Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Rule
 EPA-454/B-03-005

 September 2003

Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

Contract No. 68-D-02-0261

Work Order No. 1-06

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Emissions, Monitoring and Analysis Division

Air Quality Trends Analysis Group

Research Triangle Park, NC
DISCLAIMER

This report is a work prepared for the United States Government by Battelle. In no event

shall either the United States Government or Battelle have any responsibility or liability for any

consequences of any use, misuse, inability to use, or reliance upon the information contained

herein, nor does either warrant or otherwise represent in any way the accuracy, adequacy,

efficacy, or applicability of the contents hereof.

ACKNOWLEDGMENTS

The Environmental Protection Agency wishes to acknowledge the assistance and input

provided by the following advisors in the preparation of this guidance document: 

Rodger Ames, National Park Service, Cooperative Institute for Research in the Atmosphere

(CIRA); Shao-Hang Chu, USEPA; Rich Damberg, USEPA; Tammy Eagan, Florida Dept. of

Environmental Protection; Neil Frank, USEPA; Eric Ginsburg, USEPA; Dennis Haddow, U.S.

Fish and Wildlife Service; Ann Hess, Colorado State University; Hari Iyer, Dept. of Statistics,

Colorado State University; Mike Koerber, Lake Michigan Air Directors Consortium; Bill

Leenhouts, U.S. Fish and Wildlife Service; William Malm, National Park Service (CIRA); 

Debbie Miller, National Park Service; Tom Moore , Western Regional Air Partnership; Janice

Peterson, U.S. Department of Agriculture, Forest Service; Marc Pitchford, National Oceanic and

Atmospheric Administration, Air Resources Laboratory; Rich Poirot, State of Vermont, Dept. of

Environmental Conservation; Bruce Polkowsky, National Park Service; Tom Rosendahl, 

USEPA; David Sandberg, U.S. Department of Agriculture, Forest Service; Jim Sisler, National

Park Service (CIRA); Tim Smith USEPA; Ken Walsh, SAIC.
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

i

TABLE OF CONTENTS

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

List of Tables ................................................................v

Abbreviations and Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

Glossary of Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 1

1.1 What is regional haze? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 1

1.2 What is meant by the term “natural visibility conditions?” . . . . . . . . . . . . . . . 1 - 1

1.3 What is the purpose of the Guidance for Estimating Natural 

Visibility Conditions Under the Regional Haze Rule? . . . . . . . . . . . . . . . . . . 1 - 2

1.4 Does this guidance document apply to Tribal Class I areas as 

well as mandatory Federal Class I areas? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 3

1.5 What is the statutory and regulatory background for the 

regional haze program? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 4

1.6 What visibility metric will be used for estimating natural conditions,

 setting goals, and tracking progress? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 4

1.7 What are the key requirements and milestones for 

State implementation plans, pertaining to the estimation of 

natural visibility conditions under the regional haze rule? . . . . . . . . . . . . . . . 1 - 5

1.8 What other factors should be considered in developing progress goals? . . . . 1 - 7

1.9 What progress reviews and future SIP revisions are required 

under the regional haze rule? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 8
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

ii

1.10 Should estimates of natural visibility conditions reflect 

contemporary conditions and land use patterns, or historic conditions? . . . . . 1 - 9

1.11 What estimates of natural conditions are referenced in the regional haze

rule and preamble? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 9

1.12 How are the natural visibility conditions at a mandatory 

Federal Class I area determined? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 10

1.13 What approaches for estimating natural conditions are discussed 

in this guidance document? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 10

1.14 How are natural emissions from fire taken into account in 

estimates of natural PM and visibility levels? . . . . . . . . . . . . . . . . . . . . . . . . 1 - 12

1.15 How does the need to consider the fire component for natural 

visibility conditions interface with EPA's general policies regarding 

fire emissions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 13

1.16 Can a State delay submittal of its control strategy SIP and associated 

mandatory Federal Class I area progress goals until it has developed

a “refined” estimate of natural conditions? . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 14

2. DEFAULT APPROACH TO ESTIMATING NATURAL 

VISIBILITY CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 1

2.1 What are the default estimates of the natural concentrations for the 

PM2.5 components? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 1

2.2 What should be done if the default estimate for any naturally contributed

 species exceeds the corresponding measured concentrations? . . . . . . . . . . . . 2 - 2

2.3 How are the long-term relative humidity data used to determine 

f(RH) values? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 2
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

iii

2.4 How is the default natural light extinction at a mandatory 

Federal Class I area calculated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 4

2.5 How are the default bext values used to estimate natural 

visibility in deciview units? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 - 5

2.6 How are the 20% best visibility days and the 20% worst 

visibility days determined in the default approach? . . . . . . . . . . . . . . . . . . . . 2 - 6

3. REFINED ESTIMATION APPROACHES REGIONAL 

& SITE-SPECIFIC APPLICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1

3.1 Why might States want to use a refined approach to estimate 

natural visibility conditions? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1

3.2 What are some of the approaches that could be used by States to 

refine the default natural visibility estimates? . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 1

3.3 Which refined approach is most appropriate for States to use? . . . . . . . . . . . . 3 - 2

3.4 What should States do if they want to use a refined approach, 

rather than the default approach to estimate natural visibility conditions? . . . 3 - 2

3.5 How might an infrequent natural impact be quantified? . . . . . . . . . . . . . . . . . 3 - 3

3.6 Can natural visibility estimates be made on a 

sample-period-by-sample-period basis? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 - 3

Appendix A - Annual Average f(RH) and Monthly Average f(RH)

Values at All Mandatory Federal Class I Areas . . . . . . . . . . . . . . . . . . . . . . A - 1

Origin of Relative Humidity and f(RH) Values . . . . . . . . . . . . . . . . . . . . . . . A - 2

Appendix B - Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas . . . . . . . . . . . . . . . . . . . . . B - 1
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

iv

List of Figures

Figure 1-1 Example of method for determining mandatory Federal Class I area

 rate of progress to be analyzed in SIP development process.

 (a HI values for 2004 are based on 2000-2004 data, etc.) . . . . . . . . . . . . . . . . 1 - 6

Figure 1-2 Types of Data Used in Approach for Estimating 

Natural Visibility Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 - 11

Figure 2-1 Estimates of the Default 10% Natural Haze Index Values (in dv)

(Note different color scales for the two parts of the figure) . . . . . . . . . . . . . . 2 - 9

Figure 2-2 Estimates of the Default 90% Natural Haze Index Values (in dv)

(Note different color scales for the two parts of the figure) . . . . . . . . . . . . . 2 - 10

Figure A-1 Monthly Average f(RH) Values for February 

(all weather stations shown) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 15

Figure A-2 Monthly Average f(RH) Values for May 

(all weather stations shown) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 16

Figure A-3 Monthly Average f(RH) Values for August 

(all weather stations shown) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 17
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

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List of Tables

Table 2-1 Average Natural Levels of Aerosol Componentsa . . . . . . . . . . . . . . . . . . . . . . 2 - 2

Table A-1 Values for f(RH) determined from the growth of ammonium sulfate . . . . . . A - 5

Table A-2 Recommended Monthly Site-Specific f(RH) Values for Each 

Mandatory Federal Class I Area, Based on the Representative 

IMPROVE Site Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 6

Table A-3 Monthly Site-Specific f(RH) Values for Each Mandatory Federal 

Class I Area, Based on the Centroid of the Area

(Supplemental Information) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A - 10

Abbreviations and Acronyms
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

vi

bext - Total light extinction 

CAA – Clean Air Act

CAAA – 1990 Clean Air Act Amendments

CASTNet - Clean Air Status and Trends Network

CM - Coarse mass

EPA – United States Environmental Protection Agency

f(RH) – Relative humidity adjustment factor

IMPROVE – Interagency Monitoring of protected Visual Environments

LAC - Light absorbing carbon

Mm-1 - Inverse megameter (10-6 m-1)

NAAQS – National Ambient Air Quality Standards

NWS - National Weather Service

OC - Organic carbon

OMC - Organic carbon mass

OP - Pyrolized organics

PM – Particulate matter

PM2.5 – Particulate matter with an aerodynamic diameter less than 2.5 microns

PM10 – Particulate matter with an aerodynamic diameter less than 10 microns

RH - Relative humidity

SIP – State Implementation Plan

TOR - Thermal optical reflectance
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program

vii

Glossary of Terms

Aerosols – Suspensions of tiny liquid and/or solid particles in the air.

Coarse mass – Mass of particulate matter with an aerodynamic diameter greater than 2.5 microns but

less than 10 microns.

Deciview (dv) - The unit of measurement of haze, as in the haze index (HI) defined below.

 Default approach - The basic approach recommended by EPA to estimate the natural visibility

conditions. States may choose to adopt the default values for natural visibility conditions or, with

sufficient technical justification, propose alternatives to the basic approach or generate refined estimates. 

EPA believes that the default values that are provided in this document are adequately justified and

believes that it can propose for approval States’ use of them. However, EPA may not guarantee approval

prior to receiving and fully considering public comment on any proposed actions.

Default values - the values obtained from adopting the default approach to estimating natural visibility

conditions.

Fine particulate matter – particulate matter with an aerodynamic diameter less than 2.5 microns (PM2.5).

Fine soil – Particulate matter composed of material from the Earth’s soil, with an aerodynamic diameter

less than 2.5 microns. The fine soil mass is calculated from chemical mass measurements of fine

aluminum, fine silicon, fine calcium, fine iron, and fine titanium as well as their associated oxides.

Haze index (HI) – A measure of visibility derived from calculated light extinction measurements, that is

designed so that uniform changes in the haze index correspond to approximately uniform incremental

changes in visual perception, across the entire range of conditions from pristine to highly impaired. The

haze index [in units of deciviews (dv)] is calculated directly from the total light extinction [bext expressed

in inverse megameters (Mm-1)] as follows:

HI = 10 ln (bext/10)

Least-impaired days – The clearest, or least hazy, days.

Light absorbing carbon - Carbon particles in the atmosphere that absorb light; sometimes reported as

elemental carbon.

Light extinction – A measure of how much light is absorbed or scattered as it passes through a medium,

such as the atmosphere. The aerosol light extinction coefficient refers to the absorption and scattering by

aerosols, and the total light extinction coefficient refers to the sum of the aerosol light extinction

coefficient, the absorption coefficient of gases (such as NO2), and the atmospheric light extinction

coefficient due to molecular light scattering (Rayleigh scattering).
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

viii

Mandatory Federal Class I areas – Certain National Parks (over 6,000 acres), wilderness areas (over

5,000 acres), national memorial parks (over 5,000 acres), and international parks that were in existence as

of August 1977. Appendix A lists the mandatory Federal Class I areas. 

Most impaired days – the dirtiest, or haziest, days.

Nitrate – Solid or liquid particulate matter containing ammonium nitrate [NH4NO3] or other

nitrate salts. Atmospheric nitrate aerosols are often formed from the atmospheric oxidation of

oxides of nitrogen (NOx).

Organic carbon – Aerosols composed of organic compounds, which may result from emissions

from incomplete combustion processes, solvent evaporation followed by atmospheric

condensation, or the oxidation of some vegetative emissions.

Particulate matter – Material that is carried by liquid or solid aerosol particles with

aerodynamic diameters less than 10 microns (in the discussions of this report). The term is used

for both the in situ atmospheric suspension and the sample collected by filtration or other means.

Rayleigh scattering – Light scattering by gases in the atmosphere. At an elevation of 1.8

kilometers, the light extinction from Rayleigh scattering is approximately 10 inverse megameters

(Mm-1).

Relative humidity – The partial pressure of water vapor at the existing atmospheric temperature

divided by the saturated vapor pressure of water at that temperature, expressed as a percentage.

Sulfate – Solid or liquid particulate matter composed of sulfuric acid [H2SO4], ammonium

bisulfate [NH4HSO4], or ammonium sulfate [(NH4)2SO4], or other sulfate salts. Atmospheric

sulfate aerosols are often formed from the atmospheric oxidation of sulfur dioxide.

Total carbon – Sum of the light absorbing carbon and organic carbon.

Visibility impairment – Any humanly perceptible change in visibility (light extinction, visual range,

contrast, coloration) from that which would have existed under natural conditions. This change in

atmospheric transparency results from added particulate matter or trace gases.

1. INTRODUCTION

1.1 What is regional haze?
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 1

Regional haze is visibility impairment caused by the cumulative air pollutant emissions

from numerous sources over a wide geographic area. Visibility impairment is caused by

particles and gases in the atmosphere. Some particles and gases scatter light while others absorb

light. The net effect is called “light extinction.” The result of the scattering and absorption

processes is a reduction of the amount of light from a scene that is returned to the observer, and

scattering of other light into the sight path, creating a hazy condition. 

The primary cause of regional haze in many parts of the country is light scattering

resulting from fine particles (i.e., particulate matter less than 2.5 microns in diameter, referred to

as PM2.5) in the atmosphere. These fine particles can contain a variety of chemical species

including carbonaceous species (i.e., organics and elemental carbon), as well as ammonium

nitrate, sulfates, and soil. Additionally, coarse particles between 2.5 and 10 microns in diameter

can contribute to light extinction. Each of these components can be naturally occurring or the

result of human activity. The natural levels of these species result in some level of visibility

impairment, in the absence of any human influences, and will vary with season, daily

meteorology, and geography. 

1.2 What is meant by the term “natural visibility conditions?”

The term “natural visibility conditions” represents the ultimate goal of the regional haze

program, consistent with the national visibility goal set forth in section 169A of the Clean Air

Act (CAA). The national visibility goal is to remedy existing and prevent future human-caused

impairment of visibility in mandatory Federal Class I areas. Regional haze strategies are to

make reasonable progress towards this goal. 

Natural visibility conditions represent the long-term degree of visibility that is estimated

to exist in a given mandatory Federal Class I area in the absence of human-caused impairment. 

It is recognized that natural visibility conditions are not constant, but rather they vary with

changing natural processes (e.g., windblown dust, fire, volcanic activity, biogenic emissions). 

Specific natural events can lead to high short-term concentrations of particulate matter and its

precursors. However, for the purpose of this guidance and implementation of the regional haze

program, natural visibility conditions represents a long-term average condition analogous to the

5-year average best-and worst-day conditions that are tracked under the regional haze program. 

1.3 What is the purpose of the Guidance for Estimating Natural Visibility Conditions

Under the Regional Haze Rule? 

The purpose of this document is to provide guidance to the States in implementing the
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1

64 Federal Register 35769, July 1, 1999.

2 In the context of this guidance, the term "default" refers to the basic approach recommended by EPA to

estimate the natural visibility conditions and the values obtained from adopting this approach. States are welcome to

adopt the default values for natural visibility conditions or, with sufficient technical justification, to propose

alternatives to the basic approach or to generate refined estimates. In theabsence of refinement, EPA recommends

that the default values provided in this document be adopted.

1 - 2

regional haze program under the Clean Air Act. The regional haze regulations were published

by EPA in 1999.1

 They are designed to protect visual air quality in 156 National Parks and

wilderness areas (known as “mandatory Federal Class I areas”), across the country. As part of

the program, States will develop goals and implement strategies for improving visibility in each

mandatory Federal Class I area. Estimates of natural visibility conditions are needed by the

States for the goal development process. This guidance document describes “default”2

 and

“refined” approaches for estimating natural conditions. The EPA believes that natural conditions

estimates developed using the default approach will be adequate to satisfy the requirements of

the regional haze rule for the initial State implementation plan (SIP) submittals due no later than

2008. 

This document provides guidance to EPA Regional, State, and Tribal air quality

management authorities and the general public, on how EPA intends to exercise its discretion in

implementing Clean Air Act provisions and EPA regulations, concerning the estimation of

natural conditions under the regional haze program. The guidance is designed to implement

national policy on these issues. Sections 169A and 169B of the Clean Air Act (42) U.S.C. § §

7491,7492 and implementing regulations at 40 CFR 51.308 and 51.309 contain legally binding

requirements. This document does not substitute for those provisions or regulations, nor is it a

regulation itself. Thus, it does not impose binding, enforceable requirements on any party, nor

does it assure that EPA may approve all instances of its application, and thus the guidance may

not apply to a particular situation based upon the circumstances. The EPA and State decision

makers retain the discretion to adopt approaches on a case-by-case basis that differ from this

guidance where appropriate. Any decisions by EPA regarding a particular SIP demonstration

will only be made based on the statute and regulations, and will only be made following notice

and opportunity for public review and comment. Therefore, interested parties are free to raise 

questions and objections about the appropriateness of the application of this guidance to a

particular situation; EPA will, and States should, consider whether or not the recommendations

in this guidance are appropriate in that situation. This guidance is a living document and may be
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

3 Areas designated as mandatory Class I areas are those National Parks exceeding 6,000 acres, wilderness

areas and national memorial parks exceeding 5,000 areas, and all international parks which were in existence on

August 7, 1977. Visibility has been identified as an important value in 156 of these areas. See 40 CFR part 81,

subpart D. The extent of a Class I area includes subsequent changes in boundaries, such as park expansions. (CAA

section 162(a)). States and tribes may designate additional areas as Class I, but the requirements of the visibility

program under section 169A of the CAA apply only to "mandatory Class I areas," and do not affect these additional

areas. For the purpose of this guidance document, the term “Class I area” will be used interchangeably with

“mandatory Federal Class I area.”

4 See 63 Federal Register 7254 (February 12, 1998), and 40 CFR Part 49.

1 - 3

revised periodically without public notice. The EPA welcomes public comments on this

document at any time and will consider those comments in any future revision of this guidance

document. 

Readers of this document are cautioned not to regard statements recommending the use

of certain procedures or defaults as either precluding other procedures or information or

providing guarantees that using these procedures or defaults will result in actions that are fully

approvable. As noted above, EPA cannot assure that actions based upon this guidance will be

fully approvable in all instances, and all final actions may only be taken following notice and

opportunity for public comment.

1.4 Does this guidance document apply to Tribal Class I areas as well as mandatory

Federal Class I areas? 

Not directly, although the procedures for estimating natural conditions that are described

in this guidance can be used by Tribes if desired. The CAA and the regional haze rule call for

the protection of visibility in 156 “mandatory Federal Class I areas.”3

 Tribes can establish Class

I areas for the purposes of the prevention of significant deterioration (PSD) program, but the

CAA does not provide for the inclusion of Tribal areas as mandatory Federal Class I areas

subject to section 169A and 169B of the CAA. For this reason, progress goals and natural

conditions estimates do not have to be established for Tribal Class I areas. 

However, Tribes may find it advantageous for a number of reasons to participate in

regional planning organizations (RPO) for regional haze and to develop regional haze tribal

implementation plans (TIPs). Participation in an RPO may allow some Tribes to build capacity

and enhance their air quality management capabilities. Under the Tribal Air Rule, Tribal 

governments may elect to implement air programs in much the same way as States, including 

development of Tribal implementation plans.4

 In this way, Tribes can work with other States

and 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

5 See 45 Federal Register 80084 (December 2, 1980).

6 See 64 Federal Register 35713 (July 1, 1999). See also 40 CFR 51.300-309.

1 - 4

Tribes on the development and adoption of specific emissions reduction strategies designed to

protect air quality across a broad region including Tribal and State lands. 

1.5 What is the statutory and regulatory background for the regional haze program? 

In section 169A of the 1977 Amendments of the Clean Air Act, Congress established a

national visibility goal as the “prevention of any future, and the remedying of any existing,

impairment of visibility in mandatory Federal Class I areas which impairment results from

manmade air pollution.” States are required to develop implementation plans that make

“reasonable progress” toward this goal. 

The EPA issued initial visibility regulations in 19805

 that addressed visibility impairment

in a specific mandatory Federal Class I area that is determined to be “reasonably attributable” to

a single source or small group of sources. Regulations to address regional haze were deferred

until improved techniques could be developed in monitoring, modeling, and in understanding the

effects of specific pollutants on visibility impairment. The 1990 Clean Air Act Amendments

included language in Section 169B to focus attention on regional haze issues. That section

called for EPA to establish the Grand Canyon Visibility Transport Commission, and to issue

regional haze rules within 18 months of receipt of a final report from the Commission. The EPA

issued regional haze regulations in 1999.6 

As noted in question 1.2 above, estimates of “natural visibility conditions,” which are the

national visibility goal of the Clean Air Act, are needed as part of the implementation process for

the regional haze program. 

1.6 What visibility metric will be used for estimating natural conditions, setting goals, and

tracking progress?

According to the Regional Haze Rule, baseline visibility conditions, progress goals, and

changes in natural visibility conditions must be expressed in terms of deciview (dv) units. The

deciview is a unit of measurement of haze, implemented in a haze index (HI) that is derived from

calculated light extinction, and that is designed so that uniform changes in haziness correspond 

approximately to uniform incremental changes in perception, across the entire range of 

conditions, from pristine to highly impaired. The HI is expressed by the following formula:
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

7 Note that in the May 2002 American Corn Growers decision, the DC circuit court of appeals raised

concerns with some of the deadlines for regional haze SIPs in the 1999 regional haze rule. While these issues are

not fully resolved, EPA intends to seek solutions that will ensure that the schedule for regional haze implementation

1 - 5

HI = 10 ln(bext/10)

where 

bext represents total light extinction expressed in inverse megameters (i.e., Mm-1 = 10-6 m-1).

1.7 What are the key requirements and milestones for State implementation plans,

pertaining to the estimation of natural visibility conditions under the regional haze rule?

The regional haze rule requires States to develop SIPs that include 1) reasonable progress

goals for improving visibility in each mandatory Federal Class I area, and 2) a set of emission

reduction measures to meet these goals. A State that does not have any Class I areas will not

establish any progress goals in its SIP, but it is required to consult with nearby States having

Class I areas that may be impacted by emissions from the State. A State without any Class I

areas will also need to adopt emission reduction strategies to address its contribution to visibility

impairment problems in Class I areas located in other States. 

Specifically, a State is required to set progress goals for each Class I area in the State that: 

• provide for an improvement in visibility for the 20% most impaired (i.e., worst

visibility) days over the period of the implementation plan, and

• ensure no degradation in visibility for the 20% least impaired (i.e., best visibility)

days over the same period. 

Baseline visibility conditions for the 20% worst and 20% best days are to be determined using

monitoring data collected during calendar years 2000-2004. Baseline conditions for 2000-2004,

progress goals, and tracking changes over time are to be expressed in deciview units via the haze

index.

Most States (and Tribes as appropriate) participating in regional planning organizations

will submit regional haze implementation plans, including estimates of natural conditions and

proposed progress goals, in the 2007-2008 time frame7

. In developing any progress goal, the
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

plans is fully harmonized with the schedule for implementation plans addressing PM2.5 nonattainment. 

1 - 6

2000-4 2018 2064

29

11

Required Analysis for

1st Implementation 

Period = 4.2 deciviews 

Haze Index

(Deciview)

Estimated 

Natural

Conditions

Baseline

Conditions

Yeara

2000-4 2018 2064

29

11

Required Analysis for

1st Implementation 

Period = 4.2 deciviews 

Haze Index

(Deciview)

Estimated 

Natural

Conditions

Baseline

Conditions

Yeara

Figure 1-1 Example of method for determining mandatory Federal Class I

area rate of progress to be analyzed in SIP development process. 

( a

 HI values for 2004 are based on 2000-2004 data, etc.)

State will need to analyze and consider in its set of options the rate of improvement between

2004 (when 2000-2004 baseline conditions are set) and 2018 that, if maintained in subsequent

implementation periods, would result in achieving estimated natural conditions in 2064. In the

example in Figure 1-1, baseline conditions for the 20% worst days exceed estimated natural

conditions by 18 deciviews. The rate the State must analyze and consider for the 2018 progress

goal is equal to 18 divided by 60 years = 0.3 deciviews per year x 14 years (2004 to 2018) = 4.2

deciviews. The State must demonstrate in the SIP whether it finds that this rate is reasonable or

not, taking into consideration the relevant statutory factors. If it finds that this first rate is not

reasonable, the State shall include a demonstration supporting its finding that an alternate rate is

reasonable. 

In order to determine the 2004-2018 progress rate for this analysis, the State should calculate

baseline conditions in accordance with EPA guidance on tracking progress and use this guidance

document for estimating natural conditions.
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

8 See CAA section 169A(g).

1 - 7

1.8 What other factors should be considered in developing progress goals? 

Other important issues to be considered in developing mandatory Federal Class I area 

progress goals include the reasonable progress factors in the CAA, consultation with Tribes and

other States, and emission reductions due to other Clean Air Act programs. The reasonable

progress factors8 to consider in developing any progress goal are: 

• the costs of compliance;

• the time necessary for compliance;

• the energy and non-air quality environmental impacts of compliance; and

• the remaining useful life of any existing source subject to such requirements.

The EPA plans to develop additional guidance on how to address these factors in the goal setting

process. 

Because visibility impairment results from human activities and their emissions

transported over long distances - hundreds of miles in many cases - addressing impairment can

be effective only through efforts among multiple States. For this reason, States are required to

consult with other States (and Tribes, as appropriate) in developing mandatory Federal Class I

area progress goals and long-term strategies to meet these goals. If a State is reasonably

anticipated to cause or contribute to impairment in a mandatory Federal Class I area in another

State, it is required to consult with that State on the development of that State’s progress goals,

and it must include strategies in its SIP that address its contribution to the haze in that State’s

mandatory Federal Class I area. Emissions reductions from other States may likewise be taken

into account in setting mandatory Federal Class I area goals. The EPA supports the regional

planning organization process currently under way as the most effective means to address the

requirements of the regional haze program, and it is expected that much of the consultation,

apportionment demonstrations, and technical documentation needed for SIPs will be facilitated

and developed by the regional planning organizations.

Progress goals should also take into account any emission reduction strategies in place or 

on the way in order to meet other Clean Air Act requirements. For example, emission reduction

strategies implemented to attain the PM2.5 and ozone NAAQS, and national mobile source

measures such as the Tier II or heavy duty diesel regulations, should be taken into account in 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

9 See regional haze rule, 40 CFR Section 51.308(d) (1) (vi).

1064 Federal Register 35747 (Thursday, July 1, 1999).

1 - 8

developing mandatory Federal Class I area progress goals for regional haze. Thus, EPA does not

expect any progress goals for regional haze to be less ambitious than the level of visibility

improvement expected from other programs.9

 

1.9 What progress reviews and future SIP revisions are required under the regional haze

rule? 

After the initial SIPs are approved, States will conduct formal progress reviews (in the

form of a SIP revision) every 5 years from the date of SIP submittal (e.g., in 2013 if the initial

SIP is submitted in 2008). Progress will be reviewed in terms of changes in visibility based on

monitoring data, and in terms of the implementation of emission reduction measures contained in

the plan. If progress is not consistent with the visibility and emission reduction goals established

in the original SIP, the State must evaluate the reason for lack of progress and take any

appropriate further action. If the lack of progress is primarily due to emissions from within the

State, then the State must revise its implementation plan within 1 year to include additional

measures to make progress. If the lack of progress is primarily due to emissions from other

States, then the State must reinitiate the regional planning process to address this problem in the

next major SIP revision (e.g., in 2018). If the State finds that international emissions sources are

responsible for a substantial increase in emissions in any Class I area or causing a deficiency in

visibility progress, the State must submit a technical demonstration to EPA in support of its

finding. Similarly, the State should submit a technical demonstration if the State finds that

unusual events (e.g., large wildfires), have affected visibility progress during the 5-year period.10

Given that progress is determined based upon long-term averaging, the EPA believes that it is

unlikely that such events will have a significant effect in most cases. See Section 1.14 for

additional information about consideration of natural emissions from fire.

States will be required to conduct a comprehensive SIP revision in 2018 and every 10

years thereafter. This process will involve re-evaluating rates of progress for each mandatory

Federal Class I area within the State as noted above and establishing new visibility improvement 

goals for these areas. The revised SIP should also include any revised emission reduction

measures needed to meet the new mandatory Federal Class I area progress goals. 

1.10 Should estimates of natural visibility conditions reflect contemporary conditions and
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

11 National Acid Precipitation Assessment Program, 1991. Acid Deposition: State of Science and

Technology. Report 24. Visibility: Existing and Historical Conditions – Causes and Effects. Table 24-6, 

Washington, DC.

1 - 9

land use patterns, or historic conditions?

For the purposes of this guidance, estimates of natural visibility conditions should reflect

contemporary conditions and land use patterns. That is, estimates should attempt to calculate the

degree of visibility impairment that exists today, given current vegetative landscapes, when

human emissions contributions are removed. We believe that this is a more practical approach 

than attempting to speculate about what visibility conditions would have existed under the

vegetative landscapes that existed 3 or 4 centuries ago, i.e., prior to the arrival of European

settlers.

1.11 What estimates of natural conditions are referenced in the regional haze rule and

preamble?

Section 308(d)(2)(iii) of the regional haze rule states that “[natural visibility conditions

must be calculated by estimating the degree of visibility impairment existing under natural

conditions for the most impaired and least impaired days, based on available monitoring

information and appropriate data analysis techniques.]” In the preamble to the regional haze

rule, EPA states that “it will be appropriate to derive regional estimates of natural visibility

conditions by using estimates of natural levels of visibility-impairing pollutants in conjunction

with the IMPROVE methodology for calculating light extinction from measurements of the five

main components of fine particle mass (sulfate, nitrate, organic carbon, elemental carbon, and

crustal material).” As described elsewhere in this document, in addition to the five main

components of fine particle mass, terms for coarse particle mass and Rayleigh scattering are also

included in the calculation of light extinction.

The 1991 peer-reviewed report of the National Acid Precipitation Assessment Program

(NAPAP) provides annual average estimates of natural concentrations for these six main

components of PM for the eastern and western regions of the country.11 By applying

assumptions for average extinction efficiencies for each PM component and for the effect of

humidity, the NAPAP report also included estimates of natural visibility conditions on an annual

average basis. Those estimates are equivalent to about 9.6 deciviews in the eastern region and

5.3 deciviews in the western region of the United States. 

In the regional haze preamble, EPA used the NAPAP estimates for natural concentrations

of PM mass components, but used assumptions for average extinction efficiencies and annual 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 10

average humidity, based on updated methodologies developed under the IMPROVE program. 

Using this approach, EPA found that an appropriate estimate for natural conditions for the 20%

worst days would be approximately 11-12 deciviews in the east and 8 deciviews in the west. 

The preamble further stated that “with each subsequent SIP revision, the estimates of

natural conditions for each mandatory Federal Class I area may be reviewed and revised as

appropriate as the technical basis for estimates of natural conditions improve.” Possible

approaches for refining natural conditions estimates are discussed later in this document.

1.12 How are the natural visibility conditions at a mandatory Federal Class I area

determined?

The general approach to estimating natural visibility conditions is based on the

IMPROVE methodology for calculating visibility extinction. Using estimates of the natural

concentrations of the primary components of particulate matter, along with estimates of the

extinction efficiencies of these species, and site-specific factors to account for the effects of

relative humidity on light scattering by particles, values for the annual average light extinction at

each mandatory Federal Class I area are calculated. Figure 1-2 summarizes the approach to

estimating natural visibility conditions.

1.13 What approaches for estimating natural conditions are discussed in this guidance

document?

Chapter 2 of this guidance document describes the default approach for estimating

natural visibility conditions for each mandatory Federal Class I area. This approach (see Figure

1-2) relies on the NAPAP estimates for PM mass components and the IMPROVE methodology

for calculating light extinction. Important enhancements incorporated in this approach include

the use of 10-year average relative humidity data from more than 300 weather stations, for

development of appropriate relative humidity adjustment factors (f(RH)), and statistical

techniques for estimating values for the 20% most impaired and 20% least impaired days. The

EPA believes that this approach provides an adequate estimate of natural conditions for the

purpose of developing initial visibility improvement goals and expects to be able to propose to

approve goals in SIP submissions relying on this approach. 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 11

Estimates of Natural

Concentrations

of the Primary 

Components of PM2.5

Species Specific 

Extinction 

Efficiencies

Site-Specific

f(RH) Factors

Calculated bext

(IMPROVE 

Methodology)

Deciview 

Values

Estimates of Natural

Concentrations

of the Primary 

Components of PM2.5

Species Specific 

Extinction 

Efficiencies

Site-Specific

f(RH) Factors

Calculated bext

(IMPROVE 

Methodology)

Deciview 

Values

Figure 1-2 Types of Data Used in Approach for Estimating 

Natural Visibility Conditions

Chapter 3 of this guidance describes some alternative approaches by which States may refine

their natural conditions estimates based on additional data and analyses. For example, one

possible refined approach would involve updating the estimates of natural PM mass

concentrations for each PM component, based on recent peer-reviewed literature, rather than

using the NAPAP default values. These methods do not represent an exhaustive list and States

are free to develop alternative approaches that will provide natural visibility conditions estimates

that are technically and scientifically supportable. Any refined approach should be based on

accurate, complete, and unbiased information and should be developed using a high degree of

scientific rigor.
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 12

1.14 How are natural emissions from fire taken into account in estimates of natural PM

and visibility levels?

Because some of the fires producing particulate emissions are naturally occurring, and

would occur in the absence of human activities, the estimate of natural visibility conditions must

take fire into account. 

Appendix A of the NAPAP report discusses the approach used to estimate natural mass

levels for each PM component. The estimates are based on compilations of natural versus 

man-made emission levels, ambient measurements in remote areas, and regression studies using

man-made and/or natural tracers. Uncertainties are recognized in the estimates of each PM

component. The report recognizes that estimated natural levels of both organic carbon and

elemental carbon include contributions from fire emissions. The NAPAP report includes organic

carbon as the most significant natural PM component by mass in both the eastern and western

regions. Because most of the studies cited in the NAPAP Appendix were conducted in relatively

remote areas, it is reasonable to assume that the contribution of fire to PM mass in the NAPAP

estimates represents the natural regional contribution by fire. The NAPAP estimates included

contributions from smoke but no distinction was made at the time between natural and manmade fire. Nonetheless, these are the best estimates available in the literature for current

contributions from natural sources. Since the estimate of natural visibility conditions is a longterm (5-year) average, and because we expect to be able to further refine estimates over time

based on improved information and methods, a regional contribution by fire emissions to overall

natural visibility conditions should be adequate for the purpose of developing initial progress

goals. 

Data should be available for EPA and States to develop improved estimates of the

contribution of fire emissions to natural visibility conditions in mandatory Federal Class I areas

over time. Information from a number of additional activities and technical tools should be

available over the coming years, including:

• implementation of a coordinated fire data system or fire tracking system;

• the collection of multiple years of speciated PM data in mandatory Federal Class I

areas, and the assessment of potential contributions by natural fire events using

data from the fire tracking system;

• development of chemical analysis techniques to identify carbon attributed to fire

versus other sources;

• development of improved emissions factors and tracking of fire activity levels; 

and 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 13

• improved regional scale fire modeling, or remote sensing tools to retrospectively

determine whether smoke from a fire impacted a Class I airshed.

1.15 How does the need to consider the fire component for natural visibility conditions

interface with EPA's general policies regarding fire emissions? 

The purpose of this document is to address the identification of methodologies for States

to use in estimating natural visibility conditions, including the contribution from fire. This 

document is not intended to identify or dictate potential emission sources and control

requirements. 

The EPA acknowledges the need to allow the use of fire as an efficient and economical

land management tool. The use of fire has proven benefits in maintaining the health of firetolerant and fire-dependent plant and animal ecosystems. In some cases, fire may be the only

viable alternative to maintaining species diversity, enhancing productivity, or eliminating the

threat of disease or catastrophic wildfires. The EPA, in partnership with the U.S. Department of

Agriculture (USDA), and the Department of the Interior (DOI), will work with Federal and

private land managers to develop alternatives to fire where applicable but will allow fire as a

viable option in the maintenance of forest land and agricultural (cropland, rangeland,

pastureland) ecosystems. The EPA has participated in the review of the USDA/DOI Wildland

Fire Management Policies (1995 and 2001) and in the development of the 10-Year

Comprehensive Strategy which establishes fire management priorities. The EPA is also actively

involved with USDA and their Agricultural Air Quality Task force in addressing fire as a

management tool for crop production and rangeland management. The EPA expects to amend

the 1998 Interim Air Quality Policy on Wildland and Prescribed Fires to incorporate the final

policy on burning for agricultural crop production and rangeland management. The EPA’s

overall policy approach encourages the use of smoke management plans to minimize the impacts

of burning activities on air quality and visibility impairment and provide some flexibility to areas

with certified smoke management programs if it is determined that emissions from these fires

contribute significantly to the National Ambient Air Quality Standards (NAAQS) violations.

States/tribes are aware of their responsibility to meet air quality standards and develop

plans on how they will meet the standards. It is EPA’s view that smoke management plans are

best negotiated and implemented at the local level, taking into account regional impacts, and that

sources of emissions from burning are treated in an equitable manner. Recognizing the State’s

responsibility to meet air quality standards, EPA encourages flexibility for local decisions on

smoke management by States, locals, or tribal authorities. To address the NAAQS, reduce
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 14

human health risk or exposure, or improve visibility in Class I areas, EPA encourages State and

local air regulatory authorities to include their respective State/local agriculture, forestry, and 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 15

park management agencies in stakeholder discussions and decisions, to ensure equitable and 

appropriate viable options for maintaining cropland, rangeland, pastureland, and forest land

ecosystems while meeting air quality goals and standards.

The EPA understands the benefits of a tracking system to keep accurate accounting of

fire emissions for emissions inventory, modeling, and attainment demonstrations and for the

purposes of making sound decisions regarding burn and no burn days. The EPA is currently

working with USDA and USDOI to develop a shared data system that would allow access to

information useful to Federal, State, and local agencies. Some States or regional organizations

have already started to develop their own tracking system for the area. The EPA is not endorsing

any particular tracking system and will work with the States and regional planning organizations

to make sure their tracking systems will interface with the Federal Tracking System to be

developed. 

In some cases, regional organizations have found it useful to classify fire emissions into

two categories, natural and man-made, for the purposes of estimating natural visibility

conditions. While EPA is not expressing an opinion on the importance of classifying fires, it

supports those organizations who wish to do so for the purposes of estimating visibility

conditions. However, the EPA does not require the distinction between natural and man-made

fires. The EPA believes that it is important to recognize that any such classification of fire

should not be construed to suggest any classification of emission sources for purposes of

identifying those that are subject to control requirements. The criteria used to classify fires may

or may not be the same criteria used to determine culpable sources and potential control

requirements. Identifying culpable sources and potential control requirements to meet SIP

requirements is beyond the scope and purpose of this document. 

1.16 Can a State delay submittal of its control strategy SIP and associated mandatory

Federal Class I area progress goals until it has developed a “refined” estimate of natural

conditions?

No, States cannot use the development of a refined estimate of natural visibility

conditions as a reason for delaying the submittal of regional haze control strategy SIPs required

by statute and regulation. The EPA believes that the default approach to estimating natural

visibility conditions presented in this document is adequate for the development of progress

goals for the first implementation period under the regional haze rule. In addition, the timeline

for implementing the regional haze program already includes a significant amount of lead time

for developing these SIPs, and EPA does not believe that SIP due dates may be extended beyond

the existing regulatory requirements. The EPA expects that States will need to begin assessing

progress goals and emission reduction strategies beginning in the 2004-2005 time frame, in order 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

1 - 16

to leave adequate time for air quality modeling, analysis of the statutory factors, consultation

with other States or Tribes, development of regional recommendations, and adoption of

individual State regulations by 2007-8. Because the process of planning and implementing

strategies and evaluating progress is an iterative one, there will be future opportunities to refine

progress goals based on new information about natural visibility conditions, rates of growth and

development, and the effectiveness of controls.
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

12Trijonis, J.C., NAPAP State of Science & Technology, Vol. III, 1990.

2 - 1

2. DEFAULT APPROACH TO ESTIMATING NATURAL VISIBILITY CONDITIONS

This section of the guidance document presents the default approach to be used in

estimating the natural visibility conditions for both the 20% most and 20% least impaired days. 

2.1 What are the default estimates of the natural concentrations for the PM2.5 components?

The estimates of the annual averages for the natural levels of fine particle constituents

and of coarse particles are drawn from the 1990 report of NAPAP.12 That report draws published

data from a variety of sources and presents estimates for the natural levels of sulfates, organics,

light absorbing carbon (also referred to as elemental carbon), ammonium nitrate, soil dust, and

coarse particles for the eastern and western regions of the United States. The estimates presented

in that report include significant uncertainties which indicate that the actual natural levels for

these species are likely to fall within a range around the values reported. However, with minor

adjustments, these estimates provide the starting point for calculating natural visibility

conditions in the mandatory Federal Class I areas. 

The approach to estimating natural conditions presented in the NAPAP report defines

two separate regions of the United States: (1) the East, which consists of all the States east of the

Mississippi River, and up to one tier of States west of the Mississippi; and (2) the West,

including the desert/mountain regions of the Mountain and Pacific time zones. Geographically,

these two subregions show strong differences in haze sources, vegetation, relative humidity, and

regional haze levels. Within these two subregions, spatial variations in the natural aerosol levels

would be expected. As a result, States near the boundary between East and West should choose

which set of NAPAP estimates are most appropriate and adopt those values.

Table 2-1 presents the default estimated natural concentrations of the particulate species

for the East and the West along with estimates of the dry extinction efficiencies for each species. 

These concentration estimates are used with the respective estimates of the dry extinction

efficiencies to establish the light extinction attributed to natural sources in the East and West. As

Table 2-1 shows, the natural concentration estimates differ between the East and West only in

the concentrations of sulfate and organic species.
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 2

Table 2-1 Average Natural Levels of Aerosol Componentsa

Average Natural Concentration

Error

Factor

Dry

Extinction

Efficiency

(m2

/g) West (μg/m3

) East (μg/m3)

Ammonium sulfate b 0.12 0.23 2 3

Ammonium nitrate 0.10 0.10 2 3

Organic carbon mass c 0.47 1.40 2 4

Elemental carbon 0.02 0.02 2-3 10

Soil 0.50 0.50 11⁄2 - 2 1

Coarse Mass 3.0 3.0 11⁄2 - 2 0.6

a: After Trijonis, see footnote 12

b: Values adjusted to represent chemical species in current IMPROVE light extinction algorithm; Trijonis

estimates were 0.1 μg/m3

 and 0.2 μg/m3 of ammonium bisulfate. 

c: Values adjusted to represent chemical species in current IMPROVE light extinction algorithm; Trijonis

estimates were 0.5 μg/m3

 and 1.5 μg/m3 of organic compounds.

2.2 What should be done if the default estimate for any naturally contributed species

exceeds the corresponding measured concentrations?

Contributions by natural sources to haze are defined as "those not from man-made

sources," accordingly, neither natural nor man-made contributions to haze can exceed the total

haze levels over any period of time. The default natural concentration estimates are for longterm average conditions, and so may be larger than the measured current concentrations for short

periods, but should not exceed the average concentration over several annual cycles. If the

average measured level of any of the six particle species (for the baseline period, or for any other

5-year period), is smaller than the corresponding default natural values, then the default values

should be replaced by values that are equal to or less than the measured values. This would

constitute a refinement of the default as discussed in Section 3. 

2.3 How are the long-term relative humidity data used to determine f(RH) values?

The U.S. EPA recently sponsored a project to examine measured hourly relative humidity

data over a 10-year period (1988-1997) within the United States, to derive month-specific 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

13 U.S. EPA, Interpolating Relative Humidity Weighting Factors to Calculate Visibility Impairment and

the Effects of IMPROVE Monitor Outliers, prepared by Science Applications International Corporation, Raleigh,

NC, EPA Contract No. 68-D-98-113, August 30, 2001.

2 - 3

climatological mean humidity correction factors for each mandatory Federal Class I area.13

These relative humidity (RH) factors were calculated from available hourly relative humidity

data from 292 National Weather Service (NWS) stations across the 50 States and the District of

Columbia, as well as from 29 IMPROVE and IMPROVE protocol monitoring sites, 48 Clean Air

Status and Trends Network (CASTNet) sites, and 13 additional sites administered by the

National Park Service.

 

The hourly RH measurements from each site were converted to hourly f(RH) values

using a non-linear weighting factor curve, based on a modified ammonium sulfate growth curve

(see Appendix A), applied to the 10 years of surface relative humidity data. 

The annual average f(RH) values for all mandatory Federal Class I areas are tabulated in

Appendix A of this document. Those values are used in the default approach to establishing

natural visibility conditions. The 12 monthly averaged f(RH) values for each of these Class I

areas are also tabulated in Appendix A. In most regions there is a seasonal cycle of relative

humidity, which is evident in the appropriate monthly f(RH) values. The monthly f(RH) values

may be used in refined estimates of the natural visibility conditions (Chapter 3). Note that Table

A-2 and supplementary Table A-3 only includes f(RH) values for the designated mandatory

Federal Class I areas. However, the software program needed to calculate f(RH) values for other

sites is available for use by States, Tribes, and other agencies or interested parties, upon request

to EPA. 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 4

b f RH SULFATE

f RH NITRATE

OMC

LAC

SOIL

CM

ext = +

+

+

+

+

+

( ) ( )[ ]

( ) ( )[ ]

( )[ ]

( )[ ]

( )[ ]

( . )[ ]

3

3

4

10

1

0 6

10

(1)

b

Mm

ext = + + + ++ +

= −

3 34 0 23 3 34 01 4 14 10 0 02 1 05 0 6 30 10

215 1

( . )[ . ] ( . )[ . ] [ . ] [ . ] [ . ] . [ . ]

.

2.4 How is the default natural light extinction at a mandatory Federal Class I area

calculated?

The calculation of natural light extinction is based on the IMPROVE methodology. 

Using the values in Table 2-1, the natural light extinction can be calculated from Equation 1:

where bext is the calculated total light extinction in inverse megameters. (Note: A value of 10

Mm-1 is used for all mandatory Federal Class I areas as an estimate of the light extinction caused

by the light scattering from gas molecules, i.e., Rayleigh scattering). Relative humidity

correction factors, f(RH), are included for the sulfate and nitrate species as these are hygroscopic

(i.e., absorb water) and their extinction efficiencies change with relative humidity. Annual

average site-specific f(RH) values for 154 of the 156 mandatory Federal Class I areas (Appendix

A) have been determined from historical data and are used in the default approach to establish

site-specific natural visibility conditions. 

Example calculations with Equation 1 will illustrate the use of the default approach. 

Looking at two examples in the East, and referring to Table 2-1 for default concentrations and

Appendix A for annual f(RH) values, we see that the natural total light extinction for the Acadia

National Park (Maine) is:

Similarly, for the Everglades National Park (Florida) bext is:

b

Mm

ext = + + + ++ +

= −

3 2 7 0 23 3 2 7 01 4 14 10 0 02 1 05 0 6 30 10

208 1

( . )[ . ] ( . )[ . ] [ . ] [ . ] [ . ] . [ . ]

.

In the West, we see that Bandelier National Monument (New Mexico) has a default natural light
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 5

HI b = 10 10 ext ln( / ) (2)

extinction of:

b

Mm

ext = + + + ++ +

= −

3 19 012 3 19 01 4 0 47 10 0 02 1 05 0 6 30 10

156 1

( . )[ . ] ( . )[ . ] [ . ] [ . ] [ . ] . [ . ]

.

and Yellowstone National Park (Wyoming) has a default bext of:

b

Mm

ext = + + + ++ +

= −

3 21 012 3 21 01 4 0 47 10 0 02 1 05 0 6 30 10

158 1

( . )[ . ] ( . )[ . ] [ . ] [ . ] [ . ] . [ . ]

.

The default natural light extinction values have been calculated by this approach for 154 of the

156 mandatory Federal Class I areas and are listed in Appendix B.

2.5 How are the default bext values used to estimate natural visibility in deciview units?

The default light extinction values are used to calculate estimates for the annual average

HI values (in dv units) at each mandatory Federal Class I area. These default HI values are

determined from Equation 2:

where bext is the default total light extinction in Mm-1 as calculated by Equation 1. From the

examples above, the default annual average HI value for Acadia National Park is:

HI

dv

=

=

10 215 10

7 7

ln( . / )

. .

For the Everglades National Park, the default HI value is:

HI

dv

=

=

10 208 10

7 3

ln( . / )

. .
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

14Rodger Ames and William Malm, Recommendations for Natural Condition Deciview Variability: An

Examination of IMPROVE Data Frequency Distributions, Proceedings of “Regional Haze and Global Radiation

Balance - Aerosol Measurements and Models: Closure Reconciliation and Evaluation,” October 2-5, 2002, Bend,

Oregon. 

2 - 6

The default HI value for Bandelier National Monument is:

HI

dv

=

=

10 156 10

4 4

ln( . / )

. .

and for Yellowstone National Park the default HI is:

HI

dv

=

=

10 158 10

4 6

ln( . / )

. .

The calculated annual average HI values for each mandatory Federal Class I area are presented

in Appendix B along with the default total light extinction (bext)values.

2.6 How are the 20% best visibility days and the 20% worst visibility days determined in

the default approach?

The calculated HI value represents an estimate of the annual average of daily natural

visibility in dv units. If daily HI values for the natural background visibility in dv units were

available, those values could be arranged in order, and the averages of the best 20% and the

worst 20% of the values could be calculated to establish the regional haze rule goals for each

mandatory Federal Class I area. However, since daily natural visibility HI values are not

available, the default approach provides only an estimate of the annual average natural

background visibility, and the averages for the best and worst 20% must be estimated.

Ames and Malm14 have shown that the frequency distributions of daily calculated HI

values for sites in the East and in the West, can each be well represented by normal distributions. 

Consequently, the average HI values for the 20% best visibility days and the 20% worst

visibility days can be estimated from 10th and 90th percentile HI values, respectively. That is,
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 7

since the frequency distributions appear to behave normally, the 10th and 90th percentile HI

values (p10 and p90, respectively) for a mandatory Federal Class I area can be estimated from

the following equations:

p HI . sd 10 128 = − (3)

and,

p HI . sd 90 128 = + (4)

where sd represents the standard deviation (in dv units) of the daily HI values for that area, and

HI is the annual average of the HI values. Estimates of sd for current visibility conditions for

eastern and western sites were derived from a database of current visibility conditions. At each

site, daily HI values were calculated from the calculated light extinction values, and the mean

and standard deviation of the daily HI values were determined. Comparison of sites within the

same region showed that, in the East, the current visibility conditions have on average an HI

value of approximately 18 dv, with an average sd of approximately 5 dv. In the West, the

current visibility conditions showed an average HI of approximately 8 dv and an average sd of

approximately 2.4 dv. More important in the present context, by inspection of the relationships 

between sd and HI! , Ames and Malm14 inferred best estimates of the sd values for natural 

visibility in both the West and East. In the West this best estimate of the natural visibility sd is 2

dv, whereas in the East the best estimate of the natural visibility sd is 3 dv. 

These estimates of the standard deviation of natural contributions to visibility impairment

can be used in Equations 3 and 4 above, along with the default natural HI values, to estimate the

averages of the 20% best and 20% worst natural visibility contributions. 

For example, the calculated 10th and 90th percentile natural HI values for Acadia National Park

are:

p

p

10 7 7 128 3 38

90 7 7 128 3 115

=− =

=+ =

. . () .

. . () .

Appendix B provides the default 10th and 90th percentile natural visibility HI values in dv

units for each of the 156 mandatory Federal Class I areas. Figure 2-1 is a map of the 10th

percentile default HI at mandatory Federal Class I areas across the United States, indicating a

range from approximately 2 dv in the West to 4 dv in the East. Figure 2-2 is a map of the 90th

percentile HI, which ranges from approximately 7 dv in the west to 11 dv in the East. Note that

different color scales apply to the East and West portions of Figures 2-1 and 2-2, as indicated in
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 8

the figures. Higher natural HI values in the northwest than the southwest United States are due

to higher RH in the northwest. Higher natural condition organic carbon mass concentrations in

the East are primarily responsible for higher default 10th and 90th percentile natural HI values in

the East relative to the western United States. As noted in Section 2.1, States near the boundary

between East and West have the option of choosing which set of default natural background

conditions to use. 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

2 - 9

Figure 2-1 Estimates of the Default 10% Natural Haze Index Values (in dv)

(Note different color scales for the two parts of the figure)
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

3 - 1

Figure 2-2 Estimates of the Default 90% Natural Haze Index Values (in dv)

(Note different color scales for the two parts of the figure)

3. REFINED ESTIMATION APPROACHES REGIONAL & SITE-SPECIFIC APPLICATION
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

3 - 2

3.1 Why might States want to use a refined approach to estimate natural visibility

conditions?

There are a variety of circumstances under which States might wish to adopt a refined

approach to estimating natural visibility conditions. For example, if the default estimates of the

natural background conditions are close to the current visibility conditions, small uncertainties

can have significant impacts on States’ ability to meet SIP goals. In some regions, natural

sources are known to exhibit predictable seasonal influences on visibility. Therefore, States

might wish to use refined estimates of natural visibility conditions to account for these

influences. Also, States which receive significant visibility impacts from biomass smoke might 

wish to distinguish more explicitly between man-made and natural sources. These examples are

non-exhaustive, and there may be many other circumstances under which States find it desirable

to develop more refined estimates. In all such cases, they should be prepared to support

alternative approaches with sufficient information so that EPA and the reviewing public can

verify their accuracy and validity.

3.2 What are some of the approaches that could be used by States to refine the default

natural visibility estimates? 

A refined approach is essentially one that uses species concentration estimates that differ

from the NAPAP default values given in Table 2-1. Several possible refined approaches which

can be adopted are described in this document, and States may identify others that are more

appropriate for their own situations. 

One possible refined approach is to revise the NAPAP default estimates of the natural

concentrations of one or more of the composite components, and repeat the calculations with the

refined concentrations. This approach might be adopted where there is an offset between the

regional natural concentrations and the NAPAP default estimates. In this approach, the visibility

calculations (i.e., Equations 1-4) would be carried out using refined annual average

concentration estimates and the default annual average f(RH) values. Note that any refined

natural concentration estimates must retain the distinction between natural and anthropogenic

components. For example, the natural concentration estimate for a species can never exceed the

actual measured concentration of that species over a 5-year period.

In cases where constant values for natural species concentrations may not be appropriate,

a second possible approach could estimate natural visibility using species concentrations that

vary (e.g., seasonally, monthly, or climatologically). This approach might adopt the NAPAP

default estimates for some species, and temporally varying estimates for others. Alternatively, 

the NAPAP estimates might be used for some seasons or time periods and other technically

justified estimates or measurements for the remaining time periods. This approach would use the
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

A - 3

refined concentration estimates and if the time-varying species is hygroscopic (i.e., sulfate or

nitrate), it would also use the appropriate monthly average f(RH) values (Appendix A).

Finally, a refined approach might account for infrequent natural events, such as forest

fires or wind-blown dust, as major influences on visibility. Such an approach would require

estimating the frequency and magnitude of the natural contribution to particle concentrations

during the events. 

3.3 Which refined approach is most appropriate for States to use?

To determine which approach is most appropriate, States should first identify whether

any of the particle species concentrations are thought to deviate significantly from the NAPAP

default values. Once identified, States should classify the deviations as either a constant offset

(e.g., NAPAP sulfate values are too low near the sea coast), a systematic temporal variation (e.g.,

natural organics are seasonally higher in the summer), or an infrequent natural variation (e.g.,

dust produced by a natural sand dune area during wind events). The refinement of particle

species concentrations could follow a range of different approaches, from using different annual

average species concentrations, to using seasonal or monthly concentrations, to using different

natural concentrations for individual sample events. Such refined approaches may require

alternative methods to predict the 10th and 90th percentile natural condition HI values. The EPA

encourages flexibility in the approaches used so that default and refined annual average,

seasonal, monthly, and event-specific species concentrations may be intermingled to provide the

best estimates of natural visibility for each of the mandatory Federal Class I areas. 

3.4 What should States do if they want to use a refined approach, rather than the default

approach to estimate natural visibility conditions?

States wishing to employ a refined approach should supply demonstrations that the

refined approach is technically sound and provides regionally representative estimates of natural

visibility conditions. The proposed refined approach must be based upon particle species

classification into natural and man-made components (i.e., in any given time period, the natural

particle species concentration cannot exceed the measured concentration), and should be

submitted to EPA for approval prior to implementation. 

States wishing to adopt a refined approach based on a constant offset of the natural

concentrations of the particle species should provide technical justification for revising the

NAPAP default concentrations. Using the refined concentrations, the natural visibility condition 

should then be calculated based on an approach that is consistent with the methodology that is

used to track trends, such as the default approach.
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

3 - 3

States wishing to adopt a refined approach based on estimates of annually varying 

(seasonally, monthly, climatologically, etc.) natural particle concentrations should also provide

technical justification for the estimates of the natural particle species concentrations. For

example, if seasonal variations in particle species are the basis for the refined approach, then

estimates should be provided of natural concentrations in every season for every pertinent

species. Those particle species components that do not vary significantly should be treated using

a constant estimate of the natural concentrations (e.g., use NAPAP value for each season). 

In any case, the appropriate mechanism for putting a refined estimation approach in place

is to incorporate the approach in a new or revised SIP. The justification for the proposed refined

approach will thereby be considered as part of the normal SIP review process.

3.5 How might an infrequent natural impact be quantified?

Infrequent events affecting the visibility at specific mandatory Federal Class I areas could

be addressed by using a constant or temporally varying value for species affected by the event

during all non-event periods, and a different value for those same species for sampling periods

during the event. For example, consider a forest fire, which affects particulate organic and

elemental carbon. The contribution of the fire event to the natural levels of those species during

the fire might be estimated by assuming all of the observed increment above the mean of the

sample periods immediately pre- and post-fire event was the result of the fire. Multiple pre- and

post-event sample periods could be used to strengthen the comparison. Alternatively, an air

quality model might be used to estimate the impact of the smoke plume on particle carbon levels,

or other air quality measurements might be used to estimate the impact of the event.

3.6 Can natural visibility estimates be made on a sample-period-by-sample-period basis?

Yes, such calculations can be done, but refined concentration estimates should be

justified to support such an approach. In that case, the calculation of the current bext would first

be done for each sample day, using Equation 1, the appropriate monthly f(RH) values, and the

daily monitoring data for each species. The resulting daily bext values would then be converted to

an HI value in dv units by Equation 2. Those HI values would then be sorted, and the highest

20% and lowest 20% identified, indicating the days with the most and the least visibility

impairment, respectively. (This procedure is described in detail in a separate guidance document 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

3 - 1

for tracking progress). For each of the days in these two groups, the natural contribution to light

extinction would then be estimated. The average of each of these two groups of natural

contributions would then be calculated. 

As noted above, in any given time period the natural concentration of a species estimated

by this calculation cannot exceed the actual measured concentration. Furthermore, if this 

approach is taken, natural visibility conditions (i.e., the averages of the 20% worst and 20% best

natural HI values) should be estimated for as many years as possible to ensure that the average

results are more representative of the long-term conditions. 
A - 1

Appendix A

Annual Average f(RH) and Monthly Average f(RH)

Values at All Mandatory Federal Class I Areas
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

15 U.S. EPA, Interpolating Relative Humidity Weighting Factors to Calculate Visibility Impairment and the

Effects of IMPROVE Monitor Outliers, prepared by Science Applications International Corporation, Raleigh, NC,

EPA Contract No. 68-D-98-113, August 30, 2001.

A - 2

Appendix A 

Origin of Relative Humidity and f(RH) Values

In terms of visibility reduction caused by fine particles, it is appropriate to treat relative

humidity differently for different objectives. If the objective is the most reliable short-term

estimate of visibility, then the measured or estimated relative humidity for the specific time and

location of the aerosol speciation data is most appropriate. If the objective is to assess the longterm changes in man-made visibility impairment, it is appropriate to use relative humidity that is

the same for the baseline period and future periods. In other words, it is more appropriate to

eliminate the confounding effects of varying relative humidity, if the purpose is to track the

visibility effects of air pollution emissions over extended time periods.

A number of approaches were considered to prevent variations in the relative humidity

adjustment factor from confounding efforts to track progress related to emission controls. The

simplest approach would use the same typical or overall average adjustment factor for all Class I

areas at all times. However, this would enhance the contributions of hygroscopic particle species

in dry locations and during typically dry seasons above what they truly should be while reducing

their contributions in moist locations and seasons. Such distortions of the contributions to haze

by hygroscopic particle species are unnecessary if a set of Class I area-specific adjustment

factors are used that reflect seasonal changes in relative humidity. 

A second approach would be to review relative humidity data over a long period of time

to derive climatological estimates for relative humidity adjustment factors. These climatological

estimates would then be used to estimate visibility extinction coefficients. These estimates are

more likely to reflect “typical” relative humidity at the different mandatory Federal Class I areas

during different times of year and, thus, are more likely to be more appropriate for establishing

trends in visibility at the mandatory Federal Class I areas. 

Recently, the U.S. EPA sponsored a project to examine measured hourly relative

humidity data over a 10-year period within the United States, to derive month-specific

climatological mean humidity correction factors for each mandatory Federal Class I area.15 The

results of that work are presented in the table below and the draft report is available at:

http://www.epa.gov/ttn/naaqs/pm/pm25_tech.html
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

A - 3

These relative humidity factors have been calculated from available hourly relative humidity

data from 292 National Weather Service stations across the 50 States and District of Columbia as

well as from 29 IMPROVE and IMPROVE protocol monitor sites, 48 CASTNet sites, and 13

additional sites administered by the National Park Service. 

The hourly RH measurements from each site were converted to f(RH) values using a nonlinear weighting factor curve, based on a modified ammonium sulfate growth curve. Values

above 95% RH were set equal to the f(RH) corresponding to 95% RH. For days in which at least

16 hours of valid RH data were available, daily averages were determined from these hourly

f(RH) values at each site. Monthly averages were then calculated from the daily f(RH) averages

at each site.

The monthly average f(RH) values were interpolated at 1/4-degree increments using the

inverse distance weighting technique (with a distance interpolation exponent of 1):

f RH f RH x

x g

w wg

wg

( )

( )/

/ = ∑

∑1

where the monthly f(RH)g of the grid cell is calculated from f(RH)w at the weather station, and the

horizontal distance between the grid cell center and the weather station, xwg, summed over all the

weather stations within a 250-mile radius with valid f(RH) values for that month.

In most regions there is a seasonal cycle of relative humidity which is accounted for by

this process of appropriate f(RH) values for each month of the year from the daily-averaged

values. Thus, the 12 monthly-averaged f(RH) values determined in this way for each Class I area

should be used for all aerosol speciation data or model predictions for that location. However, a

more complicated approach has also been investigated, as described below.

The regional haze regulation requires separate tracking of visibility changes for the worst

20% and best 20% of visibility days. If there is a significant correlation in any month at any site

between daily relative humidity and the sulfate or nitrate concentrations, then use of the

monthly-averaged f(RH) will systematically over- or under-predict the contribution to visibility

impairment of the aerosol species. Fortunately, this concern can be tested at a number of

locations in all regions of the country using the IMPROVE database. If the use of monthlyaveraged values were found to cause large systematic biases in any region of the country, the

Class I areas in those regions would require two f(RH) values for each month. One value would

be the average f(RH) associated with relative humidity conditions that correspond to the worst

20% and the other value associated with relative humidity conditions that correspond to the best 
Guidance for Estimating Natural Visibility 

Conditions Under the Regional Haze Program

A - 4

20% of the light extinction values. Therefore there is the potential that some Class I area

locations could require up to 24 f(RH) values for use in calculating extinction for aerosol data.

The U.S. National Park Service has tested this possibility, by examining data for each of

the 12 months from 20 mandatory Federal Class I areas where relative humidity measurements

are made. In nearly all cases, no statistically significant correlations were found between

measured concentrations of SO4

2-, NO3-

 and [SO4

2- + NO3-

] vs. daily values of relative humidity

in a large majority of months. Furthermore, deciview calculations were made using day-specific

vs. climatological values for the relative humidity adjustment factor for each of 10 years in 15

mandatory Federal Class I areas. In 14 of the 15 areas, little if any difference was observed in

the year to year calculations for the mean deciview values for the 20% worst and 20% best days,

nor was there any difference in the trends. Some difference in the mean deciview value for the

worst 20% days was observed in one mandatory Federal Class I area. However, the overall trend

in the mean worst and best deciview values for this site was similar using the two types of f(RH)

values. These results suggest there is a relatively weak correlation between hygroscopic

components of PM and relative humidity and that the choice of a “climatological” vs. “dayspecific” method for computing f(RH) has little apparent effect on observed trends in visibility. 

Consequently, the simpler climatological approach is used in regional haze calculations.
Guidance for Estimating Natural Visibility

Conditions Under the Regional Haze Program 

A - 5

Table A-1 Values for f(RH) determined from the growth of ammonium sulfate

RH f(RH) RH f(RH) RH f(RH)

1 1.00 34 1.00 67 2.03

2 1.00 35 1.00 68 2.08

3 1.00 36 1.00 69 2.14

4 1.00 37 1.02 70 2.19

5 1.00 38 1.04 71 2.25

6 1.00 39 1.06 72 2.31

7 1.00 40 1.08 73 2.37

8 1.00 41 1.10 74 2.43

9 1.00 42 1.13 75 2.50

10 1.00 43 1.15 76 2.56

11 1.00 44 1.18 77 2.63

12 1.00 45 1.20 78 2.70

13 1.00 46 1.23 79 2.78

14 1.00 47 1.26 80 2.86

15 1.00 48 1.28 81 2.94

16 1.00 49 1.31 82 3.03

17 1.00 50 1.34 83 3.12

18 1.00 51 1.37 84 3.22

19 1.00 52 1.41 85 3.33

20 1.00 53 1.44 86 3.45

21 1.00 54 1.47 87 3.58

22 1.00 55 1.51 88 3.74

23 1.00 56 1.54 89 3.93

24 1.00 57 1.58 90 4.16

25 1.00 58 1.62 91 4.45

26 1.00 59 1.66 92 4.84

27 1.00 60 1.70 93 5.37

28 1.00 61 1.74 94 6.16

29 1.00 62 1.79 95 7.40

30 1.00 63 1.83 96 9.59

31 1.00 64 1.88 97 14.1

32 1.00 65 1.93 98 26.4

33 1.00 66 1.98
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-2 Recommended Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class 

 Based on the Representative IMPROVE Site Location

Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Code Site St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) Acadia Acadia 1 ACAD1 ME 44.38 -68.37 3.2 2.8 2.8 3.2 3.2 3.3 3.7 3.7 

Agua Tibia Agua Tibia 100 AGTI1 CA 33.38 -116.87 2.4 2.3 2.4 2.2 2.2 2.2 2.2 2.3 

Alpine Lakes Snoqualmie Pass 80 SNPA1 WA 47.38 -121.37 5.3 5.0 3.7 3.6 4.2 3.1 3.5 3.4 Anaconda - Pintler Sula 71 SULA1 MT 45.88 -114.12 3.4 3.0 2.6 2.3 2.3 2.2 1.9 1.8 

Ansel Adams Kaiser 110 KAIS1 CA 37.13 -119.12 3.0 2.7 2.5 2.1 2.0 1.7 1.7 1.7 Arches Canyonlands 50 CANY1 UT 38.38 -109.87 2.6 2.3 1.8 1.6 1.5 1.2 1.3 1.5 Badlands Badlands 59 BADL1 SD 43.63 -101.87 2.8 2.8 2.8 2.6 2.8 2.7 2.4 2.4 Bandelier Bandelier 33 BAND1 NM 35.88 -106.37 2.3 2.1 1.8 1.6 1.6 1.4 1.7 2.0 Bering Sea (a)Big Bend Big Bend 31 BIBE1 TX 29.38 -103.12 1.8 1.7 1.5 1.4 1.5 1.5 1.6 1.8 Black Canyon of the Gunnison Weminuche 55 WEMI1 CO 37.63 -107.87 2.5 2.3 2.0 1.7 1.7 1.5 1.7 2.0 Bob Marshall Monture 73 MONT1 MT 47.13 -113.12 3.3 2.9 2.6 2.4 2.4 2.4 2.1 2.0 Bosque del Apache Bosque del Apache 38 BOAP1 NM 33.88 -106.87 2.2 2.0 1.6 1.4 1.4 1.3 1.7 1.9 Boundary Waters Canoe Area Boundary Waters 23 BOWA1 MN 47.88 -91.62 2.9 2.6 2.6 2.3 2.5 2.8 3.0 3.1 Breton Breton 20 BRET1 LA 29.13 -89.12 3.5 3.3 3.3 3.3 3.4 3.6 3.8 3.8 

Bridger Bridger 65 BRID1 WY 42.88 -109.87 2.5 2.3 2.3 2.1 2.1 1.8 1.5 1.5 Brigantine Brigantine 5 BRIG1 NJ 39.38 -74.37 2.9 2.6 2.7 2.6 2.9 3.0 3.2 3.4 

Bryce Canyon Bryce Canyon 49 BRCA1 UT 37.63 -112.12 2.6 2.4 2.0 1.6 1.5 1.3 1.3 1.5 

Cabinet Mountains Cabinet Mountains 75 CABI1 MT 47.88 -115.62 3.7 3.2 2.8 2.5 2.5 2.4 2.1 2.1 Caney Creek Caney Creek 29 CACR1 AR 34.38 -94.12 3.3 3.0 2.7 2.8 3.2 3.2 3.0 3.0 Canyonlands Canyonlands 50 CANY1 UT 38.38 -109.87 2.6 2.3 1.8 1.6 1.5 1.2 1.3 1.5 Cape Romain Cape Romain 15 ROMA1 SC 32.88 -79.62 3.2 2.9 2.8 2.7 2.9 3.3 3.3 3.6 Capitol Reef Capitol Reef 52 CAPI1 UT 38.38 -111.37 2.7 2.5 2.0 1.7 1.6 1.4 1.4 1.6 Caribou Lassen Volcanic 90 LAVO1 CA 40.63 -121.62 3.7 3.1 2.8 2.4 2.3 2.1 2.0 2.0 Carlsbad Caverns Guadalupe Mountains 32 GUMO1 TX 31.88 -104.87 2.4 2.0 1.6 1.4 1.6 1.5 1.9 2.2 Chassahowitzka Chassahowitzka 18 CHAS1 FL 28.63 -82.62 3.5 3.2 3.1 3.0 3.0 3.5 3.5 3.7 

Chiricahua NM Chiricahua 39 CHIR1 AZ 32.13 -109.37 2.0 1.9 1.6 1.2 1.2 1.1 1.7 2.0 

Chiricahua W Chiricahua 39 CHIR1 AZ 32.13 -109.37 2.0 1.9 1.6 1.2 1.2 1.1 1.7 2.0 Cohutta Cohutta 12 COHU1 GA 34.88 -84.62 3.4 3.1 2.9 2.7 3.2 3.6 3.6 3.7 

Crater Lake Crater Lake 86 CRLA1 OR 42.88 -122.12 4.6 4.0 3.7 3.5 3.2 2.9 2.6 2.7 Craters of the Moon Craters of the Moon 69 CRMO1 ID 43.38 -113.62 3.1 2.7 2.3 2.0 2.0 1.8 1.4 1.4 Cucamonga San Gabriel 93 SAGA1 CA 34.38 -118.12 2.6 2.5 2.5 2.2 2.2 2.1 2.2 2.2 Denali Denali 102 DENA1 AK 63.75 -148.75 2.5 2.3 2.1 1.9 1.8 2.1 2.5 2.9 

Desolation Bliss 95 BLIS1 CA 38.88 -120.12 3.2 2.8 2.5 2.0 1.9 1.6 1.5 1.5 Diamond Peak Crater Lake 86 CRLA1 OR 42.88 -122.12 4.6 4.0 3.7 3.5 3.2 2.9 2.6 2.7 

Dolly Sods Dolly Sods 8 DOSO1 WV 39.13 -79.37 3.0 2.7 2.7 2.5 3.5 3.1 3.2 3.5 Dome Land Dome Land 109 DOME1 CA 35.63 -118.12 2.6 2.3 2.2 1.9 1.9 1.8 1.8 1.8 Eagle Cap Starkey 76 STAR1 OR 45.13 -118.62 4.3 3.8 3.2 2.9 2.7 2.4 2.0 2.1 Eagles Nest White River 56 WHRI1 CO 39.13 -106.87 2.2 2.2 2.0 2.0 2.0 1.7 1.8 2.1 Emigrant Yosemite 96 YOSE1 CA 37.63 -119.62 3.0 2.9 2.7 2.2 2.1 1.7 1.5 1.5 Everglades Everglades 19 EVER1 FL 25.38 -80.62 2.6 2.5 2.5 2.3 2.3 2.6 2.5 2.8 Fitzpatrick Bridger 65 BRID1 WY 42.88 -109.87 2.5 2.3 2.3 2.1 2.1 1.8 1.5 1.5 

Flat Tops White River 56 WHRI1 CO 39.13 -106.87 2.2 2.2 2.0 2.0 2.0 1.7 1.8 2.1 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-2 Recommended Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class 

 Based on the Representative IMPROVE Site Location

Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Code Site St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

Galiuro Chiricahua 39 CHIR1 AZ 32.13 -109.37 2.0 1.9 1.6 1.2 1.2 1.1 1.7 2.0 Gates of the Mountains Gates of the Mountains 74 GAMO1 MT 46.88 -111.62 2.8 2.5 2.4 2.3 2.3 2.2 2.0 1.9 Gearhart Mountain Crater Lake 86 CRLA1 OR 42.88 -122.12 4.6 4.0 3.7 3.5 3.2 2.9 2.6 2.7 Gila Gila Cliffs 42 GICL1 NM 33.13 -108.12 2.1 1.9 1.6 1.3 1.3 1.2 1.9 1.9 Glacier Glacier 72 GLAC1 MT 48.63 -114.12 3.9 3.4 3.1 2.9 3.0 3.0 2.5 2.5 Glacier Peak North Cascades 81 NOCA1 WA 48.63 -121.12 4.5 4.1 3.6 3.4 3.3 3.0 2.9 3.1 Goat Rocks White Pass 79 WHPA1 WA 46.63 -121.37 4.8 4.2 3.8 3.6 3.4 3.1 2.9 3.0 

Grand Canyon Grand Canyon, Hance 48 GRCA2 AZ 35.88 -111.87 2.5 2.4 2.0 1.6 1.4 1.2 1.4 1.7 Grand Teton Yellowstone 66 YELL2 WY 44.63 -110.37 2.5 2.3 2.2 2.1 2.1 1.9 1.7 1.6 

Great Gulf Great Gulf 4 GRGU1 NH 44.38 -71.12 2.8 2.6 2.6 2.8 2.9 3.0 3.3 3.5 Great Sand Dunes Great Sand Dunes 53 GRSA1 CO 37.63 -105.62 2.4 2.3 2.0 1.9 1.9 1.7 1.9 2.3 Great Smoky Mountains Great Smoky Mountains 10 GRSM1 TN 35.63 -83.87 3.6 3.0 3.0 2.8 3.2 3.6 3.6 3.6 Guadalupe Mountains Guadalupe Mountains 32 GUMO1 TX 31.88 -104.87 2.4 2.0 1.6 1.4 1.6 1.5 1.9 2.2 Haleakala Haleakala 108 HALE1 HI 20.75 -156.25 2.7 2.6 2.5 2.5 2.4 2.3 2.4 2.4 Hawaii Volcanoes Hawaii Volcanoes 107 HAVO1 HI 19.25 -155.25 3.0 2.9 2.9 2.9 2.9 2.9 3.0 3.0 Hells Canyon Hells Canyon 77 HECA1 OR 44.88 -116.87 3.7 3.1 2.4 2.1 2.0 1.8 1.5 1.4 Hercules - Glade Hercules - Glade 28 HEGL1 MO 36.63 -92.87 3.2 2.9 2.6 2.6 3.0 3.0 3.0 3.0 Hoover Hoover 97 HOOV1 CA 38.13 -119.12 3.1 2.7 2.5 2.0 1.9 1.6 1.5 1.5 

Isle Royale Isle Royale 25 ISLE1 MI 47.38 -88.12 3.1 2.6 2.7 2.5 2.4 2.9 3.2 3.4 James River Face James River Face 7 JARI1 VA 37.63 -79.62 2.9 2.7 2.6 2.4 2.9 3.1 3.2 3.3 Jarbidge Jarbidge 68 JARB1 NV 41.88 -115.37 2.9 2.6 2.1 2.1 2.2 2.0 1.6 1.4 

John Muir Kaiser 110 KAIS1 CA 37.13 -119.12 3.0 2.7 2.5 2.1 2.0 1.7 1.7 1.7 

Joshua Tree Joshua Tree 101 JOSH1 CA 34.13 -116.37 2.4 2.3 2.3 2.0 2.0 1.9 1.5 2.0 

Joyce Kilmer - Slickrock Great Smoky Mountains 10 GRSM1 TN 35.63 -83.87 3.6 3.0 3.0 2.8 3.2 3.6 3.6 3.6 Kaiser Kaiser 110 KAIS1 CA 37.13 -119.12 3.0 2.7 2.5 2.1 2.0 1.7 1.7 1.7 

Kalmiopsis Kalmiopsis 89 KALM1 OR 42.63 -124.12 4.5 3.9 3.7 3.5 3.3 3.1 2.9 3.0 Kings Canyon Sequoia 98 SEQU1 CA 36.38 -118.87 2.9 2.6 2.5 2.2 2.1 1.8 1.7 1.7 La Garita Weminuche 55 WEMI1 CO 37.63 -107.87 2.5 2.3 2.0 1.7 1.7 1.5 1.7 2.0 

Lassen Volcanic Lassen Volcanic 90 LAVO1 CA 40.63 -121.62 3.7 3.1 2.8 2.4 2.3 2.1 2.0 2.0 

Lava Beds Lava Beds 87 LABE1 CA 41.63 -121.62 4.0 3.4 3.1 2.8 2.6 2.4 2.2 2.2 

Linville Gorge Linville Gorge 13 LIGO1 NC 35.88 -81.87 3.2 3.0 2.9 2.7 3.2 3.6 3.6 3.9 

Lostwood Lostwood 62 LOST1 ND 48.63 -102.37 3.0 2.9 3.0 2.3 2.2 2.5 2.5 2.3 Lye Brook Lye Brook 3 LYBR1 VT 43.13 -73.12 2.8 2.6 2.7 2.6 2.8 2.9 3.1 3.3 Mammoth Cave Mammoth Cave 9 MACA1 KY 37.13 -86.12 3.3 3.0 2.9 3.0 4.1 4.7 4.6 3.5 Marble Mountain Trinity 104 TRIN1 CA 40.88 -122.87 4.0 3.4 3.2 2.9 2.8 2.6 2.5 2.6 

Maroon Bells - Snowmass White River 56 WHRI1 CO 39.13 -106.87 2.2 2.2 2.0 2.0 2.0 1.7 1.8 2.1 Mazatzal Ike's Backbone 46 IKBA1 AZ 34.38 -111.62 2.2 2.0 1.8 1.4 1.3 1.2 1.4 1.7 

Medicine Lake Medicine Lake 63 MELA1 MT 48.38 -104.37 3.0 2.9 2.9 2.2 2.2 2.4 2.4 2.1 Mesa Verde Mesa Verde 54 MEVE1 CO 37.13 -108.37 2.8 2.6 2.2 1.7 1.7 1.3 1.7 2.0 Mingo Mingo 26 MING1 MO 36.88 -90.12 3.2 2.9 2.7 2.6 2.9 3.0 3.1 3.1 Mission Mountains Monture 73 MONT1 MT 47.13 -113.12 3.3 2.9 2.6 2.4 2.4 2.4 2.1 2.0 

Mokelumne Bliss 95 BLIS1 CA 38.88 -120.12 3.2 2.8 2.5 2.0 1.9 1.6 1.5 1.5 

Moosehorn Moosehorn 2 MOOS1 ME 45.13 -67.37 3.0 2.7 2.7 2.9 2.9 3.1 3.5 3.6 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-2 Recommended Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class 

 Based on the Representative IMPROVE Site Location

Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Code Site St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

Mount Adams White Pass 79 WHPA1 WA 46.63 -121.37 4.8 4.2 3.8 3.6 3.4 3.1 2.9 3.0 

Mount Baldy Mount Baldy 43 BALD1 AZ 34.13 -109.37 2.2 2.1 1.7 1.4 1.3 1.2 1.6 1.9 Mount Hood Mount Hood 85 MOHO1 OR 45.38 -121.87 4.6 4.1 3.7 3.6 3.2 3.0 2.7 2.8 Mount Jefferson Three Sisters 84 THSI1 OR 44.38 -122.12 5.3 4.6 4.4 4.3 3.8 3.4 2.7 2.7 

Mount Rainier Mount Rainier 78 MORA1 WA 46.88 -122.12 5.3 4.7 4.4 4.3 3.9 3.7 3.4 3.6 

Mount Washington Three Sisters 84 THSI1 OR 44.38 -122.12 5.3 4.6 4.4 4.3 3.8 3.4 2.7 2.7 

Mount Zirkel Mount Zirkel 58 MOZI1 CO 40.63 -106.62 2.2 2.2 2.0 2.1 2.2 1.8 1.7 1.8 

Mountain Lakes Crater Lake 86 CRLA1 OR 42.88 -122.12 4.6 4.0 3.7 3.5 3.2 2.9 2.6 2.7 

North Absaroka North Absoraka 67 NOAB1 WY 44.63 -109.37 2.4 2.2 2.2 2.1 2.1 1.9 1.6 1.5 

North Cascades North Cascades 81 NOCA1 WA 48.63 -121.12 4.5 4.1 3.6 3.4 3.3 3.0 2.9 3.1 Okefenokee Okefenokee 16 OKEF1 GA 30.63 -82.12 3.3 3.0 3.2 3.0 3.2 3.8 3.4 3.6 Olympic Olympic 83 OLYM1 WA 48.13 -122.87 4.2 3.9 3.6 3.5 2.9 3.2 2.7 3.3 

Otter Creek Dolly Sods 8 DOSO1 WV 39.13 -79.37 3.0 2.7 2.7 2.5 3.5 3.1 3.2 3.5 Pasayten Pasayten 82 PASA1 WA 48.38 -119.87 4.6 4.1 3.5 3.3 3.2 2.9 2.8 2.9 

Pecos Wheeler Peak 35 WHPE1 NM 36.63 -105.37 2.4 2.2 1.9 1.8 1.8 1.6 1.8 2.1 Petrified Forest Petrified Forest 41 PEFO1 AZ 35.13 -109.87 2.4 2.1 1.7 1.4 1.3 1.2 1.5 1.8 

Pine Mountain Ike's Backbone 46 IKBA1 AZ 34.38 -111.62 2.2 2.0 1.8 1.4 1.3 1.2 1.4 1.7 

Pinnacles Pinnacles 92 PINN1 CA 36.38 -121.12 3.4 3.4 3.5 2.6 2.4 2.2 2.1 2.2 

Point Reyes Point Reyes 91 PORE1 CA 38.13 -122.87 3.6 3.2 3.1 2.6 2.5 2.3 2.4 2.4 

Presidential Range - Dry River Great Gulf 4 GRGU1 NH 44.38 -71.12 2.8 2.6 2.6 2.8 2.9 3.0 3.3 3.5 Rawah Mount Zirkel 58 MOZI1 CO 40.63 -106.62 2.2 2.2 2.0 2.1 2.2 1.8 1.7 1.8 

Red Rock Lakes Yellowstone 66 YELL2 WY 44.63 -110.37 2.5 2.3 2.2 2.1 2.1 1.9 1.7 1.6 

Redwood Redwood 88 REDW1 CA 41.63 -124.12 3.8 3.6 3.8 3.6 3.8 3.9 4.2 4.2 

Rocky Mountain Rocky Mountain 57 ROMO1 CO 40.38 -105.62 1.9 2.0 2.0 2.1 2.3 2.0 1.9 1.9 Roosevelt Campobello Moosehorn 2 MOOS1 ME 45.13 -67.37 3.0 2.7 2.7 2.9 2.9 3.1 3.5 3.6 

Saguaro Saguaro 40 SAGU1 AZ 32.13 -110.62 1.8 1.6 1.4 1.1 1.1 1.0 1.4 1.7 

Saint Marks Saint Marks 17 SAMA1 FL 30.13 -84.12 3.5 3.3 3.2 3.1 3.2 3.6 3.8 3.8 

Salt Creek Salt Creek 36 SACR1 NM 33.38 -104.37 2.2 1.9 1.5 1.5 1.6 1.5 1.7 1.9 

San Gabriel San Gabriel 93 SAGA1 CA 34.38 -118.12 2.6 2.5 2.5 2.2 2.2 2.1 2.2 2.2 San Gorgonio San Gorgonio 99 SAGO1 CA 34.13 -116.87 2.5 2.6 2.4 2.1 2.1 1.8 1.7 1.8 San Jacinto San Gorgonio 99 SAGO1 CA 34.13 -116.87 2.5 2.6 2.4 2.1 2.1 1.8 1.7 1.8 

San Pedro Parks San Pedro Parks 34 SAPE1 NM 36.13 -106.87 2.4 2.2 1.9 1.6 1.6 1.4 1.7 2.0 San Rafael San Rafael 94 RAFA1 CA 34.63 -120.12 3.0 2.8 2.8 2.5 2.5 2.4 2.5 2.6 

Sawtooth Sawtooth 70 SAWT1 ID 44.13 -114.87 3.3 2.8 2.3 2.0 2.0 1.8 1.4 1.4 Scapegoat Monture 73 MONT1 MT 47.13 -113.12 3.3 2.9 2.6 2.4 2.4 2.4 2.1 2.0 Selway - Bitterroot Sula 71 SULA1 MT 45.88 -114.12 3.4 3.0 2.6 2.3 2.3 2.2 1.9 1.8 

Seney Seney 22 SENE1 MI 46.38 -85.87 3.3 2.8 2.9 2.7 2.6 3.0 3.3 3.6 

Sequoia Sequoia 98 SEQU1 CA 36.38 -118.87 2.9 2.6 2.5 2.2 2.1 1.8 1.7 1.7 

Shenandoah Shenandoah 6 SHEN1 VA 38.63 -78.37 2.9 2.6 2.7 2.4 2.9 3.1 3.2 3.5 

Shining Rock Shining Rock 11 SHRO1 NC 35.38 -82.87 3.3 3.0 2.9 2.7 3.2 3.6 3.6 3.9 Sierra Ancha Sierra Ancha 45 SIAN1 AZ 34.13 -110.87 2.2 2.0 1.7 1.4 1.3 1.1 1.5 1.8 Simeonof Simeonof 105 SIME1 AK 55.25 -160.75 4.2 4.2 3.8 4.0 4.2 4.6 5.0 5.2 Sipsey Sipsey 21 SIPS1 AL 34.38 -87.37 3.3 3.0 2.8 2.7 3.1 3.4 3.5 3.5 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-2 Recommended Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class 

 Based on the Representative IMPROVE Site Location

Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Code Site St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

South Warner Lava Beds 87 LABE1 CA 41.63 -121.62 4.0 3.4 3.1 2.8 2.6 2.4 2.2 2.2 

Strawberry Mountain Starkey 76 STAR1 OR 45.13 -118.62 4.3 3.8 3.2 2.9 2.7 2.4 2.0 2.1 Superstition Tonto 44 TONT1 AZ 33.63 -111.12 2.1 1.9 1.6 1.3 1.2 1.1 1.4 1.7 Swanquarter Swanquarter 14 SWAN1 NC 35.38 -76.12 2.9 2.7 2.6 2.4 2.7 3.0 3.1 3.2 Sycamore Canyon Sycamore Canyon 47 SYCA1 AZ 35.13 -111.87 2.4 2.4 2.0 1.6 1.5 1.2 1.5 2.0 Teton Yellowstone 66 YELL2 WY 44.63 -110.37 2.5 2.3 2.2 2.1 2.1 1.9 1.7 1.6 Theodore Roosevelt Theodore Roosevelt 61 THRO1 ND 46.88 -103.37 2.9 2.8 2.8 2.4 2.4 2.5 2.4 2.2 Thousand Lakes Lassen Volcanic 90 LAVO1 CA 40.63 -121.62 3.7 3.1 2.8 2.4 2.3 2.1 2.0 2.0 Three Sisters Three Sisters 84 THSI1 OR 44.38 -122.12 5.3 4.6 4.4 4.3 3.8 3.4 2.7 2.7 

Tuxedni Tuxedni 103 TUXE1 AK 59.75 -152.75 3.6 3.4 2.9 2.8 2.8 2.9 3.6 3.9 UL Bend UL Bend 64 ULBE1 MT 47.63 -108.62 2.6 2.4 2.4 2.3 2.2 2.1 1.9 1.8 Upper Buffalo Upper Buffalo 27 UPBU1 AR 35.88 -93.12 3.2 2.9 2.6 2.7 3.1 3.1 3.0 3.0 

Ventana Pinnacles 92 PINN1 CA 36.38 -121.12 3.4 3.4 3.5 2.6 2.4 2.2 2.1 2.2 Virgin Islands (b) Virgin Islands 106 VIIS1 VI 18.75 -155.75

Voyageurs Voyageurs 24 VOYA2 MN 48.38 -92.87 2.7 2.4 2.3 2.2 2.2 2.8 2.5 2.7 

Washakie North Absoraka 67 NOAB1 WY 44.63 -109.37 2.4 2.2 2.2 2.1 2.1 1.9 1.6 1.5 

Weminuche Weminuche 55 WEMI1 CO 37.63 -107.87 2.5 2.3 2.0 1.7 1.7 1.5 1.7 2.0 

West Elk White River 56 WHRI1 CO 39.13 -106.87 2.2 2.2 2.0 2.0 2.0 1.7 1.8 2.1 Wheeler Peak Wheeler Peak 35 WHPE1 NM 36.63 -105.37 2.4 2.2 1.9 1.8 1.8 1.6 1.8 2.1 White Mountain White Mountain 37 WHIT1 NM 33.38 -105.62 2.2 1.9 1.6 1.5 1.5 1.4 1.7 1.9 

Wichita Mountains Wichita Mountains 30 WIMO1 OK 34.63 -98.62 2.8 2.6 2.4 2.4 2.7 2.5 2.2 2.4 Wind Cave Wind Cave 60 WICA1 SD 43.63 -103.37 2.5 2.5 2.5 2.5 2.6 2.5 2.2 2.2 

Wolf Island Okefenokee 16 OKEF1 GA 30.63 -82.12 3.3 3.0 3.2 3.0 3.2 3.8 3.4 3.6 

Yellowstone Yellowstone 66 YELL2 WY 44.63 -110.37 2.5 2.3 2.2 2.1 2.1 1.9 1.7 1.6 Yolla Bolly - Middle Eel Trinity 104 TRIN1 CA 40.88 -122.87 4.0 3.4 3.2 2.9 2.8 2.6 2.5 2.6 Yosemite Yosemite 96 YOSE1 CA 37.63 -119.62 3.0 2.9 2.7 2.2 2.1 1.7 1.5 1.5 

a: No particulate matter sampling or visibility monitoring is conducted in the Bering Sea Wilderness.b: f(RH) values for Virgin Islands National Park were not calculated because of the limited RH data available.
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-3 Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class I Area

Based on the Centroid of the Area (Supplemental Information)

Site Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Map ID Code St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) Acadia Acadia 1 ACAD1 ME 44.37 68.26 3.3 2.9 2.8 3.4 3.1 3.0 3.4 3.8 Agua Tibia Agua Tibia 100 AGTI1 CA 33.41 116.98 2.4 2.4 2.4 2.2 2.2 2.2 2.3 2.3 

Alpine Lakes Snoqualmie Pass 80 SNPA1 WA 47.42 121.42 4.3 3.8 3.5 3.9 2.9 3.2 2.9 3.1 Anaconda - Pintler Sula 71 SULA1 MT 45.98 113.42 3.3 2.9 2.5 2.4 2.4 2.3 2.0 1.9 Ansel Adams Kaiser 110 KAIS1 CA 37.65 119.20 3.0 2.7 2.4 2.1 1.9 1.7 1.6 1.6 Arches Canyonlands 50 CANY1 UT 38.64 109.58 2.6 2.3 1.8 1.6 1.6 1.3 1.4 1.5 Badlands Badlands 59 BADL1 SD 43.74 101.94 2.6 2.7 2.6 2.4 2.8 2.7 2.5 2.4 Bandelier Bandelier 33 BAND1 NM 35.78 106.27 2.2 2.1 1.8 1.6 1.6 1.4 1.7 2.1 Bering Sea (a) 60.45 172.79Big Bend Big Bend 31 BIBE1 TX 29.31 103.19 2.0 1.9 1.6 1.5 1.6 1.6 1.7 2.0 Black Canyon of the Gunnison Weminuche 55 WEMI1 CO 38.58 107.70 2.4 2.2 1.9 1.9 1.9 1.6 1.7 1.9 Bob Marshall Monture 73 MONT1 MT 47.75 113.38 3.6 3.1 2.8 2.6 2.7 2.7 2.3 2.2 Bosque del Apache Bosque del Apache 38 BOAP1 NM 33.79 106.83 2.1 1.9 1.6 1.4 1.4 1.3 1.8 2.0 Boundary Waters Canoe Area Boundary Waters 23 BOWA1 MN 47.95 91.50 3.0 2.6 2.7 2.4 2.3 2.9 3.1 3.4 Breton Breton 20 BRET1 LA 29.73 88.88 3.7 3.5 3.7 3.6 3.8 4.0 4.3 4.3 Bridger Bridger 65 BRID1 WY 42.98 109.76 2.5 2.4 2.3 2.2 2.1 1.8 1.5 1.5 Brigantine Brigantine 5 BRIG1 NJ 39.46 74.45 2.8 2.6 2.7 2.6 3.0 3.2 3.4 3.7 Bryce Canyon Bryce Canyon 49 BRCA1 UT 37.62 112.17 2.6 2.4 1.9 1.6 1.5 1.3 1.3 1.5 Cabinet Mountains Cabinet Mountains 75 CABI1 MT 48.21 115.71 3.8 3.3 2.9 2.6 2.7 2.7 2.3 2.2 Caney Creek Caney Creek 29 CACR1 AR 34.41 94.08 3.4 3.1 2.9 3.0 3.6 3.6 3.4 3.4 Canyonlands Canyonlands 50 CANY1 UT 38.46 109.82 2.6 2.3 1.7 1.6 1.5 1.2 1.3 1.5 Cape Romain Cape Romain 15 ROMA1 SC 32.94 79.66 3.3 3.0 2.9 2.8 3.2 3.7 3.6 4.1 Capitol Reef Capitol Reef 52 CAPI1 UT 38.36 111.05 2.7 2.4 2.0 1.7 1.6 1.4 1.4 1.6 Caribou Lassen Volcanic 90 LAVO1 CA 40.50 121.18 3.7 3.1 2.8 2.5 2.4 2.2 2.1 2.1 Carlsbad Caverns Guadalupe Mountains 32 GUMO1 TX 32.14 104.48 2.1 2.0 1.6 1.5 1.6 1.6 1.8 2.1 Chassahowitzka Chassahowitzka 18 CHAS1 FL 28.75 82.55 3.8 3.5 3.4 3.2 3.3 3.9 3.9 4.2 Chiricahua NM Chiricahua 39 CHIR1 AZ 32.01 109.39 2.0 2.0 1.6 1.3 1.3 1.1 1.8 2.1 Chiricahua W Chiricahua 39 CHIR1 AZ 31.84 109.27 2.0 1.9 1.6 1.2 1.3 1.1 1.8 2.1 

Cohutta Cohutta 12 COHU1 GA 34.92 84.58 3.3 3.1 3.0 2.8 3.4 3.8 4.0 4.2 

Crater Lake Crater Lake 86 CRLA1 OR 42.90 122.13 4.6 3.9 3.7 3.4 3.2 3.0 2.8 2.9 

Craters of the Moon Craters of the Moon 69 CRMO1 ID 43.47 113.55 3.1 2.7 2.3 2.0 2.0 1.8 1.4 1.4 Cucamonga San Gabriel 93 SAGA1 CA 34.25 117.57 2.5 2.4 2.4 2.2 2.1 2.1 2.1 2.2 Denali Denali 102 DENA1 AK 63.72 148.97 2.5 2.3 2.1 1.9 1.9 2.2 2.5 3.0 Desolation Bliss 95 BLIS1 CA 38.98 120.12 3.2 2.8 2.4 2.0 1.8 1.6 1.5 1.6 Diamond Peak Crater Lake 86 CRLA1 OR 43.53 122.10 4.5 4.0 3.6 3.7 3.2 3.1 2.9 2.9 

Dolly Sods Dolly Sods 8 DOSO1 WV 39.11 79.43 3.0 2.8 2.8 2.6 3.1 3.4 3.5 3.9 

Dome Land Dome Land 109 DOME1 CA 35.70 118.19 2.5 2.3 2.2 1.9 1.8 1.8 1.8 1.8 Eagle Cap Starkey 76 STAR1 OR 45.10 117.29 3.8 3.2 2.5 2.1 2.0 1.9 1.6 1.6 

Eagles Nest White River 56 WHRI1 CO 39.69 106.25 2.2 2.2 2.0 2.0 2.1 1.9 1.8 2.0 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-3 Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class I Area

Based on the Centroid of the Area (Supplemental Information)

Site Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Map ID Code St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

Emigrant Yosemite 96 YOSE1 CA 38.20 119.75 3.2 2.8 2.5 2.1 1.9 1.7 1.5 1.6 

Everglades Everglades 19 EVER1 FL 25.39 80.68 2.7 2.6 2.6 2.4 2.4 2.7 2.6 2.9 

Fitzpatrick Bridger 65 BRID1 WY 43.27 109.57 2.5 2.3 2.2 2.1 2.1 1.8 1.5 1.5 

Flat Tops White River 56 WHRI1 CO 39.97 107.25 2.3 2.2 2.0 2.0 2.0 1.8 1.7 1.9 Galiuro Chiricahua 39 CHIR1 AZ 32.56 110.32 2.0 1.8 1.5 1.2 1.2 1.1 1.5 1.8 Gates of the Mountains Gates of the Mountains 74 GAMO1 MT 46.87 111.81 2.9 2.6 2.4 2.3 2.3 2.3 2.0 1.9 Gearhart Mountain Crater Lake 86 CRLA1 OR 42.49 120.85 4.0 3.4 3.1 2.8 2.7 2.5 2.3 2.3 Gila Gila Cliffs 42 GICL1 NM 33.22 108.25 2.1 1.9 1.6 1.3 1.4 1.2 2.1 2.0 Glacier Glacier 72 GLAC1 MT 48.51 114.00 4.0 3.5 3.2 3.1 3.2 3.4 2.8 2.6 Glacier Peak North Cascades 81 NOCA1 WA 48.21 121.04 4.2 3.7 3.4 3.8 2.9 3.2 2.9 3.1 Goat Rocks White Pass 79 WHPA1 WA 46.54 121.48 4.3 3.8 3.4 4.2 2.8 3.4 3.0 3.2 

Grand Canyon Grand Canyon, Hance 48 GRCA2 AZ 35.97 111.98 2.4 2.3 1.9 1.5 1.4 1.2 1.4 1.7 Grand Teton Yellowstone 66 YELL2 WY 43.68 110.73 2.6 2.4 2.2 2.1 2.1 1.8 1.5 1.5 Great Gulf Great Gulf 4 GRGU1 NH 44.31 71.22 2.8 2.6 2.6 2.8 2.9 3.2 3.5 3.8 

Great Sand Dunes Great Sand Dunes 53 GRSA1 CO 37.73 105.52 2.4 2.3 2.0 1.9 1.9 1.8 1.9 2.3 

Great Smoky Mountains Great Smoky Mountains 10 GRSM1 TN 35.63 83.94 3.3 3.0 2.9 2.7 3.2 3.9 3.8 4.0 Guadalupe Mountains Guadalupe Mountains 32 GUMO1 TX 31.83 104.80 2.0 2.0 1.6 1.5 1.6 1.5 1.9 2.2 Haleakala Haleakala 108 HALE1 HI 20.81 156.28 2.7 2.6 2.6 2.5 2.4 2.3 2.5 2.4 

Hawaii Volcanoes Hawaii Volcanoes 107 HAVO1 HI 19.43 155.27 3.2 2.9 3.0 3.0 3.0 2.9 3.1 3.2 

Hells Canyon Hells Canyon 77 HECA1 OR 45.34 116.57 3.7 3.1 2.5 2.2 2.1 2.0 1.6 1.6 Hercules - Glade Hercules - Glade 28 HEGL1 MO 36.69 92.90 3.2 2.9 2.7 2.7 3.3 3.3 3.3 3.3 Hoover Hoover 97 HOOV1 CA 38.14 119.45 3.1 2.8 2.5 2.1 1.9 1.6 1.5 1.5 

Isle Royale Isle Royale 25 ISLE1 MI 47.99 88.83 3.1 2.5 2.7 2.4 2.2 2.6 3.0 3.2 

James River Face James River Face 7 JARI1 VA 37.62 79.48 2.8 2.6 2.7 2.4 3.0 3.3 3.4 3.7 

Jarbidge Jarbidge 68 JARB1 NV 41.89 115.43 3.0 2.6 2.1 2.1 2.2 2.2 1.6 1.4 John Muir Kaiser 110 KAIS1 CA 37.39 118.84 2.9 2.6 2.4 2.1 1.9 1.7 1.7 1.7 

Joshua Tree Joshua Tree 101 JOSH1 CA 34.03 116.18 2.4 2.3 2.2 2.0 2.0 1.9 2.0 2.0 

Joyce Kilmer - Slickrock Great Smoky Mountains 10 GRSM1 TN 35.43 84.00 3.3 3.1 2.9 2.7 3.3 3.8 4.0 4.2 Kaiser Kaiser 110 KAIS1 CA 37.28 119.18 3.0 2.7 2.5 2.1 1.9 1.7 1.6 1.7 Kalmiopsis Kalmiopsis 89 KALM1 OR 42.27 123.93 4.5 3.9 3.8 3.5 3.5 3.3 3.2 3.2 

Kings Canyon Sequoia 98 SEQU1 CA 36.82 118.76 2.8 2.6 2.4 2.1 1.9 1.8 1.7 1.7 La Garita Weminuche 55 WEMI1 CO 37.96 106.81 2.3 2.2 1.9 1.8 1.8 1.6 1.7 2.1 

Lassen Volcanic Lassen Volcanic 90 LAVO1 CA 40.54 121.57 3.8 3.2 2.9 2.5 2.4 2.2 2.1 2.1 

Lava Beds Lava Beds 87 LABE1 CA 41.71 121.34 4.0 3.4 3.1 2.7 2.6 2.4 2.3 2.3 

Linville Gorge Linville Gorge 13 LIGO1 NC 35.89 81.89 3.3 3.0 3.0 2.7 3.3 3.9 4.1 4.5 Lostwood Lostwood 62 LOST1 ND 48.60 102.48 3.0 2.9 2.9 2.3 2.3 2.6 2.7 2.4 Lye Brook Lye Brook 3 LYBR1 VT 43.15 73.12 2.7 2.6 2.6 2.6 2.8 3.0 3.3 3.6 

Mammoth Cave Mammoth Cave 9 MACA1 KY 37.22 86.07 3.4 3.1 2.9 2.6 3.2 3.5 3.7 3.9 

Marble Mountain Trinity 104 TRIN1 CA 41.52 123.21 4.4 3.8 3.7 3.3 3.4 3.2 3.2 3.2 

Maroon Bells - Snowmass White River 56 WHRI1 CO 39.15 106.82 2.2 2.1 2.0 2.0 2.1 1.7 1.9 2.2 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-3 Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class I Area

Based on the Centroid of the Area (Supplemental Information)

Site Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Map ID Code St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

Mazatzal Ike's Backbone 46 IKBA1 AZ 33.92 111.43 2.1 1.9 1.7 1.3 1.3 1.1 1.5 1.7 Medicine Lake Medicine Lake 63 MELA1 MT 48.50 104.29 3.0 2.9 2.9 2.3 2.2 2.5 2.5 2.2 

Mesa Verde Mesa Verde 54 MEVE1 CO 37.20 108.49 2.5 2.3 1.9 1.5 1.5 1.3 1.6 2.0 

Mingo Mingo 26 MING1 MO 36.98 90.20 3.3 3.0 2.8 2.6 3.0 3.2 3.3 3.5 

Mission Mountains Monture 73 MONT1 MT 47.40 113.85 3.6 3.1 2.7 2.5 2.6 2.6 2.3 2.2 Mokelumne Bliss 95 BLIS1 CA 38.58 120.03 3.2 2.8 2.4 2.0 1.9 1.6 1.5 1.6 Moosehorn Moosehorn 2 MOOS1 ME 45.12 67.26 3.0 2.7 2.7 3.0 3.0 3.1 3.4 3.8 

Mount Adams White Pass 79 WHPA1 WA 46.19 121.50 4.3 3.8 3.4 4.4 2.9 3.5 3.1 3.3 

Mount Baldy Mount Baldy 43 BALD1 AZ 34.12 109.57 2.2 2.0 1.7 1.4 1.3 1.2 1.6 1.9 

Mount Hood Mount Hood 85 MOHO1 OR 45.38 121.69 4.3 3.8 3.5 3.9 3.0 3.2 2.9 3.0 Mount Jefferson Three Sisters 84 THSI1 OR 44.55 121.83 4.4 3.9 3.6 3.7 3.1 3.1 2.9 2.9 

Mount Rainier Mount Rainier 78 MORA1 WA 46.76 122.12 4.4 4.0 3.6 4.7 3.1 3.7 3.3 3.5 

Mount Washington Three Sisters 84 THSI1 OR 44.30 121.87 4.4 3.9 3.6 3.7 3.1 3.1 3.0 2.9 

Mount Zirkel Mount Zirkel 58 MOZI1 CO 40.55 106.70 2.2 2.2 2.0 2.1 2.2 1.9 1.7 1.9 

Mountain Lakes Crater Lake 86 CRLA1 OR 42.34 122.11 4.3 3.6 3.3 3.0 2.9 2.6 2.5 2.5 

North Absaroka North Absoraka 67 NOAB1 WY 44.77 109.78 2.4 2.3 2.2 2.2 2.1 1.9 1.7 1.6 North Cascades North Cascades 81 NOCA1 WA 48.54 121.44 4.1 3.7 3.4 3.7 2.9 3.2 2.9 3.2 

Okefenokee Okefenokee 16 OKEF1 GA 30.74 82.13 3.5 3.2 3.1 3.0 3.6 3.7 3.7 4.1 Olympic Olympic 83 OLYM1 WA 47.32 123.35 4.5 4.1 3.8 4.1 3.2 3.5 3.1 3.5 Otter Creek Dolly Sods 8 DOSO1 WV 39.00 79.65 3.0 2.8 2.8 2.6 3.2 3.5 3.7 4.1 Pasayten Pasayten 82 PASA1 WA 48.85 120.52 4.2 3.7 3.4 3.7 2.9 3.2 2.9 3.2 Pecos Wheeler Peak 35 WHPE1 NM 35.93 105.64 2.3 2.1 1.8 1.7 1.7 1.5 1.8 2.1 

Petrified Forest Petrified Forest 41 PEFO1 AZ 35.08 109.77 2.4 2.2 1.7 1.4 1.3 1.2 1.5 1.8 

Pine Mountain Ike's Backbone 46 IKBA1 AZ 34.31 111.80 2.2 2.0 1.7 1.4 1.3 1.1 1.4 1.8 

Pinnacles Pinnacles 92 PINN1 CA 36.49 121.16 3.2 2.8 2.6 2.4 2.3 2.0 2.0 2.1 

Point Reyes Point Reyes 91 PORE1 CA 38.12 122.90 3.6 3.3 3.1 2.7 2.5 2.3 2.5 2.6 Presidential Range - Dry River Great Gulf 4 GRGU1 NH 44.21 71.35 2.8 2.6 2.6 2.8 3.0 3.4 3.7 4.0 Rawah Mount Zirkel 58 MOZI1 CO 40.70 105.94 2.1 2.1 2.0 2.1 2.3 2.0 1.8 2.0 

Red Rock Lakes Yellowstone 66 YELL2 WY 44.67 111.70 2.7 2.5 2.3 2.1 2.1 1.9 1.7 1.6 

Redwood Redwood 88 REDW1 CA 41.56 124.08 4.4 3.9 4.6 3.9 4.5 4.7 4.9 4.7 Rocky Mountain Rocky Mountain 57 ROMO1 CO 40.28 105.55 1.7 1.9 1.9 2.1 2.3 2.0 1.8 2.0 

Roosevelt Campobello Moosehorn 2 MOOS1 ME 44.88 66.95 3.0 2.7 2.7 3.0 3.0 3.1 3.4 3.8 

Saguaro Saguaro 40 SAGU1 AZ 32.25 110.73 1.8 1.6 1.4 1.1 1.1 1.1 1.4 1.8 

Saint Marks Saint Marks 17 SAMA1 FL 30.12 84.08 3.7 3.4 3.4 3.4 3.5 4.0 4.1 4.4 

Salt Creek Salt Creek 36 SACR1 NM 33.61 104.37 2.1 1.9 1.5 1.5 1.7 1.6 1.8 2.0 San Gabriel San Gabriel 93 SAGA1 CA 34.27 117.94 2.5 2.5 2.4 2.2 2.2 2.1 2.2 2.2 San Gorgonio San Gorgonio 99 SAGO1 CA 34.18 116.90 2.7 2.8 2.6 2.3 2.2 1.9 1.8 1.9 San Jacinto San Gorgonio 99 SAGO1 CA 33.75 116.65 2.5 2.4 2.4 2.2 2.1 2.0 2.1 2.1 San Pedro Parks San Pedro Parks 34 SAPE1 NM 36.11 106.81 2.3 2.1 1.8 1.6 1.6 1.4 1.7 2.0 

San Rafael San Rafael 94 RAFA1 CA 34.78 119.83 2.8 2.7 2.7 2.4 2.3 2.3 2.5 2.5 
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

Table A-3 Monthly Site-Specific f(RH) Values for Each Mandatory Federal Class I Area

Based on the Centroid of the Area (Supplemental Information)

Site Jan Feb Mar Apr May Jun Jul Aug 

Class I Area Site Name Map ID Code St LAT LONG f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) f(RH) 

Sawtooth Sawtooth 70 SAWT1 ID 44.18 114.93 3.3 2.9 2.3 2.0 2.0 1.8 1.4 1.4 Scapegoat Monture 73 MONT1 MT 47.17 112.73 3.2 2.8 2.6 2.4 2.5 2.4 2.1 2.0 Selway - Bitterroot Sula 71 SULA1 MT 45.86 114.00 3.5 3.0 2.6 2.3 2.4 2.3 1.9 1.9 

Seney Seney 22 SENE1 MI 46.26 86.03 3.3 2.8 2.9 2.7 2.6 3.1 3.6 4.0 

Sequoia Sequoia 98 SEQU1 CA 36.50 118.82 2.5 2.4 2.4 2.2 1.9 1.8 1.7 1.6 Shenandoah Shenandoah 6 SHEN1 VA 38.52 78.44 3.1 2.8 2.8 2.5 3.1 3.4 3.5 3.9 Shining Rock Shining Rock 11 SHRO1 NC 35.39 82.78 3.3 3.0 2.9 2.7 3.4 3.9 4.1 4.5 

Sierra Ancha Sierra Ancha 45 SIAN1 AZ 33.82 110.88 2.1 2.0 1.7 1.3 1.3 1.1 1.5 1.8 

Simeonof Simeonof 105 SIME1 AK 54.92 159.28 4.3 4.1 3.6 3.9 3.9 4.3 5.0 5.2 Sipsey Sipsey 21 SIPS1 AL 34.34 87.34 3.4 3.1 2.9 2.8 3.3 3.7 3.9 3.9 

South Warner Lava Beds 87 LABE1 CA 41.33 120.20 3.6 3.1 2.7 2.4 2.3 2.1 1.9 1.9 

Strawberry Mountain Starkey 76 STAR1 OR 44.30 118.73 3.9 3.3 2.8 2.9 2.3 2.4 2.0 2.0 

Superstition Tonto 44 TONT1 AZ 33.63 111.10 2.1 1.9 1.6 1.3 1.3 1.1 1.5 1.7 Swanquarter Swanquarter 14 SWAN1 NC 35.31 76.28 2.9 2.7 2.6 2.5 2.9 3.2 3.4 3.5 Sycamore Canyon Sycamore Canyon 47 SYCA1 AZ 34.03 116.18 2.4 2.3 2.2 2.0 2.0 1.9 2.0 2.0 

Teton Yellowstone 66 YELL2 WY 44.09 110.18 2.5 2.4 2.2 2.1 2.1 1.9 1.6 1.5 

Theodore Roosevelt Theodore Roosevelt 61 THRO1 ND 47.30 104.00 2.9 2.8 2.8 2.3 2.3 2.5 2.4 2.2 Thousand Lakes Lassen Volcanic 90 LAVO1 CA 40.70 121.58 3.8 3.2 2.9 2.5 2.4 2.2 2.1 2.1 Three Sisters Three Sisters 84 THSI1 OR 44.29 122.04 4.5 4.0 3.6 3.7 3.1 3.1 3.0 2.9 

Tuxedni Tuxedni 103 TUXE1 AK 60.15 152.60 3.5 3.3 2.9 2.7 2.7 2.9 3.6 4.0 

UL Bend UL Bend 64 ULBE1 MT 47.55 107.87 2.7 2.5 2.5 2.3 2.2 2.2 2.0 1.8 Upper Buffalo Upper Buffalo 27 UPBU1 AR 35.83 93.21 3.3 3.0 2.7 2.8 3.4 3.4 3.4 3.4 Ventana Pinnacles 92 PINN1 CA 36.22 121.59 3.2 2.9 2.8 2.4 2.3 2.1 2.2 2.3 Virgin Islands (b) Virgin Islands 106 VIIS1 VI 18.33 64.79

Voyageurs Voyageurs 24 VOYA2 MN 48.59 93.17 2.8 2.4 2.4 2.3 2.3 3.1 2.7 3.0 Washakie North Absoraka 67 NOAB1 WY 43.95 109.59 2.5 2.3 2.2 2.1 2.1 1.8 1.6 1.5 Weminuche Weminuche 55 WEMI1 CO 37.65 107.80 2.4 2.2 1.9 1.7 1.7 1.5 1.6 2.0 West Elk White River 56 WHRI1 CO 38.69 107.19 2.3 2.2 1.9 1.9 1.9 1.7 1.8 2.1 

Wheeler Peak Wheeler Peak 35 WHPE1 NM 36.57 105.42 2.3 2.2 1.9 1.8 1.8 1.6 1.8 2.2 

White Mountain White Mountain 37 WHIT1 NM 33.49 105.83 2.1 1.9 1.6 1.5 1.5 1.4 1.8 2.0 Wichita Mountains Wichita Mountains 30 WIMO1 OK 34.74 98.59 2.7 2.6 2.4 2.4 3.0 2.7 2.3 2.5 

Wind Cave Wind Cave 60 WICA1 SD 43.55 103.48 2.5 2.5 2.5 2.5 2.7 2.5 2.3 2.3 

Wolf Island Okefenokee 16 OKEF1 GA 31.31 81.30 3.4 3.1 3.1 3.0 3.3 3.7 3.7 4.1 Yellowstone Yellowstone 66 YELL2 WY 44.55 110.40 2.5 2.4 2.3 2.2 2.2 1.9 1.7 1.6 Yolla Bolly - Middle Eel Trinity 104 TRIN1 CA 40.11 122.96 4.0 3.4 3.1 2.8 2.7 2.5 2.4 2.5 

Yosemite Yosemite 96 YOSE1 CA 37.71 119.70 3.3 3.0 2.8 2.3 2.1 1.8 1.5 1.5 Zion Zion 51 ZION1 UT 37.25 113.01 2.7 2.4 2.0 1.6 1.5 1.3 1.2 1.4 

a: No particulate matter sampling or visibility monitoring is conducted in the Bering Sea Wilderness.b: f(RH) values for Virgin Islands National Park were not calculated because of the limited RH data available.
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

A - 15

Figure A-1 Monthly Average f(RH) Values for February 

(all weather stations shown)
Guidance for Estimating Natural Visibility Conditions Under the Regional Haze Rule

A - 16

Figure A-2 Monthly Average f(RH) Values for May 

(all weather stations shown)
Guidance for Estimating Natural Visiblity Conditions Under the Regional Haze Program

A - 17

 Figure A-3 Monthly Average f(RH) Values for August 

(all weather stations shown)
B - 1

Appendix B

Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas
B - 2

Appendix B 

 Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas

Mandatory Federal Class I Area State Lat. Lon. bext

 (Mm-1)

Ann. Avg.

(dv)

Best Days

(dv) (a)

Worst Days

(dv) (a)

Acadia NP ME 44.35 -68.24 21.40 7.61 3.77 11.45

Agua Tibia Wilderness CA 33.42 -116.99 15.86 4.61 2.05 7.17

Alpine Lake Wilderness WA 47.55 -121.16 16.99 5.30 2.74 7.86

Anaconda-Pintler Wilderness MT 45.95 -113.5 16.03 4.72 2.16 7.28

Arches NP UT 38.73 -109.58 15.58 4.43 1.87 6.99

Badlands NP SD 43.81 -102.36 16.06 4.74 2.18 7.30

Bandelier NM NM 35.79 -106.34 15.62 4.46 1.90 7.02

Bering Sea AK 60.46 -172.75

Big Bend NP TX 29.33 -103.31 15.48 4.37 1.81 6.93

Black Canyon of the Gunnison NM CO 38.57 -107.75 15.68 4.50 1.94 7.06

Bob Marshall Wilderness MT 47.68 -113.23 16.17 4.80 2.24 7.36

Bosque del Apache NM 33.79 -106.85 15.54 4.41 1.85 6.97

Boundary Waters Canoe Area MN 48.06 -91.43 20.89 7.37 3.53 11.21

Breton LA 29.87 -88.82 21.57 7.69 3.85 11.53

Bridger Wilderness WY 42.99 -109.49 15.71 4.52 1.96 7.08

Brigantine NJ 39.49 -74.39 21.05 7.44 3.60 11.28

Bryce Canyon NP UT 37.57 -112.17 15.58 4.43 1.87 6.99

Cabinet Mountains Wilderness MT 48.18 -115.68 16.27 4.87 2.31 7.43

Caney Creek Wilderness AR 34.41 -94.08 21.14 7.49 3.65 11.33

Canyonlands NP UT 38.23 -109.91 15.60 4.45 1.89 7.01

Cape Romain SC 32.99 -79.49 21.22 7.52 3.68 11.36

Capitol Reef NP UT 38.06 -111.15 15.63 4.47 1.91 7.03

Caribou Wilderness CA 40.49 -121.21 16.05 4.73 2.17 7.29

Carlsbad Caverns NP NM 32.12 -104.59 15.61 4.46 1.90 7.02

Chassahowitzka FL 28.69 -82.66 21.46 7.63 3.79 11.47

Chiricahua NM AZ 32.01 -109.34 15.47 4.36 1.80 6.92

Chiricahua Wilderness AZ 31.86 -109.28 15.45 4.35 1.79 6.91

Cohutta Wilderness GA 34.93 -84.57 21.39 7.60 3.76 11.44

Crater Lake NP OR 42.92 -122.13 16.74 5.15 2.59 7.71

Craters of the Moon NM ID 43.39 -113.54 15.80 4.57 2.01 7.13

Cucamonga Wilderness CA 34.24 -117.59 15.85 4.61 2.05 7.17

Denali Preserve NP AK 63.31 -151.19 16.27 4.86 2.30 7.42

Desolation Wilderness CA 38.9 -120.17 15.80 4.57 2.01 7.13

Diamond Peak Wilderness OR 43.53 -122.1 16.84 5.21 2.65 7.77

Dolly Sods Wilderness WV 39 -79.37 21.13 7.48 3.64 11.32

Dome Land Wilderness CA 35.84 -118.23 15.70 4.51 1.95 7.07

Eagle Cap Wilderness OR 45.22 -117.37 16.12 4.78 2.22 7.34
Appendix B 

 Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas

Mandatory Federal Class I Area State Lat. Lon. bext

 (Mm-1)

Ann. Avg.

(dv)

Best Days

(dv) (a)

Worst Days

(dv) (a)

B - 3

Eagles Nest Wilderness CO 39.67 -106.29 15.72 4.52 1.96 7.08

Emigrant Wilderness CA 38.18 -119.77 15.81 4.58 2.02 7.14

Everglades NP FL 25.35 -80.98 20.77 7.31 3.47 11.15

Fitzpatrick Wilderness WY 43.24 -109.6 15.73 4.53 1.97 7.09

Flat Tops Wilderness CO 39.95 -107.3 15.70 4.51 1.95 7.07

Galiuro Wilderness AZ 32.6 -110.39 15.40 4.32 1.76 6.88

Gates of the Mountains Wilderness MT 46.86 -111.82 15.93 4.66 2.10 7.22

Gearhart Mountain Wilderness OR 42.51 -120.86 16.33 4.90 2.34 7.46

Gila Wilderness NM 33.21 -108.47 15.51 4.39 1.83 6.95

Glacier NP MT 48.64 -113.84 16.48 5.00 2.44 7.56

Glacier Peak Wilderness WA 48.21 -121 16.88 5.24 2.68 7.80

Goat Rocks Wilderness WA 46.52 -121.47 16.93 5.26 2.70 7.82

Grand Canyon NP AZ 36.3 -112.79 15.51 4.39 1.83 6.95

Grand Teton NP WY 43.82 -110.71 15.74 4.53 1.97 7.09

Great Gulf Wilderness NH 44.3 -71.28 21.10 7.47 3.63 11.31

Great Sand Dunes NM CO 37.77 -105.57 15.74 4.54 1.98 7.10

Great Smoky Mountains NP TN 35.6 -83.52 21.39 7.60 3.76 11.44

Guadalupe Mountains NP TX 31.91 -104.85 15.64 4.47 1.91 7.03

Haleakala NP HI 20.71 -156.16 16.02 4.71 2.15 7.27

Hawaii Volcanoes NP HI 19.41 -155.34 16.33 4.91 2.35 7.47

Hells Canyon Wilderness OR 45.54 -116.59 16.09 4.76 2.20 7.32

Hercules-Glades Wilderness MO 36.68 -92.9 21.03 7.43 3.59 11.27

Hoover Wilderness CA 38.11 -119.37 15.78 4.56 2.00 7.12

Isle Royale NP MI 48.01 -88.83 20.91 7.38 3.54 11.22

James River Face Wilderness VA 37.59 -79.44 20.96 7.40 3.56 11.24

Jarbidge Wilderness NV 41.77 -115.35 15.75 4.54 1.98 7.10

John Muir Wilderness CA 36.97 -118.88 15.80 4.58 2.02 7.14

Joshua Tree NM CA 33.92 -115.88 15.72 4.52 1.96 7.08

Joyce-Kilmer-Slickrock Wilderness TN 35.44 -83.99 21.40 7.61 3.77 11.45

Kaiser Wilderness CA 37.28 -119.17 15.80 4.57 2.01 7.13

Kalmiopsis Wilderness OR 42.26 -123.92 16.74 5.15 2.59 7.71

Kings Canyon NP CA 36.92 -118.61 15.79 4.57 2.01 7.13

La Garita Wilderness CO 37.95 -106.83 15.69 4.50 1.94 7.06

Lassen Volcanic NP CA 40.49 -121.41 16.08 4.75 2.19 7.31

Lava Beds NM CA 41.76 -121.52 16.37 4.93 2.37 7.49

Linville Gorge Wilderness NC 35.88 -81.9 21.36 7.59 3.75 11.43

Lostwood ND 48.59 -102.46 16.11 4.77 2.21 7.33
Appendix B 

 Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas

Mandatory Federal Class I Area State Lat. Lon. bext

 (Mm-1)

Ann. Avg.

(dv)

Best Days

(dv) (a)

Worst Days

(dv) (a)

B - 4

Lye Brook Wilderness VT 43.13 -73.02 20.99 7.41 3.57 11.25

Mammoth Cave NP KY 37.2 -86.15 21.58 7.69 3.85 11.53

Marble Mountain Wilderness CA 41.51 -123.21 16.65 5.10 2.54 7.66

Maroon Bells-Snowmass Wilderness CO 39.1 -107.02 15.70 4.51 1.95 7.07

Mazatzal Wilderness AZ 34.13 -111.56 15.44 4.35 1.79 6.91

Medicine Lake MT 48.49 -104.35 16.07 4.74 2.18 7.30

Mesa Verde NP CO 37.25 -108.45 15.73 4.53 1.97 7.09

Minarets Wilderness CA 37.74 -119.19 15.78 4.56 2.00 7.12

Mingo MO 37 -90.19 21.03 7.43 3.59 11.27

Mission Mountains Wilderness MT 47.48 -113.87 16.21 4.83 2.27 7.39

Mokelumne Wilderness CA 38.57 -120.06 15.80 4.58 2.02 7.14

Moosehorn ME 45.09 -67.29 21.22 7.52 3.68 11.36

Mount Adams Wilderness WA 46.2 -121.49 16.86 5.22 2.66 7.78

Mount Baldy Wilderness AZ 33.95 -109.54 15.51 4.39 1.83 6.95

Mount Hood Wilderness OR 45.37 -121.73 16.83 5.21 2.65 7.77

Mount Jefferson Wilderness OR 44.61 -121.84 16.91 5.25 2.69 7.81

Mount Rainier NP WA 46.86 -121.72 17.05 5.34 2.78 7.90

Mount Washington Wilderness OR 44.3 -121.88 17.03 5.33 2.77 7.89

Mount Zirkel Wilderness CO 40.75 -106.68 15.71 4.52 1.96 7.08

Mountain Lakes Wilderness OR 42.33 -122.11 16.50 5.01 2.45 7.57

North Absaroka Wilderness WY 44.74 -109.8 15.74 4.53 1.97 7.09

North Cascades NP WA 48.83 -121.35 16.86 5.22 2.66 7.78

Okefenokee GA 30.82 -82.33 21.41 7.61 3.77 11.45

Olympic NP WA 47.77 -123.74 17.02 5.32 2.76 7.88

Otter Creek Wilderness WV 38.99 -79.65 21.14 7.49 3.65 11.33

Pasayten Wilderness WA 48.89 -120.44 16.84 5.21 2.65 7.77

Pecos Wilderness NM 35.9 -105.62 15.65 4.48 1.92 7.04

Petrified Forest NP AZ 34.99 -109.79 15.54 4.41 1.85 6.97

Pine Mountain Wilderness AZ 34.31 -111.8 15.47 4.36 1.80 6.92

Pinnacles NM CA 36.48 -121.19 16.12 4.78 2.22 7.34

Point Reyes NS CA 38.06 -122.9 16.20 4.83 2.27 7.39

Presidential Range-Dry River Wilderness NH 44.2 -71.34 21.15 7.49 3.65 11.33

Rainbow Lake Wilderness WI 46.42 -91.31 20.99 7.42 3.58 11.26

Rawah Wilderness CO 40.69 -105.95 15.72 4.52 1.96 7.08

Red Rock Lakes MT 44.64 -111.78 15.81 4.58 2.02 7.14

Redwood NP CA 41.44 -124.03 16.90 5.25 2.69 7.81

Rocky Mountain NP CO 40.35 -105.7 15.67 4.49 1.93 7.05
Appendix B 

 Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas

Mandatory Federal Class I Area State Lat. Lon. bext

 (Mm-1)

Ann. Avg.

(dv)

Best Days

(dv) (a)

Worst Days

(dv) (a)

B - 5

Roosevelt Campobello International Park ME 44.85 -66.94 21.22 7.52 3.68 11.36

Saguaro NM AZ 32.17 -110.61 15.35 4.28 1.72 6.84

Salt Creek NM 33.6 -104.41 15.58 4.43 1.87 6.99

San Gabriel Wilderness CA 34.27 -117.94 15.86 4.61 2.05 7.17

San Gorgonio Wilderness CA 34.12 -116.84 15.74 4.54 1.98 7.10

San Jacinto Wilderness CA 33.75 -116.64 15.78 4.56 2.00 7.12

San Pedro Parks Wilderness NM 36.11 -106.81 15.63 4.47 1.91 7.03

San Rafael Wilderness CA 34.76 -119.81 16.03 4.72 2.16 7.28

Sawtooth Wilderness ID 43.99 -115.06 15.82 4.59 2.03 7.15

Scapegoat Wilderness MT 47.16 -112.74 16.05 4.73 2.17 7.29

Selway-Bitterroot Wilderness ID 46.12 -114.86 16.09 4.76 2.20 7.32

Seney MI 46.25 -86.09 21.23 7.53 3.69 11.37

Sequoia NP CA 36.51 -118.56 15.79 4.57 2.01 7.13

Shenandoah NP VA 38.47 -78.49 20.98 7.41 3.57 11.25

Shining Rock Wilderness NC 35.38 -82.85 21.40 7.61 3.77 11.45

Sierra Ancha Wilderness AZ 33.85 -110.9 15.46 4.36 1.80 6.92

Simeonof AK 54.91 -159.28 17.21 5.43 2.87 7.99

Sipsey Wilderness AL 34.32 -87.44 21.28 7.55 3.71 11.39

South Warner Wilderness CA 41.31 -120.2 16.09 4.76 2.20 7.32

St. Marks FL 30.11 -84.15 21.54 7.67 3.83 11.51

Strawberry Mountain Wilderness OR 44.29 -118.74 16.37 4.93 2.37 7.49

Superstition Wilderness AZ 33.5 -111.27 15.40 4.32 1.76 6.88

Swanquarter NC 35.39 -76.39 20.91 7.38 3.54 11.22

Sycamore Canyon Wilderness AZ 35.01 -112.09 15.53 4.40 1.84 6.96

Teton Wilderness WY 44.04 -110.17 15.74 4.53 1.97 7.09

Theodore Roosevelt NP ND 46.96 -103.46 16.08 4.75 2.19 7.31

Thousand Lakes Wilderness CA 40.7 -121.58 16.10 4.76 2.20 7.32

Three Sisters Wilderness OR 44.04 -121.91 17.01 5.31 2.75 7.87

Tuxedni AK 60.14 -152.61 16.58 5.06 2.50 7.62

UL Bend MT 47.54 -107.89 15.87 4.62 2.06 7.18

Upper Buffalo Wilderness AR 36.17 -92.41 21.04 7.44 3.60 11.28

Ventana Wilderness CA 36.21 -121.6 16.09 4.76 2.20 7.32

Virgin Islands NP (b) VI 18.35 -64.74

Voyageurs NP MN 48.47 -92.8 20.64 7.25 3.41 11.09

Washakie Wilderness WY 44.1 -109.57 15.73 4.53 1.97 7.09

Weminuche Wilderness CO 37.61 -107.25 15.68 4.50 1.94 7.06

West Elk Wilderness CO 38.75 -107.21 15.71 4.51 1.95 7.07
Appendix B 

 Default Natural bext, dv, and 10th and 90th Percentile 

dv Values at All Mandatory Federal Class I Areas

Mandatory Federal Class I Area State Lat. Lon. bext

 (Mm-1)

Ann. Avg.

(dv)

Best Days

(dv) (a)

Worst Days

(dv) (a)

B - 6

Wheeler Peak Wilderness NM 36.57 -105.4 15.70 4.51 1.95 7.07

White Mountain Wilderness NM 33.48 -105.85 15.56 4.42 1.86 6.98

Wichita Mountains OK 34.75 -98.65 20.60 7.23 3.39 11.07

Wind Cave NP SD 43.58 -103.47 15.97 4.68 2.12 7.24

Wolf Island GA 31.33 -81.3 21.33 7.58 3.74 11.42

Yellowstone NP WY 44.63 -110.51 15.77 4.56 2.00 7.12

Yolla Bolly Middle Eel Wilderness CA 40.09 -122.96 16.25 4.85 2.29 7.41

Yosemite NP CA 37.85 -119.54 15.81 4.58 2.02 7.14

Zion NP UT 37.32 -113.04 15.56 4.42 1.86 6.98

(a) Values for the best and worst days are estimated from a statistical approach described in Section 2.6 of this document.

(b) f(RH) values for Virgin Islands National Park were not calculated because of the limited RH data available. As such no

estimates for Natural Visibility Conditions are presented at this time.
TECHNICAL REPORT DATA

(Please read Instructions on reverse before completing)

1. REPORT NO.

 EPA-454/B-03-005

2. 3. RECIPIENT'S ACCESSION NO.

4. TITLE AND SUBTITLE

Guidance for Estimating Natural Visibility Conditions Under the

Regional Haze Program

5. REPORT DATE

 September 2003

6. PERFORMING ORGANIZATION CODE

7. AUTHOR(S)

U.S. EPA/OAR/OAQPS/EMAD/AQTAG

8. PERFORMING ORGANIZATION REPORT NO.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

Office of Air Quality Planning and Standards

Emissions, Monitoring and Analysis Division

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

10. PROGRAM ELEMENT NO.

11. CONTRACT/GRANT NO.

68-D-02-0261

Work Order No. 1-06 

12. SPONSORING AGENCY NAME AND ADDRESS

Director

Office of Air Quality Planning and Standards

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

13. TYPE OF REPORT AND PERIOD COVERED

Guidance document 

14. SPONSORING AGENCY CODE

EPA/200/04

15. SUPPLEMENTARY NOTES

16. ABSTRACT

The purpose of this document is to provide guidance to the States in implementing the regional haze program under the

Clean Air Act. As part of the program, States will develop goals and implement strategies for improving visibility in

each mandatory Federal Class I area. Estimates of natural visibility conditions are needed by the States for the goal

development process. This guidance document describes “default”16 and “refined” approaches for estimating natural

conditions. EPA believes that natural conditions estimates developed using the default approach will be adequate to

satisfy the requirements of the regional haze rule for the initial SIP submittals due no later than 2008. This document

provides guidance to EPA Regional, State, and Tribal air quality management authorities and the general public, on

how EPA intends to exercise its discretion in implementing Clean Air Act provisions and EPA regulations, concerning

the estimation of natural conditions under the regional haze program. 

17. KEY WORDS AND DOCUMENT ANALYSIS

a. DESCRIPTORS b. IDENTIFIERS/OPEN ENDED TERMS c. COSATI Field/Group

Regional Haze, Haze Index Air Pollution control

18. DISTRIBUTION STATEMENT

 Unlimited

19. SECURITY CLASS (Report)

 Unclassified

21. NO. OF PAGES

20. SECURITY CLASS (Page)

 Unclassified

22. PRICE

EPA Form 2220-1 (Rev. 4-77) PREVIOUS EDITION IS OBSOLETE
United States Office of Air Quality Planning and Standards Publication No. EP

Environmental Protection Emissions, Monitoring and Analysis Division September 2003Agency Research Triangle Park, NC

Postal information in this section where appropriate.