activity, sea spray, and other factors. 

The NPS and FWS have determined that a total increase in deposition, from all sources 

over time, greater than fifty percent of natural background deposition would trigger 

management concerns. Therefore, the natural background value (BN or BS) is multiplied 

by 0.5, or 50%. 

Use of a Cumulative Factor 

There is an FLM concern that, over time, cumulative deposition from emissions sources 

may produce impacts upon Class I areas. It is beneficial to the FLMs, the permitting 

authority, and the applicant to determine what amount, if any, a new source could 

contribute to total deposition while having a reasonable assurance that cumulative 

deposition from all new sources would not exceed 50% of natural background. In 

developing the 1996 proposal for New Source Review Reform, the U.S. Environmental 

Protection Agency (EPA) determined that, as long as no individual source contribution 

exceeds 4% of a Class I increment, it is unlikely that the accumulation of sources over 

time will exceed that increment. The FLMs have applied the 4% value used in Class I 

increment significant impact levels to these new deposition analysis thresholds. By 

incorporating this value into the DAT equations, new sources whose modeled deposition 

amounts are below the DATs are not likely to significantly contribute to cumulative 

impacts from N or S deposition. 

Deposition Analysis Threshold Equation 

The DAT for a specific Class I area is calculated as: 

Nitrogen DAT = BN(0.5) * 0.04 

Sulfur DAT = BS(0.5) * 0.04 

Where: BN = natural background nitrogen deposition value. 

 Eastern Class I areas: BN = 0.50 kg/ha/yr 

 Western Class I areas: BN = 0.25 kg/ha/yr 

 BS = natural background sulfur deposition value. 

 Eastern Class I areas: BS = 0.50 kg/ha/yr 

 Western Class I areas: BS = 0.25 kg/ha/yr 

0.5 = Variability Factor 

0.04 = Cumulative Factor 

This equation incorporates a 0.5 Variability Factor and a 0.04 Cumulative Factor. The 

value of 0.04 represents a four percent safety factor to protect Class I areas from 

cumulative deposition impacts. BN or BS is multiplied by 0.5 to result in a value that is 

fifty percent of the natural background deposition. The NPS and FWS consider an 
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increase in deposition (resulting from all sources over time) that is greater than fifty 

percent of the BN or BS value to be a threshold that triggers management concerns. The 

use of both factors is explained in more detail below. 

Therefore, DATs for nitrogen and sulfur in Eastern and Western Class I parks and 

refuges are: 

East DAT: (0.50 kg/ha/yr N or S * 0.5) * 0.04 = 0.010 kg/ha/yr N or S 

West DAT: (0.25 kg/ha/yr N or S * 0.5) * 0.04 = 0.005 kg/ha/yr N or S 

Discussion 

The DAT is a deposition threshold, not necessarily an adverse impact threshold. The 

DAT is the additional amount of deposition that triggers a management concern, not 

necessarily the amount that constitutes an adverse impact to the environment. Both the 

NPS and the FWS utilize a case-by-case approach to permit review. Adverse impact 

determinations will be considered on a case-by-case basis for modeled deposition values 

that are higher than the DAT. This approach considers the best scientific information 

available for each park or refuge to assess existing as well as potential future deposition 

impacts. The magnitude of the deposition that an individual source would contribute as 

well as the sensitivity of the ecosystem must be considered. At present there is no 

equation that would, in all situations, allow an FLM to determine whether or not a source 

of N or S deposition would cause or contribute to an adverse impact. Therefore, FLMs 

will continue to use scientific data and information, in conjunction with modeling, to 

evaluate whether or not an adverse impact would occur. FLMs must also take into 

account site-specific information for each Class I area. This would include evaluating the 

potential deposition impacts from a source not just in relation to the DAT, but with other 

factors as well, such as whether adverse impacts resulting from deposition have been 

documented, or are suspected, in that specific Class I area. 

Coastal ecosystems have evolved under naturally higher sulfur deposition rates due to 

contribution from oceanic sources. This factor will be considered by the NPS and FWS 

when making the case-by-case determination as to whether S deposition from a proposed 

source will adversely impact a Class I area containing coastal ecosystems. 

While the values used in the DAT equation reflect current NPS/FWS guidance and the 

scientific information available, it is important to note that these values could be updated 

as new changes in effects-related information becomes available. These DATs replace 

any previous screening level values or deposition thresholds that may have been utilized 

by the NPS or FWS prior to the development of these DATs. The NPS and FWS will 

work closely with permit applicants to implement these DATs, and applicants are 

encouraged to contact the NPS or FWS at all stages of the application process. 
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References

Literature used to determine the BN values:

Dentener, F. J. Personal communication with Tamara Blett, National Park Service. 

Globally modeled nitrogen maps for 1860. April 19, 2001. 

Galloway, J.N., W.H. Schlesinger, H. Levy II, A. Michaels, and J.L. Schnoor. 1995. 

Nitrogen fixation: Anthropogenic enhancement – environmental response. Global 

Biogeochemical Cycles 9(2): 235-252. 

Galloway, J.N., W.C. Keene, and G.E. Likens. 1996. Processes controlling the 

composition of precipitation at a remote Southern hemisphere location: Torres del Paine 

National Park, Chile. Journal of Geophysical Research 101(D3): 6883-6897. 

National Atmospheric Deposition Program (NRSP-3)/National Trends Network 2000. 

Illinois State Water Survey. NADP/NTN Coordination Office, 2204 Griffith Drive, 

Champaign, IL 61820. 

U.S. National Acid Precipitation Assessment Program: Acidic Deposition: State of 

Science and Technology. 1991. P.M. Irving, ed. Volume I: Emissions, Atmospheric 

Processes and Deposition. Washington D.C. 

Literature used to determine the BS values:

Bates, T.S. and B.K. Lamb. 1992. Natural sulfur emissions to the atmosphere of the 

continental United States. Global Biogeochemical Cycles, Vol 6(4):431-435. 

Dentener, F. J. Personal communication with Tamara Blett, National Park Service. 

Modeling data for global sulfur deposition in 1860. July 9, 2001. 

Galloway, J.N., W.C. Keene, and G.E. Likens. 1996. Processes controlling the 

composition of precipitation at a remote southern hemispheric location: Torres del Paine 

National Park, Chile. Journal of Geophysical Research Vol. 101(D3): 6883-6897. 

Galloway, J.N., G. E. Likens, W.C. Keene, and J.M. Miller. 1982. The composition of 

precipitation in remote areas of the World. Journal of Geophysical Research Vol. 87(11): 

8771-8786. 

Galloway, J.N., and D.M. Whepdale. 1980. An atmospheric sulfur budget for Eastern 

North America. Atmospheric Environment Vol. 14: 409-417. 
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National Atmospheric Deposition Program (NRSP-3)/National Trends Network 2000. 

Illinois State Water Survey. NADP/NTN Coordination Office, 2204 Griffith Drive, 

Champaign, IL 61820. 

Supporting Literature used in developing N and S DATs:

Bates, T.S., B.K. Lamb, A. Guenther, J. Dignon, and R.E. Stoiber. 1992. Sulfur emissions 

to the atmosphere from natural sources. Journal of Atmospheric Chemistry 14:315-337. 

Bower, J.L. and I. Valiela. 2001. Historical changes in atmospheric nitrogen deposition to 

Cape Cod, Massachusetts, USA. Atmospheric Environment 35: 1039-1051. 

Dentener, F.J. and P.J. Crutzen. 1994. A global 3D model of the ammonia cycle. Journal 

of Atmospheric Chemistry 19: 331-369. 

Fenn, M.E., M.A. Poth, J.D. Aber, J.S. Baron, B.T. Bormann, D.W. Johnson, A.D. Lenly, 

S.G. Mcnulty, D.F. Ryan and R. Stottlemeyer. 1998. Nitrogen excess in North American 

ecosystems: Predisposing factors, ecosystem responses, and management strategies. 

Ecological Applications 8(3): 1039-1051. 

Hedin, L.O., J.J. Amestro, and A.H. Johnson. 1995. Patterns of nutrient loss from 

unpolluted old growth temperate forests: Evaluation of biogeochemical theory. Ecology 

76(2): 493-509. 

Holland, E.A., B.H. Braswell, T.F. Lamasque, A. Townsend, T. Sulzman, T.F. Muller, F. 

Dentener, G. Brasseur, H. Levy II, T.E. Penner, and G.T. Roelofs. 1997. Variations in the 

predicted spatial distribution of atmospheric nitrogen deposition and their impact on 

carbon uptake by terrestrial ecosystems. Journal of Geophysical Research, 

102(D13):15849-15866. 

Kasgnoc, A.D. 1998. A time averaged inventory of subaerial volcanic sulfur emissions. 

Journal of Geophysical Research 103(25):251-262. 

Lelieveld, J.and F. Dentener. 2000. What controls tropospheric ozone? Journal of 

Geophysical Research: 3531-3551. 

U.S. National Acid Precipitation Assessment Program: Acidic Deposition: State of 

Science and Technology. 1991. P.M. Irving, ed. Volume I: Emissions, Atmospheric 

Processes and Deposition. Washington D.C. 

Van Aardenne, J. A., F.J. Dentener, C.G.M. Klijn Goldewijk, J. Lelieveld, and J.G. J. 

Olivier. A 1 ̊-1 ̊ resolution dataset of historical anthropogenic trace gas emissions for the 

period 1890-1990. Submitted to Global Biogeochemical Cycles, 1999. 

Vitousek, P.M., J. Piber, R.W. Howarth, G.E. Likens, P.A. Matson, D.W. Schindler, W.S. 

Scheslinger, and G.D. Tilman. 1997. Human Alteration of the Global Nitrogen Cycle: 
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Causes and Consequences. Issues in Ecology (1), Spring 1997. Ecological Society of 

America.