National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 

Technical Guidance on Assessing Impacts to Air 

Quality in NEPA and Planning Documents 

Natural Resource Report NPS/NRPC/ARD/NRR—2010/271 

 
ON THE COVER 

Hiker photographing distant vistas in the Needles District at Canyonlands National Park, Utah. 

Credit: National Park Service. 

ON THIS PAGE 

Good (top) and poor (bottom) visibility at Yosemite National Park, California. 

Credit: National Park Service. 
Technical Guidance on Assessing Impacts to Air 

Quality in NEPA and Planning Documents 

Natural Resource Report NPS/NRPC/ARD/NRR—2010/271 

Natural Resource Program Center 

Air Resources Division 

PO Box 25287 

Denver, Colorado 80225 

 

December 2010 

U.S. Department of the Interior 

National Park Service 

Natural Resource Program Center 

Denver, Colorado 
The National Park Service (NPS), Natural Resource Program Center publishes a range of reports that 

address natural resource topics of interest and applicability to a broad audience in the NPS and others in 

natural resource management, including scientists, conservation and environmental constituencies, and the 

public. 

The Natural Resource Report Series is used to disseminate high-priority, current natural resource 

management information with managerial application. The series targets a general, diverse audience, and 

may contain NPS policy considerations or address sensitive issues of management applicability. 

All manuscripts in the series receive the appropriate level of peer review to ensure that the information is 

scientifically credible, technically accurate, appropriately written for the intended audience, and designed 

and published in a professional manner. This technical guidance has undergone review by the NPS Air 

Resources Division, the Environmental Quality Division, and the Natural Resource Program Center’s 

Planning Technical Advisory Group, along with the Department of the Interior Solicitor’s Office. It consists 

of technical air quality information subsequent to, and consistent with, the intentions and purpose of the 

Federal Land Managers’ Air Quality Related Values Work Group Report (commonly referred to as FLAG 

2010 [USFS, NPS, and FWS 2010]), which underwent formal public notification (Federal Register Notice) 

and review. 

Views, statements, findings, conclusions, recommendations, and data in this report do not necessarily reflect 

views and policies of the National Park Service, U.S. Department of the Interior. Mention of trade names or 

commercial products does not constitute endorsement or recommendation for use by the U.S. Government. 

This report is available from the Air Resources Division of the NPS (www.nature.nps/air/planning) and the 

Natural Resource Publications Management Web site (http://www.nature.nps.gov/publications/NRPM) on 

the Internet. 

Please cite this publication as: 

National Park Service, Air Resources Division. 2010. Technical guidance on assessing impacts to air quality 

in NEPA and planning documents. Natural Resource Report NPS/NRPC/ARD/NRR—2010/271. National 

Park Service, Denver, Colorado. 

NPS 999/106176, December 2010 
National Park Service iii

Contents 

1. Purpose ....................................................................................................................................................................... 1

2. Legal Framework for Air Quality Assessment ......................................................................................................... 2

2.1. NPS Organic Act and Wilderness Act ................................................................................................................... 2

2.2. Clean Air Act .......................................................................................................................................................... 2

2.2.1. National Ambient Air Quality Standards (NAAQS) .......................................................................................... 2

2.2.2. Clean Air Act Conformity Requirements for Nonattainment Areas ................................................................ 3

2.2.3. Prevention of Significant Deterioration (PSD) Program .................................................................................. 3

2.2.4. Protection of Visibility ....................................................................................................................................... 3

2.2.5. Federal, State, and Local Pollution Emissions Requirements and Measures ................................................. 4

3. Management Framework ........................................................................................................................................... 5

3.1. NPS Management Policies .................................................................................................................................... 5

3.2. Air Quality Analysis Under NEPA .......................................................................................................................... 6

4. Impact Assessments and Impairment of Human Health, Visibility, 

 and Natural Resources from Air Pollutants ............................................................................................................. 7

4.1. Air Quality Background .......................................................................................................................................... 7

4.2. Assessing Air Quality Impacts ............................................................................................................................... 7

4.2.1. Determine the Level of Analysis ...................................................................................................................... 7

4.2.2. Obtain Air Quality Information ......................................................................................................................... 9

4.2.3. Assess Impact Levels ...................................................................................................................................... 9

4.3. Assessing Impacts to Human Health .................................................................................................................. 11

4.3.1. Health Effects Thresholds.............................................................................................................................. 11

4.4. Assessing Impacts to Air Quality Related Values from Airborne Pollutants ....................................................... 12

4.4.1. Ozone Effects Thresholds ............................................................................................................................. 14

4.4.2. Deposition Impacts Thresholds ..................................................................................................................... 14

4.4.3. Visibility Impacts Thresholds ......................................................................................................................... 14

4.5. Impairment Considerations ................................................................................................................................. 14

References .................................................................................................................................................................... 16

 
iv Air Quality Impairment Guidance (2010) 

Tables 

Table 1. Emission analysis screening thresholds for attainment areas ....................................................................... 8

Table 2. Examples of emissions levels calculated for selected activities in parks ...................................................... 9

Table 3. Air quality information sources ..................................................................................................................... 10

Table 4. Assessing cumulative impacts to human health .......................................................................................... 11

Table 5. Assessing impacts of proposed actions to air quality and AQRVs .............................................................. 13

 
National Park Service 1

Photo of healthy (top) and ozone-injured (bottom) ponderosa pine 

needles from Southwest U.S. 

Credit: National Park Service. 

1. Purpose 

This paper provides an overview of approaches to assess 

impacts to air quality and resources affected by air pollution. 

It specifically applies to projects assessed under the 

National Environmental Policy Act (NEPA) and other 

planning initiatives affecting the National Park System. This 

technical air quality guidance for park staff, and technical 

specialists is intended for use in developing initial air 

pollution emissions estimates, determining the appropriate 

level of air quality analysis, assessing whether air pollution 

impacts are likely, and describing the degree and severity of 

those impacts to resources in National Park System units. It 

identifies technical air quality modeling and analysis tools 

and information resources that are available to conduct air 

quality impact assessments on park resources. This guidance 

is also intended to provide National Park Service (NPS) 

managers with air quality information for consideration 

when assessing compliance with the no-impairment 

mandate of the NPS Organic Act (NPS Management 

Policies 2006; Section 1.4.5). 

This guidance was developed to facilitate assessment of air 

pollution impacts to natural resources on NPS lands. While 

approval of projects generating air pollutants within 

National Park System boundaries is within the purview of 

the NPS, it is acknowledged that many of the pollution 

sources affecting parks are located outside park boundaries. 

For these projects NPS staff can work with other agencies, 

industries, or groups with project approval oversight to 

communicate impact and impairment concerns. 

While this document provides guidance on determining 

impairment under the NPS Organic Act, it does not establish 

a “bright line” for the amount of air pollution or air quality 

impacts which are acceptable or unacceptable. It is clearly 

inappropriate to consider all actions that fail to rise to the 

level of impairment as desirable or even acceptable. 

National Park System units are not “managed to” the 

threshold of being impaired. On the contrary, they are 

managed with the intent of being fully functional 

landscapes, worthy of being considered “special places” to 

the American people. 

This document is subject to revision in the future as 

additional planning guidance is developed within NPS. 

Material in this technical guidance has been adapted from 

an earlier document, Interim Technical Guidance on 

Assessing Impacts and Impairments to Natural Resources

(NPS 2003). The 2003 interim guidance document also 

encompasses other natural resources of concern (e.g., 

biological, geologic, watersheds, lightscapes, soundscapes, 

and ecosystems). 

The policies and recommendations contained within this 

document are only intended to facilitate the decision 

making process for evaluating the severity of air quality 

impacts from internal and external activities, and improve 

the internal air resources management of the NPS. These 

policies and recommendations are not intended to, and do 

not, create any right or benefit, substantive or procedural, 

enforceable at law or equity by a party against the United 

States, its departments, agencies, instrumentalities or 

entities, its officers or employees, or any other person. 
2 Air Quality Impairment Guidance (2010) 

Wet deposition monitoring at Acadia National Park, Maine. 

Credit: National Park Service.

2. Legal Framework for Air Quality Assessment 

2.1. NPS Organic Act and Wilderness Act 

The National Park Service Organic Act of 1916 states that 

the NPS: 

[S]hall promote and regulate the use of the Federal areas 

known as national parks, monuments, and reservations 

hereinafter specified ... by such means and measures as 

conform to the fundamental purpose of the said parks, 

monuments, and reservations, which purpose is to 

conserve the scenery and the natural and historic objects 

and the wild life therein and to provide for the 

enjoyment of the same in such manner and by such 

means as will leave them unimpaired for the enjoyment 

of future generations (16 U.S.C. §1; italics added). 

Congress reaffirmed this mandate in 1978 when it directed 

the following: 

The authorization of activities shall be construed and the 

protection, management, and administration of these 

areas shall be conducted in light of the high public value 

and integrity of the National Park System and shall not 

be exercised in derogation of the values and purposes for 

which these various areas have been established, except 

as may have been or shall be directly and specifically 

provided by Congress (Act amending the Act of October 

2, 1968 [commonly called Redwoods Act], March 27, 

1978, P.L. 95–250, 92 Stat. 163, 16 USC §§1a–1, 79a–q).

The no-impairment mandate of the Organic Act is one of 

many legal requirements managers must consider and 

comply with when authorizing activities in parks. In some 

cases, requirements of air quality or other environmental 

laws and regulations might prohibit certain impacts on 

natural resources or values, irrespective of whether NPS 

managers would consider the impacts to rise to the level of 

“impairment.” In other cases, impacts technically allowed by 

law might be prohibited in a park because they would be 

considered by NPS managers to be an impairment of park 

resources. Generally, the most stringent test should be 

applied prior to approving an activity. 

The Wilderness Act of 1964 defines wilderness as: 

[A]n area where the earth and its community of life are 

untrammeled by man, where man himself is a visitor who 

does not remain ... an area of undeveloped Federal Land 

retaining its primeval character and influence ... which 

is protected and managed so as to preserve its natural 

conditions (16 U.S.C. 1131[c]). 

In many cases, the specific language of the Wilderness Act 

may prohibit activities before an impairment determination 

must be made, thereby making an impairment decision 

unnecessary. In other cases, the Wilderness Act may provide 

supporting legal context for the impact assessment or 

impairment decision. 

In addition to avoiding impairment, NPS managers must 

always seek ways to avoid, or to minimize to the greatest 

degree practicable, adverse impacts on park resources and 

values. However, the laws do give NPS the management 

discretion to allow certain impacts to park resources and 

values when necessary and appropriate to fulfill the 

purposes of a park, so long as the impact does not constitute 

impairment of the affected resources and values. 

2.2. Clean Air Act 

2.2.1. National Ambient Air Quality Standards (NAAQS) 

NPS air resource management policy has been developed in 

conjunction with requirements in the Clean Air Act (CAA) 

and the Environmental Protection Agency’s (EPA’s) 

regulations. The level of protection afforded some park 

resources and values by the CAA may be the determining 

factor when deciding whether air quality impacts are 

acceptable. Air pollution sources within park boundaries, 

must, by law, comply with all federal, state, and local 

regulations to the same extent as other entities. Air pollution 

sources outside park boundaries are also subject to varying 

federal, state, and local regulations depending upon land 

ownership and the type and size of pollution source. The 

CAA established National Ambient Air Quality Standards 

(NAAQS) to protect the public health and welfare from air 

pollution. Mitigation measures would likely be required 

under the CAA if emissions from an activity caused or 
National Park Service 3

contributed to a NAAQS violation. This issue is discussed 

further under the “conformity requirements” section below. 

The NAAQS describe thresholds for monitored air 

chemistry concentrations of six “criteria pollutants:” 

nitrogen dioxide (NO2); sulfur dioxide (SO2); carbon 

monoxide (CO); lead (Pb); ozone (O3); and particulate 

matter (PM10 and PM2.5). Threshold concentrations for 

these pollutants designed to protect human health are called 

“primary standards” and are intended to protect human 

health rather than natural resources (see Table 3 for a link to 

the NAAQS). EPA has also established “secondary” NAAQS 

to protect public welfare, including ecosystems. However, in 

most cases the secondary NAAQS are identical to the 

primary NAAQS and are not protective of sensitive 

ecosystems. EPA is currently reviewing and revising 

secondary NAAQS to provide appropriate protection to 

natural resources. Therefore, in addition to comparing air 

quality monitored or modeled values to the NAAQS, NEPA 

analyses for National Park System units will involve specific 

natural resource impacts analyses, as described in Section 

4.4. 

2.2.2. Clean Air Act Conformity Requirements for 

Nonattainment Areas 

Areas of the country that do not meet the NAAQS for any 

pollutant are designated as “nonattainment areas.” Areas 

that were once designated nonattainment, but are now 

achieving the NAAQS are termed “maintenance areas.” 

Areas which have air pollution levels below the NAAQS are 

termed “attainment areas.” In nonattainment areas, states 

must develop plans to reduce emissions and bring the area 

back into attainment of the NAAQS. There are stringent 

requirements for activities conducted by federal agencies in 

nonattainment and maintenance areas, to ensure that 

proposed pollution increases from new activities will not 

impede a state’s ability to achieve the NAAQS in the future. 

Therefore, when parks are assessing project emissions for 

potential impacts, it is important for a park to first 

determine whether it is located in a nonattainment or 

maintenance area. 

Parks located in areas that exceed the NAAQS 

(nonattainment areas) or whose resources are already being 

adversely affected by current ambient air quality levels 

require a greater degree of consideration and scrutiny when 

management actions are considered by NPS managers. 

Section 176 of the CAA states: 

No department, agency, or instrumentality of the 

Federal Government shall engage in, support in any way 

or provide financial assistance for, license or permit, or 

approve, any activity which does not conform to an 

[State] implementation plan ... [T]he assurance of 

conformity to such a plan shall be an affirmative 

responsibility of the head of such department, agency or 

instrumentality. 

In making decisions regarding activities or projects within a 

designated nonattainment area, park managers should 

discuss their plans with the appropriate state air pollution 

control agency to determine the applicability of conformity 

requirements. 

2.2.3. Prevention of Significant Deterioration (PSD) 

Program 

The CAA also established the Prevention of Significant 

Deterioration (PSD) of Air Quality program to protect the 

air in relatively clean areas. One purpose of the PSD 

program is to protect public health and welfare, including 

natural resources, from adverse effects that might occur 

even though NAAQS are not violated. Another purpose is to 

preserve, protect, and enhance the air quality in national 

parks, national wilderness areas, national monuments, 

national seashores, and other areas of special national or 

regional natural, recreational, scenic or historic value 

(42 U.S.C. 7401 et seq.). The PSD program includes a 

classification approach for controlling air pollution. Class I 

areas are afforded the greatest degree of air quality 

protection. Very little deterioration of air quality is allowed 

in these areas. Class I areas include international parks, 

national wilderness areas and national memorial parks in 

excess of 5,000 acres, and national parks in excess of 6,000 

acres that were in existence as of August 7, 1977, when the 

CAA was amended. Currently, there are 48 areas in the 

National Park System designated as Class I. NPS areas that 

are not designated Class I are Class II, and the CAA allows 

only moderate air quality deterioration in these areas. 

However, pollution increases causing a violation of any of 

the NAAQS are not permissible in Class I or Class II areas. 

The PSD regulatory program generally consists of 

permitting and planning requirements to limit air quality 

deterioration and to prevent adverse impacts on Air Quality 

Related Values in Class I areas. The PSD program focuses 

primarily on large stationary sources of air pollution which 

would be located outside of park boundaries. 

Under the PSD program, the park superintendent and the 

assistant secretary for Fish and Wildlife and Parks (the 

Federal Land Manager (FLM) for the U.S. Department of 

the Interior) have an affirmative responsibility to protect 

visibility and all other Class I area Air Quality Related Values 

from the adverse effects of air pollution. A new stationary 

pollution source proposing to locate near a Class I area must 

apply for a PSD permit from the appropriate regulatory 

agency, most often the state. The park superintendent, with 

technical assistance from the NPS Air Resources Division, 

then reviews the permit proposal for potential adverse 

impacts to park resources and provides comments to the 

permitting authority regarding permit conditions and 

approval of air pollution emissions from that source. Given 

the CAA goal and NPS resource protection objectives, the 

NPS may object to permits being issued for construction of 

new pollution sources outside park boundaries if these 

sources will contribute to adverse impacts within parks. 

Regardless of classification for PSD permit review purposes 

into Class I or Class II areas, all parks enjoy the same level of 

Organic Act protection, and the impact levels for NEPA 

project review listed below should be applied consistently 

regardless of Class I or Class II area designation. 

2.2.4. Protection of Visibility 

Beyond the NAAQS and PSD programs, the CAA 

established a national goal of preventing any future, and 

remedying any existing, human-made visibility impairment 
4 Air Quality Impairment Guidance (2010) 

in Class I areas. “Visibility impairment” under the CAA 

visibility protection regulations is defined as “any humanly 

perceptible change in visibility.” Here it is important to note 

that “visibility impairment” carries a specific regulatory 

meaning which does not necessarily correspond to the 

meaning of impairment under the Organic Act. While the 

Clean Air Act has a very low threshold for “visibility 

impairment” (any change), additional considerations come 

into play in determining whether the impact is adverse. 

EPA’s visibility protection regulations define an adverse 

impact on visibility as: 

visibility impairment which interferes with the 

management, protection, preservation, or enjoyment of 

the visitor's visual experience of the Federal Class I area. 

This determination must be made on a case-by-case 

basis taking into account the geographic extent, 

intensity, duration, frequency and time of visibility 

impairments, and how these factors correlate with 

(1) times of visitor use of the Federal Class I area, and 

(2) the frequency and timing of natural conditions that 

reduce visibility. This term does not include effects on 

integral vistas (40 C.F.R. 51.301). 

Therefore, the CAA “visibility impairment” definition may 

be considered by some to be more stringent than the 

impairment prohibited by the Organic Act. However, when 

“visibility impairment” interferes with the management, 

protection, preservation, or enjoyment of visitor’s visual 

experience in any park unit, it may constitute an adverse 

impact that is unacceptable under the Organic Act. 

All National Park System units experience “visibility 

impairment” under the CAA definition. Recognizing that 

visibility degradation occurring in parks has resulted from 

the cumulative effect of numerous sources, large and small, 

nearby and far away, both the NPS and EPA’s regional haze 

programs focus on sources that exceed certain impact 

thresholds (e.g., 5% change in extinction constitutes a 

significant contribution to visibility impairment; 10% 

change constitutes visibility impairment). These criteria are 

also useful when assessing impacts from projects, plans, and 

activities in NEPA documents. 

In 1999, EPA finalized the regional haze regulations which 

require states to develop plans for making reasonable 

progress toward eliminating visibility impairment in Class I 

areas, including strategies for steadily reducing emissions 

along a 60-year timeframe leading to natural conditions. 

Through these collaborative regional and state planning 

processes, park managers can ensure that park emissions are 

accounted for in plans now being developed and that 

activities inside parks do not compromise or delay progress 

toward eliminating visibility impairment. 

2.2.5. Federal, State, and Local Pollution Emissions 

Requirements and Measures 

In addition to the legal requirements discussed above, state 

and local regulatory agencies may have source-specific 

emission standards or mandated management practices or 

control measures that park managers should be aware of. 
National Park Service 5

Lake sediment sampling at Sleeping Bear Dunes National 

Lakeshore, Michigan. Diatoms in sediments can be used to 

develop critical loads for air pollution. 

Credit: National Park Service. 

3. Management Framework 

3.1. NPS Management Policies 

Under the NPS Management Policies, the NPS will: “seek to 

perpetuate the best possible air quality in parks to: (1) 

preserve natural resources and systems; (2) preserve cultural 

resources; and (3) sustain visitor enjoyment, human health, 

and scenic vistas” (NPS Management Policies 2006; Section 

4.7.1). The NPS Management Policies further state that the 

NPS will assume an aggressive role in promoting and 

pursuing measures to protect Air Quality Related Values 

from the adverse impacts of air pollution. While Air Quality 

Related Values (AQRVs) is a term originating from the 

Clean Air Act (as described above), it is often used by NPS 

generally to refer to all resources that may be affected by air 

pollutants. In cases of doubt as to the impacts of existing or 

potential air pollution on park resources, the NPS “will err 

on the side of protecting air quality and related values for 

future generations” (NPS Management Policies 2006; 

Section 4.7.1). 

Note that the Organic Act and Management Policies apply 

equally to all NPS-managed areas, regardless of CAA 

designation. Therefore, the NPS will actively protect 

resources at Class II areas as well as Class I designated units, 

including using similar analysis techniques and criteria for 

evaluating effects to AQRVs in all National Park System 

units. Furthermore, the NPS Organic Act and Management 

Policies provide additional protection from that afforded by 

the CAA's NAAQS alone because NPS has documented that 

specific park AQRVs can be adversely affected at levels 

below the NAAQS or by pollutants for which no NAAQS 

exist. 

Many projects and sources of air pollution affecting air 

quality within a park originate outside park boundaries. 

NPS policies direct park superintendents and other 

managers to actively participate in external planning and 

regulatory processes that may adversely affect the park 

resources and to seek to mitigate those impacts to the extent 

possible. NPS Management Policies for External NEPA 

Review, Chapter 1 states: 

1.4.5. What Constitutes Impairment of Park Resources and 

Values

Impairment may also result from sources or activities 

outside the park. This will be addressed consistent with 

sections 1.6 and 1.7 Cooperative Conservation and Civic 

Engagement. 

1.6. Cooperative Conservation Beyond Park Boundaries

Cooperative conservation beyond park boundaries is 

necessary as the NPS strives to fulfill its mandate to 

preserve the natural and cultural resources of parks 

unimpaired for future generations. Ecological processes 

cross park boundaries, and park boundaries may not 

incorporate all of the natural resources, cultural sites, 

and scenic vistas that relate to park resources or the 

quality of the visitor experience. Therefore, activities 

proposed for adjacent lands may significantly affect park 

programs, resources, and values. Such local and regional 

cooperation may involve other federal agencies; tribal, 

state, and local governments. 

The Service will use all available tools to protect park 

resources and values from unacceptable impacts. The 

Service will also seek to advance opportunities for 

conservation partnerships. Superintendents will monitor 

land use proposals, changes to adjacent lands, and 

external activities for their potential impacts on park 

resources and values. It is appropriate for 

superintendents to engage constructively with the 

broader community in the same way that any good 

neighbor would. Superintendents will encourage 

compatible adjacent land uses and seek to avoid and 

mitigate potential adverse impacts on park resources and 

values by actively participating in the planning and 

regulatory processes of other federal agencies and tribal, 

state, and local governments having jurisdiction over 

property affecting, or affected by, the park. If a decision 

is made or is imminent that will result in unacceptable 

impacts on park resources, superintendents must take 

appropriate action, to the extent possible within the 

Service’s authorities and available resources, to manage 

or constrain the use to minimize impacts. 

Impairment decisions need to be made in the appropriate 

context. This means considering the proposed action and its 

effects on air quality and other resources in light of the 

purposes for which the park was established, the 

management objectives, and desired conditions. The 

manager should also consider existing conditions of air 

quality and air pollution sensitive resources in the park, the 

relative impacts from activities within and outside the park, 

and the incremental increase to the cumulative effects from 

proposed and ongoing activities. When deciding whether 
6 Air Quality Impairment Guidance (2010) 

impacts from internal projects might constitute an 

impairment, park managers should remain cognizant of the 

effect such decisions might have on their ability to protect 

park resources and values from impacts caused by activities 

outside park boundaries. 

3.2. Air Quality Analysis Under NEPA 

The National Environmental Policy Act (NEPA) of 1969 

requires federal agencies, including the NPS, to assess the 

impact of proposals on the quality of the human 

environment. The DO-12 Conservation Planning, 

Environmental Impact Analysis, and Decision-Making (NPS 

NEPA Handbook) indicates that the impact assessment 

should lay out a methodology for assessing each impact 

topic, including the criteria or thresholds used to draw a 

conclusion on the context, intensity, and duration of the 

impact. Based on these assessments, impacts may be 

characterized as “negligible,” “minor,” “moderate,” or 

“major.” These impact characterizations, in turn, provide a 

foundation for assessing whether or not the impact is likely 

to result in an impairment of park resources or values. 

Although impairment determinations are made under the 

Organic Act rather than as a NEPA requirement, NPS policy 

is to make impairment determinations concurrent with the 

environmental planning and assessment process of NEPA. 

Section 1.4.7 of the NPS Management Policies directs 

decision-makers to “consider any environmental 

assessments or environmental impact statements required 

by NEPA; relevant scientific studies and other sources of 

information; and public comments” in making impairment 

determinations. 

The NPS has specific responsibilities for determining 

whether impairment of park resources would result from 

any action under NEPA. The NPS Environmental Quality 

Division (EQD) has developed the Interim Guidance for 

Impairment Determinations in NPS NEPA Documents (July 

2010) which describes the process park managers should 

follow to assess and document impairment determinations 

resulting from in-park and outside-of-park activities. While 

this Technical Guidance on Assessing Impacts to Air Quality in 

NEPA and Planning Documents provides some general 

background on impairment as context for utilizing air 

quality information in making impairment determinations; 

NPS EQD’s current or future guidance should be 

considered primary in outlining the process NPS managers 

should use in making impairment determinations . 

In the following sections, this air quality technical guidance 

tracks the NPS NEPA Handbook approach by laying out 

methodologies, criteria, and threshold levels for 

characterizing impacts along a sliding scale (“negligible” to 

“major”). It does not specify the impact level that might 

constitute an impairment; however it does outline important 

factors to consider that may assist NPS managers in 

evaluating whether an impact is more or less likely to be an 

impairment. Impact levels/thresholds are used to identify 

and evaluate the resource impacts of the action in the NEPA 

analyses, and they also assist in defining the nature and 

severity of the impacts. 

In addition to considering air quality impacts for internal 

NPS projects, there are also many federal actions that occur 

outside of National Park System boundaries which can have 

a significant effect on the air quality or Air Quality Related 

Values in parks. To the extent practical, superintendents 

and NPS staff should be engaged in external planning 

processes and work with agencies and partners to minimize 

the effects to air quality and Air Quality Related Values 

within parks. 

While NEPA documents disclosing air quality impacts are 

prepared largely by technical staff and contractors, 

determination of whether an impact constitutes an 

impairment should only be made in consultation with the 

park manager or other NPS decision-maker. Ultimately, 

park managers will need to determine whether or not the 

impact is the “unavoidable result, which cannot reasonably 

be further mitigated, of an action necessary to preserve or 

restore the integrity of park resources or values.” If the 

effects to park resources are the result of another agency’s 

decision outside of NPS jurisdiction and control, the 

superintendent, in consultation with resource managers, 

should first determine whether the impacts constitute 

impairment or are unacceptable. Then to the extent 

practicable, superintendents should consult and work with 

the lead agency to mitigate or eliminate any adverse effects. 

Considering “connected,” “cumulative,” “similar,” and 

“reasonably foreseeable future” actions (CEQ NEPA 

Regulations: Section 1508.25) is particularly important when 

conducting NEPA analyses for air quality, because air 

pollutants can easily cross park boundaries and impacts 

from all sources can easily aggregate. The CEQ regulations 

provide specific guidance on assessing the scope of a project 

to determine when the impacts of actions must be analyzed 

together. 

Further, for NEPA projects that could result in air quality 

effects in a National Park System unit, the NPS should be a 

cooperating agency in accordance with DOI regulations, 40 

CFR 1501.6. The lead agency should “use the proposals and 

environmental analysis” of agencies with “special expertise 

or legal jurisdiction” to the extent possible. Wherever 

possible, the NPS should encourage other agencies and their 

stakeholders to adequately evaluate and consider air quality 

effects and impacts in the parks and where necessary, 

implement adequate mitigation measures. 

Despite having different requirements and outcomes, both 

NEPA and the CAA PSD program provide for the 

assessment and evaluation of environmental effects, as well 

as requirements for evaluating the “severity” of those 

effects. A detailed description of the methods and 

background for air quality assessments that can be used in 

both PSD and NEPA is found in the found in the FLAG 

document, the FLM’s air quality guidance document (FLAG 

2010).

 
National Park Service 7

A scientist uses a grid plot to monitor alpine vegetation at Logan 

Pass in Glacier National Park, Montana. Studies such as these 

have been used to develop critical loads of nitrogen deposition for 

sensitive alpine plants. 

Credit: National Park Service. 

4. Impact Assessments and Impairment of Human Health, 

Visibility, and Natural Resources from Air Pollutants 

4.1. Air Quality Background 

Many resources and values of the National Park System are 

affected by air pollution. For example, the ability to 

appreciate scenic vistas is highly dependent on clean, clear 

air. Poor visibility caused by air pollution suggests that there 

may be other impacts occurring to resources that are not 

readily apparent. Air pollution may cause or exacerbate 

respiratory symptoms for visitors and employees at NPS 

areas. Human-made pollution can also injure trees and 

other plants or animals, acidify streams and lakes, and leach 

nutrients from soils. 

Air pollutants emitted in and near parks may affect visibility 

(nitrogen oxides, sulfur oxides, and particulates); human 

health (hydrocarbons, ozone precursors, nitrogen oxides, 

sulfur dioxide, particulates, and air toxics); and ecosystems 

(ozone, acidic deposition of nitrogen and sulfur, nitrogen 

nutrient enrichment and air toxics). When air pollution 

concentrations and effects from a project are quantified and 

assessed it is important early on to identify mitigation 

measures, and assess strategies for potential emissions 

reductions. This is consistent with NPS policy to “seek to 

perpetuate the best possible air quality in parks,” and 

consistent with the way NPS addresses impacts of sources 

located outside park boundaries by promoting the use of 

“best available control technology” (BACT) or the most 

effective emissions reductions feasible. 

Point sources (industrial), area sources (oil and gas 

development, vehicles, agricultural, urban), and mobile 

sources (on and off road vehicles, ships) produce air 

pollution emissions that can be transported hundreds of 

miles into parks. Therefore, air pollution effects on 

resources, visitor enjoyment, and human health almost 

always have cumulative, as well as project-specific, impacts 

to consider. The existing impact of air pollutants on parks 

will have bearing on impact assessments and impairment 

determinations from a proposed project in several ways. For 

example, (1) if the air quality is very clean, the 

characteristics of visibility are such that it would only take a 

very small amount of pollution to create visibility 

degradation perceptible to the human eye; (2) if acidic 

deposition impacts have been occurring for several years, 

ecosystem tolerances may be at or above critical load levels 

so that even small amounts of additional emissions may 

cause significant ecosystem changes; (3) if air pollution 

concentrations in parks exceed or are close to exceeding 

any NAAQS, federal law (and NPS policy) may prohibit 

additional emissions that would increase these pollution 

concentrations. 

4.2. Assessing Air Quality Impacts 

Although there are no standard methodologies that can be 

applied to determine impacts or impairment in every 

situation, in general there are a number of tasks that will be 

useful for assessing impacts to parks: 

• Determine the level of air quality analysis necessary, 

based on the type and amount of project emissions 

(Table 1), distance of project to the park, current air 

quality conditions in the park, and other relevant 

information. 

• Obtain air quality information from appropriate data 

sources (Table 3). 

• Assess impacts on air quality, human health, and Air 

Quality Related Values in the NEPA context 

(Tables 4–5). 

These tasks are fairly interdependent and may be iterative, 

as some of the same information that can be used to 

determine the necessary level of analysis can also be used to 

interpret the severity of the effects. Each of these tasks is 

described in detail below. 

4.2.1. Determine the Level of Analysis 

The level of air quality analyses needed for the proposed 

action can range from a simple qualitative discussion of 

potential air quality impacts and emissions calculations, 

which may be conducted by park staff, to the use of a 

regional photochemical grid model, best conducted by an 

air quality modeling specialist (often a contracted 

consultant). The following actions may be useful in 

determining the appropriate level of analysis: 

• Estimate the emissions from the proposed action and its 

alternatives. 

• Determine the distance (in km) of the project to the 

park unit(s) of concern (applicable only to projects 

external to National Park System boundaries). 
8 Air Quality Impairment Guidance (2010) 

• Describe/quantify the current air quality condition in 

the park. For example, based on data from on-site or 

nearby monitors, how close is the ambient air quality to 

exceeding the NAAQS? Is the project located in a 

nonattainment or maintenance area, therefore requiring 

a “conformity determination?” Are there nearby 

sources of pollutants that will need to be accounted for 

in a cumulative analysis? 

• Describe the types of pollutants (such as SO2, O3, NOx, 

PM2.5) that will be emitted, and relate them to the air 

quality issues of concern, such as visibility impairment 

or deposition. 

The emissions analysis screening thresholds presented in 

Table 1 are intended to be used in context with other 

information relevant to the park (e.g., projects with 

relatively low emissions may sometimes warrant a more 

quantitative or complex level of analysis than shown in the 

table). However, Table 1 is intended as a first cut for 

projects to determine the appropriate level of air quality 

impact analysis in attainment areas (analysis for 

nonattainment areas is discussed in more detail below). 

Both emission quantity and duration should be considered 

in determining the level of analysis needed. If the project is 

likely to result in a small amount of emissions (< 50 tons per 

year [TPY]), then no air quality modeling may be necessary, 

and a simple description of the emissions calculations 

methods may be sufficient. If the project results in emissions 

levels > 50 TPY, additional air quality modeling should be 

considered. In circumstances where the project or proposed 

action is likely to result in high levels of emissions (> 250 

TPY), more complex air quality modeling will be needed in 

most cases. The emissions analysis thresholds should be 

used only in the context of current air quality and applicable 

connected or cumulative actions. For example if a project is 

occurring in or near an area where the ambient 

concentrations are close (> 80%) to exceeding the NAAQS, 

modeling should be considered to determine if the project is 

likely to contribute to a NAAQS exceedance. Note that 

significant contribution to an existing NAAQS exceedance 

by emissions from a new federal project would not be 

allowed under the General Conformity Rule under the 

Clean Air Act. If the project is in a nonattainment area, 

emissions are restricted to a much greater extent, and would 

generally not be considered “negligible” unless they 

constitute a net decrease. In nonattainment areas, emissions 

are not permitted to increase above “de minimus” levels 

(Table 3 lists where to find more information about 

conformity and de minimus levels). 

There are various air quality emissions, atmospheric 

transport, visibility, and pollution dispersion models 

available and each model varies in degree of complexity and 

information needs. Examples of types of modeling and 

analysis tools from the least complex and resource intensive 

to the most sophisticated and resource intensive are: 

• Emissions Calculations (e.g., AP-42; MOBILE 6; 

forestry fuel loading and fuel consumption models such 

as CONSUME, FOFEM, FEPS, E&P Tank, etc.), 

• Screening and near-field dispersion models 

(e.g., VISCREEN, AERMOD), 

• Long-range transport models (e.g., CALPUFF), and 

• Photochemical Grid Models (e.g., CAMx, CMAQ). 

Simple emissions calculations can be used to estimate 

emissions from certain activities. Air quality models use 

emissions calculations, as well as meteorological inputs, to 

predict resulting air pollutant concentrations. Air quality 

modeling can also provide useful information about 

potential magnitude, duration, and location of air 

pollutants. Since air quality standards are generally 

expressed as concentrations, air quality modeling results 

can be used to evaluate impacts on NAAQS, potential effects 

to human health, or increments for criteria pollutants (an 

increment is the additional, cumulative amount of pollution 

from all new sources allowed by the PSD program in a Class 

I or II area). The specific air quality issues (such as visibility 

or deposition or ozone effects) and type and magnitude of 

potential emissions should be considered when selecting an 

air quality model. 

In addition to using Table 1 as a guide for the type of air 

quality analysis recommended for in-park projects, a quick 

screening process is available to determine when external 

projects (originating outside NPS lands) may be exempted 

from further air quality analysis. The Federal Land 

Managers’ Air Quality Related Values Work Group (FLAG) 

developed a screening calculation to identify projects that 

can be excused from additional analyses either because of 

their low emissions levels or their distance from a park. This 

screening tool is based specifically on visibility impairing 

pollutants and is known as the “Q/d Screen.” “Q” is the sum 

of the visibility impairing pollutants emitted in tons per year, 

divided by “d” the distance from the project to the park in 

kilometers. If the Q/d is ≤ 10, the project is unlikely to result 

in significant visibility or deposition concerns. If the Q/d is > 

Table 1. Emission analysis screening thresholds for 

attainment areas 

(Emissions categories are loosely based on EPA’s Prevention of 

Significant Deterioration (PSD) air pollution permitting thresholds)

AQ analysis level Proposed action 

(emissions) 

Emissions duration 

Qualitative 

description may be 

sufficient. 

< 50 tons per year 

(TPY) (any 

pollutant) 

One to several days 

or very low daily 

emissions over an 

annual period.

Screening modeling 

may be needed. 

> 50 and <100 

TPY (any 

pollutant) 

Several days to 

weeks or very low 

daily emissions over 

an annual period. 

Near-field or longrange transport 

modeling may be 

needed. 

> 100 TPY (any 

pollutant) 

Several weeks to 

months. 

Complex air quality 

modeling is likely 

needed. 

> 250 TPY (any 

pollutant) 

Long term, one year 

to several years. 
National Park Service 9

10, a visibility and/or deposition analysis should be 

performed. Details of the approach are outlined in the 

revised FLAG guidance (FLAG 2010). 

For both in-park and external projects where emissions may 

be small or intermittent, it is important to note that air 

quality analysis may still be needed for “connected,” 

“cumulative,” and/or “reasonably foreseeable future” 

actions. Screening out an individual project in no way 

removes the obligation to consider its emissions or conduct 

a detailed cumulative air quality analysis where necessary. In 

other words, it is not appropriate to segment numerous 

similar actions which individually may not cause air quality 

concerns and could be “screened” from analysis, but 

cumulatively may result in significant regional effects in the 

airshed. 

The magnitude of emissions should be calculated for any 

project, unless it is anticipated that a project will not result 

in emissions of any air pollutant, or emissions will be very 

small (e.g., < 5 TPY in an attainment area). A few examples 

of types and amounts of emissions from pollution sources 

commonly found within National Park System units are 

provided in Table 2. Actual emissions for each of these types 

of activities will vary considerably depending on the details 

of the activity. 

Emissions for a variety of park activities can easily be 

calculated using the references listed in Table 3. If the 

results of the emissions calculations demonstrate that more 

information is needed, or there are complex air quality 

issues in the region or project area, then additional air 

quality analyses should be considered. The types of 

pollutants that will be emitted, the distance to the 

receptors/park(s) of concern, magnitude of these emissions 

and current air quality conditions can assist in determining 

the type of analysis necessary. For instance, if visibility 

impairing (or precursor) pollutants will be emitted (SO4, 

PM10, PM2.5, NOx, SO2) at levels above 50 TPY, or at lower 

levels but where visibility is a particular concern, then a 

screening or dispersion model that can simulate visibility 

effects should be considered (e.g., VISCREEN or 

CALPUFF). If ozone is a significant concern in the region, 

and ozone precursor pollutants (NOx and volatile organic 

compounds [VOCs]) will be emitted from the project, a 

photochemical grid model (currently the only type of model 

that can simulate ozone production) should be considered. 

Each decision for the appropriate analysis tool should be 

bounded by the duration, timing and frequency of the 

anticipated impacts. In other words, are the impacts 

anticipated to last only a number of hours or days, or will 

they continue for several months to years? What time of 

year will the impacts occur; will they coincide with times of 

high visitor use? A more extensive, but not exhaustive list 

of these tools is provided under “Information Needs” in 

Table 3. Finally, more detailed information on the types of 

models that should be used for air quality assessments, how 

they should be used and when they should be applied can be 

found in the FLAG 2010 guidance document and applicable 

EPA modeling guidance (such as the Interagency 

Workgroup on Air Quality Modeling [IWAQM]). In any 

analysis, it is important to document the data sources and 

rationale for the type of analysis that was selected. 

4.2.2. Obtain Air Quality Information 

There are many excellent sources of information available 

for estimating project emissions, selecting the types of air 

analysis tools that will be used, understanding existing air 

quality conditions, and assessing impacts of a project on 

human health and AQRVs in specific National Park System 

units. Table 3 lists many of these information sources, along 

with links to relevant web sites. 

4.2.3. Assess Impact Levels 

The final task is to evaluate or interpret the level of effects. 

The four impact categories “negligible, minor, moderate, 

and major” are discussed below in two parts; (1) Assessing 

Impacts Based on Human Health and (2) Assessing Impacts 

to Air Quality Related Values. 

Table 2. Examples of emissions levels calculated for 

selected activities in parks (for the year 2000) 

Park Activity Park-wide emissions 

(tons per year) 

Mesa Verde Visitor Vehicles 

49 TPY PM10

35 TPY VOC 

26 TPY NOx

Yosemite Campfires 

6.5 TPY PM10

7 TPY VOC 

1.5 TPY NOx

Golden Gate Watercraft 

exhaust 

8 TPY PM10

13.5 TPY VOC 

165 TPY NOx

7 TPY SO2

Grand Canyon Aircraft 

17 TPY PM10

69 TPY VOC 

84 TPY NOx

6.5 TPY SO2

Channel 

Islands Generators 

0.8 TPY PM10

0.9 TPY VOC 

11 TPY NOx

0.7 TPY SO2

Redwood Prescribed Fire 

125 TPY PM10

29.6 TPY VOC 

2.2 TPY NOx
10 Air Quality Impairment Guidance (2010) 

 Table 3. Air quality information sources 

Assessment 

component 

Information needs Sources of information 

Air pollution 

emissions, 

regulations, and 

atmospheric 

transport 

Calculated air pollutant emissions from the 

proposed project and cumulative emissions from 

other sources contributing to impacts on park 

resources. This requires information on current 

and proposed activity levels within the park. 

Options and data needs for air quality modeling. 

Information about applicable Federal, state, and 

local regulations including emissions limits, 

performance standards or management practices 

and permitting requirements. 

National Ambient Air Quality Standards (NAAQS), 

and General Conformity de minimus levels. 

Check with NPS-ARD for information on how to calculate project 

emissions, to determine which cumulative sources should be 

included, or whether modeling is recommended. 

Check with local and state air permitting authorities for current emissions 

levels of adjacent sources, and for non-attainment area boundaries. 

EPA emission factors AP-42: 

http://www.epa.gov/ttn/chief/ap42/index.html.

EPA monitoring and air emissions data: 

http://www.epa.gov/air/data/index.html. 

USFS/SAMI/VISTAS emissions tool: 

http://webcam.srs.fs.fed.us/emissions/. 

Park records may include information about activity levels (e.g., the 

number of visitors using personal water crafts, the number of vehicles 

entering the park per day). Emissions inventories for certain parks: 

http://www.nature.nps.gov/air/AQBasics/sources.cfm#parks and 

http://www.nps.gov/climatefriendlyparks/. 

In-park emissions can be estimated using the CLIP (Climate Leadership 

in Parks) tool: http://www.nps.gov/climatefriendlyparks/. 

FLAG (Federal Land Managers’ Air Quality Related Values Work Group) 

Guidance on air quality modeling and analyses: 

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

NAAQS table: http://www.epa.gov/air/criteria.html. 

Many fire emissions calculation tools are available through the Fire and 

Environmental Research Applications Team (FERA) website: 

http://www.fs.fed.us/pnw/fera/publications/index.shtml

EPA General Conformity de minimus levels: 

Air chemistry 

and deposition 

Ambient air quality data (O3, PM, SO2, NOx) 

Atmospheric deposition data. 

Air toxics data. 

EPA, state, local, or park ambient monitoring equipment for air quality 

data. 

National Atmospheric Deposition Program (NADP) for wet deposition: 

http://nadp.sws.uiuc.edu/. 

EPA-Clean Air Status and Trends Network (CASTNet) for dry 

deposition: http://www.epa.gov/castnet/. 

Park specific air chemistry and deposition: 

http://www.nature.nps.gov/air/monitoring/index.cfm. 

For parks without on-site monitors, interpolated data: 

http://www.nature.nps.gov/air/Maps/AirAtlas/index. 

Mercury Deposition Network (MDN) for mercury deposition: 

http://nadp.sws.uiuc.edu/mdn/. 

Air toxics: http://www.epa.gov/ttn/atw/index.html. 

Air pollution 

impacts on 

human health 

Current and future air quality levels relative to the 

NAAQS to help assess health impacts to visitors 

and employees. 

Air Quality Index scores by state and county to 

assess health impacts of current and cumulative 

air quality conditions. 

Ozone NAAQS exceedances in parks: 

http://www.nature.nps.gov/air/Monitoring/exceed.cfm. 

Ozone health advisories in parks: 

http://www.nature.nps.gov/air/data/O3AdvisSum.cfm. 

EPA Air Quality Index for interpreting human health impacts: 

http://airnow.gov/index.cfm?action=static.aqi

http://www.airsis.com/usfs/aqi.asp. 

Air pollution 

impacts on 

natural 

resources 

(AQRVs) 

Current condition and future projected condition 

related to air quality impacts on park AQRVs 

(visibility, flora, fauna, soils, water or cultural 

resources). This may include information on 

resources highly sensitive to air pollution 

impacts, as well as documented current impacts. 

Visibility trends, natural background visibility, current 

condition, and historic conditions. 

Visibility impacts from particulate matter, nitrogen 

oxides and sulfur dioxides. 

Vegetation impacts from ozone. 

Deposition sensitive aquatic ecosystems (water and 

aquatic biota). 

AQRV data available from NPS-ARD, scientific literature and reports, 

Inventory and Monitoring (I&M) Networks: 

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

Visibility Information Exchange Web System (VIEWS) for visibility 

photos; transmissometer or nephelometer data; fine and coarse mass 

from particle filters. 

Ozone risk assessment for vegetation: 

http://www.nature.nps.gov/air/permits/aris/networks/ozonerisk.cfm. 

Water quality in parks: http://www.nature.nps.gov/water/infodata.cfm. 
National Park Service 11

4.3. Assessing Impacts to Human Health 

Air quality impacts to human health occur from all 

pollutants cumulatively. Therefore, when assessing whether 

the impact levels are negligible, minor, moderate, or major, 

current air quality conditions need to be considered along 

with the estimated project-specific changes in air quality. 

The National Ambient Air Quality Standards (NAAQS) have 

been developed primarily to protect human health from air 

pollutants to the same degree nationwide. However, 

concentrations below the NAAQS can also affect human 

health, particularly in sensitive individuals, and disclosure of 

these impacts is important. Air quality modeling, as 

discussed in the previous section, can be used to develop 

quantitative estimates to better assess how the proposed 

project emissions, in combination with existing air quality 

conditions, may impact human health. 

The EPA has developed an Air Quality Index (AQI) that 

correlates criteria pollutant concentrations to associated 

health concern categories. NPS recommended cumulative 

impact levels for assessing human health concerns in 

planning processes are found in Table 4. Impact levels in 

this table are based on a combination of the AQI ranges 

established by EPA, and NAAQS thresholds. The AQI index 

is widely used by EPA to communicate air quality conditions 

to the public. The scale is non-linear in the upper numbers 

and the concentration at which the NAAQS is reached is 

always an AQI of 100 (unhealthy for sensitive groups). 

According to the AQI, concentrations below 50% of the 

NAAQS represent “Good” air quality and concentrations 

between 50% and 99% of the NAAQS represent 

“Moderate” air quality. 

4.3.1. Health Effects Thresholds 

In Table 4, the “Negligible” level is the range of 

concentrations for each pollutant that is the highest 

estimated policy relevant background (PRB) concentration 

as determined by EPA in its criteria pollutant documents 

and pollutant assessments. Concentrations in this range are 

indistinguishable from variations in the background 

concentrations that are of natural and long-range transport 

origin. Some National Park System units maintain air quality 

in this background range most of the time. The source for 

background concentrations is EPA’s Integrated Science 

Assessment documents for each pollutant 

(http://www.epa.gov/ttn/naaqs/). The PRB represents what 

is currently thought to be the natural background plus 

human pollution transported from outside of North 

America. The “Minor” level follows the AQI scale and 

corresponds to concentrations from the PRB up to an 

additional 50% of the difference between the PRB and the 

NAAQS. The “Moderate” level is from 51% to 99% of the 

NAAQS. The “Major” level in Table 4 corresponds to the 

NAAQS for each pollutant. Pollutant concentrations at or 

above the NAAQS describe “nonattainment” status and 

represents unhealthy air quality for park visitors and 

employees. The formal process for state and federal air 

regulatory agencies to designate an area as nonattainment 

for the NAAQS is complex (e.g., 98% or 99% of the available 

data surpasses these levels in 2–4 instances over a three year 

period; where the specific criteria varies by pollutant). For 

the impact levels discussed here, NPS recommends a 

simplified scheme that can be applied across all impact 

categories, for all pollutants: compare the 99th percentile of 

the monitored or modeled air concentration data available 

for each pollutant in (or representative of) the park over the 

most recent 3-year period to the values in Table 4. Data 

representing “exceptional events” (e.g., fire) need not be 

included, and if less than three years of data are available 

then the most recent year can be used instead. Consultation 

with NPS-ARD staff regarding how to apply this process to 

a specific case is always recommended.

Table 4. Assessing cumulative impacts to human health 

(Concentrations are expressed in ppm or μg/m3 following the practice by EPA) 

Impact level 8-hr Ozone*

(ppm) 

8-hr Carbon 

Monoxide (ppm) 

1-hr Sulfur 

Dioxide (ppm) 

24-hr PM10

(μg/m3) 

24-hr PM2.5

(μg/m3) 

1-hr Nitrogen 

Dioxide (ppm) 

Negligible 0–0.045 0–0.2 0–0.001 0–11 0–5 0–0.001 

Minor 0.046–0.064 0.3–4.4 0.002–0.034 12–77 6–15 0.002–0.049 

Moderate 0.065–0.075 4.5–9.0 0.035–0.074 78–155 16–34 0.050–0.099 

Major > 0.075 > 9.0 ≥ 0.075 > 155 ≥ 35 ≥ 0.100 

* In January 2010, EPA proposed strengthening the ozone NAAQS to a value between 0.060–0.070 ppm. EPA is expected to finalize the rule in 

December 2010. At that time, the ozone concentrations designating impact levels will need to be adjusted.
12 Air Quality Impairment Guidance (2010) 

Field technicians collect plant specimens for nutrient analysis to assess the effects of nitrogen 

deposition in Great Smoky Mountains National Park 

Credit: National Park Service, 2008. 

4.4. Assessing Impacts to Air Quality 

Related Values (visibility, flora, fauna, soils, 

water) from Airborne Pollutants 

As discussed earlier, air pollution regulatory standards, such 

as the NAAQS, do not protect natural resources or visibility 

from air pollution impacts. However, there are other air 

quality and resource-based thresholds that can be used to 

assess these potential impacts. 

In many cases, the degree to which air pollution is currently 

affecting park resources will dictate the amount of 

additional air pollution that can be added before impact or 

“impairment” to park resources occurs. In the case of 

visibility, a very clean current condition means that a small 

amount of added air pollution emissions could produce a 

perceptible visual impact. With atmospheric deposition, a 

high amount of current deposition (relative to natural 

background deposition levels) increases the likelihood that 

additional deposition from new emissions would contribute 

to cumulative resource impacts in the soils, waters, and 

vegetation. Because parks and ecosystems are so diverse, 

there are no “one size fits all” quantitative measures for 

determining impact levels, therefore the information below 

provides more general guidelines rather than hard and fast 

rules. 

The recommended AQRV-based impact levels in Table 5 

utilize a variety of qualitative and quantitative measures 

based on published data, policy, and professional judgment. 

The bullets under each of the four impact levels describe: 

• examples of potential predicted impacts from the 

proposed action and/or cumulative actions to natural 

resources (e.g., exceedance of deposition significance 

thresholds , or visibility impact thresholds), and 

• current air quality conditions in the park (e.g., W126 

ozone concentrations, annual wet deposition averages, 

annual visibility averages or other cumulative 

thresholds) at each impact level that may increase the 

likelihood of harm to park resources from additional 

emissions. 

The examples provided are not necessarily comprehensive 

or expected to occur to all resources simultaneously in 

order t o merit designation at an impact level. Impact levels 

selected for proposed or cumulative actions may differ 

between parks and projects depending on the existing 

(current baseline) air quality condition and existing impacts 

to natural resources. 

For visibility analyses, it is important to note that the FLAG 

thresholds should be applied using estimated natural 

background visibility conditions rather than current 

conditions, as this is consistent with national policy goals for 

visibility improvement. 
National Park Service 13

Table 5. Assessing impacts of proposed and cumulative actions to air quality and AQRVs 

Negligible Impacts 

Examples of predicted Impact: 

 No perceptible visibility impacts likely (no visible smoke, plume, or haze). 

 Predicted (i.e., modeled) visibility impacts, and nitrogen and sulfur deposition loading levels 

are below thresholds listed in NPS FLAG and Deposition Analysis Threshold (DAT) 

guidance.

Current Air Quality Condition: 

 Ozone W126* ≤ 7ppm-hrs or SUM06** ≤

8 ppm-hrs 

 Wet deposition < 1 kg/ha/yr for both 

nitrogen and sulfur 

 Visibility† no more than 2dv above 

natural background condition

Minor Impacts 

Examples of predicted Impact: 

 Predicted (i.e., modeled) visibility impacts and nitrogen and sulfur deposition loading levels 

are approaching (between 90–100%) of thresholds listed in NPS FLAG and DAT guidance. 

 Perceptible visibility impacts occur, but are only visible from a small area of the park, are of 

short duration (less than one day per year) and visible to only a few park visitors on the days 

that they occur.

Current Air Quality Condition: 

 Ozone W126* ≤ 7ppm-hrs or SUM06**

≤ 8 ppm-hrs 

 Wet deposition < 1 kg/ha/yr for both 

nitrogen and sulfur 

 Visibility† no more than 2 dv above 

natural background condition

Moderate Impacts 

Examples of predicted Impact: 

 Predicted (i.e., modeled) visibility impacts exceed thresholds listed in NPS FLAG guidance, 

but perceptible impacts are minimal (see next bullet); and/or nitrogen and sulfur deposition 

loading levels exceed screening thresholds in the DAT guidance but available scientific 

information indicates deposition will not, or is not, harming integrity of the resources in the 

park. 

 Perceptible visibility impacts occur and are visible from several areas of the park, occur 

between one and several days per year, and many park visitors may observe them on the 

days that they occur. 

Note: impacts that exceed the FLAG thresholds may still constitute a “Major” impact regardless 

of current condition, taking into account the magnitude, frequency and duration of the 

impact, as described in the FLAG document.‡

Current Air Quality Condition: 

 Ozone W126* from 7–13 ppm-hrs or 

SUM06** ozone 8–15 ppm-hrs 

 Wet deposition is from 1–3 kg/ha/yr for 

both nitrogen and sulfur, and sensitive 

ecosystems are present in the park 

that could likely be impacted in some 

way (change to physical, chemical or 

biological processes) from deposition 

 Visibility† from 2–8 dv above natural 

background condition

Major Impacts 

Examples of predicted Impact: 

 Predicted (i.e., modeled) visibility impacts exceed thresholds listed in NPS FLAG guidance 

(see next bullet); and/or nitrogen and sulfur deposition loading levels exceed screening 

thresholds listed in NPS DAT guidance, and available scientific information indicates that 

deposition may harm the integrity of resources in the park. 

 Perceptible visibility impacts occur and are visible from many areas of the park, occur many 

days over the course of a year, or are visible to a majority of park visitors on the days that 

they occur. 

Note: impacts that exceed the FLAG thresholds may still constitute a “Major” impact regardless 

of current condition, taking into account the magnitude, frequency and duration of the 

impact, as described in the FLAG document.‡

Current Air Quality Condition: 

 Ozone W126* > 13 ppm-hrs or 

SUM06** > 15 ppm-hrs 

 Wet deposition > 3 kg/ha/yr for either 

nitrogen or sulfur or deposition impacts 

to AQRVs have been documented in 

the park 

 Visibility† greater than 8 dv above 

natural background condition 

 Visibility conditions are worsening 

(trending downward based on ARD’s 

10 yr trends info) at the park 

* The W126 preferentially weights the higher ozone concentrations most likely to affect plants and sums all weighted concentrations during daylight hours over three 

months during the growing season, giving a cumulative metric expressed in ppm-hrs. 

** The SUM06 sums all hourly ozone concentrations ≥ 0.060 ppm during the daylight hours over three months during the growing season, giving a cumulative metric 

expressed in ppm-hrs. 

† Visibility condition = Group 50 visibility - annual average natural conditions, where Group 50 is the mean of the 40th–60th percentiles of observed conditions in 

deciview (dv). 

‡ It is important to note that the FLAG thresholds should be applied using estimated natural background visibility conditions rather than current conditions, as this is 

consistent with national policy goals for visibility improvement. For the purposes of this guidance, the current condition information is used only as a “modifier” to 

determine whether more weight should be given to modeled impacts predicted in the moderate or minor category in parks where current condition is worsening. 
14 Air Quality Impairment Guidance (2010) 

4.4.1. Ozone Effects Thresholds 

Ozone effects thresholds are based on information from 

EPA’s recent review of the ozone standards. EPA recognized 

that plants respond to cumulative ozone exposures rather 

than short-term concentrations. Cumulative exposures can 

be expressed as either the W126 or the SUM06, defined in 

Table 5. EPA has considered the recommendations from an 

expert workgroup who noted that for growth effects to tree 

seedlings in natural forest stands, a W126 range of 7–13 

ppm-hrs would be protective; a W126 of 5–9 ppm-hrs, 

would protect plants in natural ecosystems against foliar 

injury (Heck and Cowling 1997; EPA 2007). Therefore, NPS 

considers that W126 exposures ≤7 ppm-hrs represent a 

minor impact to ozone-sensitive vegetation, while W126 

exposures > 13 ppm-hrs represent a major impact. 

Equivalent SUM06 exposures from Table 5 may also be 

used to evaluate impact levels. 

4.4.2. Deposition Impacts Thresholds 

Nitrogen (N) and sulfur (S) deposition can cause 

acidification of lakes, streams, and soils; N deposition can 

also cause fertilization and eutrophication, leading to 

unwanted changes in species abundance, composition and a 

loss of biodiversity. Several factors are considered in 

evaluating the potential for deposition impacts from a 

proposed project, including natural background deposition 

estimates and available information on deposition effects on 

ecosystems. Estimates of natural background deposition for 

total deposition are approximately 0.25–0.50 kilograms per 

hectare per year (kg/ha/yr) for N or S (Bates et al. 1992; 

Galloway et al. 1995; Galloway et al. 1982; Holland et al. 

1999). For wet deposition only, this is roughly equivalent to 

0.13–0.25 kg/ha/yr. Certain sensitive ecosystems respond to 

very low levels of deposition (e.g., 3 kg/ha/yr total 

deposition, or about 1.5 kg/ha/yr wet deposition) (Baron 

2006). “Critical loads” is a term used to describe the amount 

of deposition below which ecosystems are protected from 

harm, according to current knowledge. An overview of the 

current research on empirical nitrogen critical loads for 

most U.S. ecosystems is in Pardo et al. 2010. 

Evidence is not currently available that indicates that wet 

deposition amounts less than 1 kg/ha/yr for either N or S 

cause ecosystem harm. Therefore, if current baseline wet 

deposition of N or S < 1 kg/ha/yr, and the proposed 

project’s predicted contribution to deposition is below the 

screening thresholds indentified in the DAT guidance, the 

impact to deposition-sensitive resources is likely to be 

negligible. If current wet deposition of N or S is > 3 kg/ha/yr, 

or applicable critical loads values or other scientific 

information is available that suggests the ecosystem is being 

harmed by current deposition levels, and the proposed 

project’s contribution to deposition is above the DAT 

screening levels, the impact is likely to be moderate or 

major. It is important to remember that the DATs only 

represent significance thresholds, below which the amount 

of deposition is considered “insignificant,” rather than any 

indication that there will be impact to resources. Therefore, 

an exceedance of the DAT indicates only that deposition to 

the park may be of concern, and predicted or modeled 

deposition increases should be considered in the context of 

their magnitude and intensity, as well as current knowledge 

regarding the sensitivity of the ecosystem and resources of 

concern, including any documented deposition impacts that 

are already or reasonably believed to be occurring. This 

guidance is consistent with the criteria and methodology 

used by the NPS to evaluate progress toward air quality 

performance goals in the NPS annual report to Congress, as 

well as decision making criteria for impacts to AQRVs 

identified in the FLAG 2010 guidance. 

4.4.3. Visibility Impacts Thresholds 

Potential impacts to visibility take into account current 

conditions and predicted changes to visibility. Visibility can 

be expressed in terms of visual range (units of length e.g. 

kilometers), light extinction (units of inverse length e.g. 

inverse megameters [Mm-1]), or haze index (deciview [dv]). 

The terms are mathematically related. If the current average 

light extinction is no more than 2 dv greater than estimated 

natural conditions and changes to visibility from the 

proposed project (and /or cumulative sources) are less than 

the thresholds given in the FLAG guidance, the impact is 

likely to be negligible. If current visibility condition is more 

than 8 dv higher than the estimated natural conditions or

changes to visibility from the proposed project (and /or 

cumulative sources) exceed the thresholds1

 given in the 

FLAG guidance, then visibility is degraded in the park, and 

the additional impact is more likely to be major. This 

guidance is consistent with the criteria and methodology 

used by the NPS to evaluate progress toward air quality 

performance goals in the NPS Air Quality in National Parks: 

Annual Performance and Progress Report (Annual Report) 

and the FLAG 2010 guidance. 

4.5. Impairment Considerations 

If a potential adverse impact were identified from the impact 

categories above, then the park’s enabling legislation and 

management objectives (relative to resource protection, 

human health, and the opportunity for visitors to view 

scenic vistas) could be considered by the park 

superintendent in making a final impairment determination. 

The following questions are useful for NPS managers to 

consider in making impairment determinations. 

• Are projected air quality concentrations likely to 

adversely affect visitor or employee health? Note, if air 

quality concentrations are projected to significantly 

contribute to a violation of the NAAQS, or impede the 

States’ progress towards achieving the NAAQS, the 

action cannot be approved without mitigations or 

offsets. 

• Are human-caused emissions in/near a park likely to 

adversely affect visibility conditions such that they 

detract from the view of scenic vistas? Where very clean 

air quality conditions exist for the “best visibility days” 

 

1. It is important to note that the FLAG thresholds should be applied 

using estimated natural background visibility conditions rather than 

current conditions, as this is consistent with national policy goals for 

visibility improvement. 
National Park Service 15

A scientist in Sequoia National Park studies physiological 

responses of pine trees to ozone. 

Credit: Jeanne Panek, 2003. 

in a park, a small addition in emissions may be more 

likely to result in perceptible changes in visibility. 

• Are human-caused emissions likely to create a visible 

smoke, haze, or plume? 

• Are human-caused emissions in/near a park likely to 

create adverse impacts to resources/values that are: 

specifically mentioned in enabling legislation, key to 

natural or cultural integrity or opportunities for 

enjoyment of park, identified in the park General 

Management Plan or other planning document, or are 

AQRVs in Class I areas or wilderness areas? 

• Are projected resource impacts above air-quality 

“concern thresholds” for visibility, nitrogen or sulfur 

deposition (as posted on the NPS-ARD web site in the 

FLAG or DAT guidance documents)? 

• Are projected increases in emissions likely to exacerbate 

current stresses from air pollution on visibility, flora, 

fauna, soil, or water (e.g., exceeds critical loads)? 
16 Air Quality Impairment Guidance (2010) 

References 

Bates, T.S, and B.K. Lamb. 1992. Natural Sulfur Emissions to 

the Atmosphere of the Continental United States. Global 

Biogeochemical Cycles. (4): 431–435. 

Baron, J.S. 2006. Hindcasting nitrogen deposition to 

determine an ecological critical load. Ecological 

Applications. 16(2). 

EPA 2007. Policy Assessment of Scientific and Technical 

Information: OAQPS Staff Paper. EPA-452/R-07-007. 

[FLAG] 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 Resources Report. 

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

Denver, Colorado. Available at: 

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

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, JN, GE Likens, WC Keene and JM Miller. 1982. 

The Composition of Precipitation in Remote Areas of 

the World. Journal of Geophysical Research. 87 (11): 

8771–8786. 

Heck, W., Cowling, E.B. 1997. The need for a long-term 

cumulative secondary ozone standard—an ecological 

perspective. Environmental Management (January): 

23–33. 

Holland, E.A., Dentener, F.J., Braswell, B.H., Sulzman, J.M. 

1999. Contemporary and pre-industrial global reactive 

nitrogen budgets. Biogeochemistry. 46: 7–43. 

National Park Service. 2001a. DO-12, Conservation 

Planning, Environmental Impact Analysis, and Decisionmaking. 

National Park Service. 2003. Interim Technical Guidance on 

Assessing Impacts and Impairment to Natural 

Resources. NPS Natural Resources Program Center. July 

2003. 

National Park Service. 2006. Management Policies. 

Washington DC. Available at: 

www.nps.gov/policy/mp2006.pdf.

Pardo, L.H., Geiser, L.J. Goodale, C.L., Driscoll, C.T., Fenn, 

M., Allen, E. Baron, J., Bobbink, R., Bowman, B., Clark, 

C., Emmett, B., Gilliam, F., Greaver, T., Hall, S.J., 

Lilleskov, E.A., Liu, L., Lynch, J., Nadelhoffer, K, 

Perakis, S., Stoddard, J., Weathers, K. In press. 2010. 

Assessment of N deposition effects and empirical critical 

loads of N for ecoregions of the United States. USDA 

Forest Service General Technical Report.
 

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

and other information about those resources; and honors its special responsibilities to American Indians, Alaska Natives, and 

affiliated Island Communities. 

NPS 999/106176, December 2010
National Park Service 

U.S. Department of the Interior 

Natural Resource Program Center 

Air Resources Division 

PO Box 25287 

Denver, Colorado 80225 

www.nature.nps.gov/air 

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