1

Attribution of Haze Workgroup’s 

Technical Recommendations on Monitoring Metrics for Regional Haze Planning 

9/22/06 DRAFT 

Executive Summary 

The Attribution of Haze Workgroup recommends the following forms of monitoring metrics be used by 

states and tribes for assessing all visibility-protected Class I Federal areas of the WRAP region in the 

December 2007 regional haze implementation plans: 

• The revised IMPROVE light extinction equation as developed and approved in 2005 by the 

IMPROVE Steering Committee to convert from mass concentration measurements to light extinction 

for visibility analysis and regional haze planning; 

• The 2000-04 baseline period IMPROVE visibility monitoring data reported using the revised 

IMPROVE equation, as available on the Technical Support System; 

• The alternative 2064 natural visibility conditions estimates of total light extinction and individual 

light extinction species using the revised IMPROVE equation, as developed and recommended by the 

Inter-RPO Monitoring & Data Analysis Discussion Group; and 

• The 2018 planning milestone projected visibility value (using the Relative Reduction Factor or RRF), 

as calculated using a scaling factor from 2000-04 baseline period monitoring data (WRAP method) 

applied to a ratio of historic and projected regional gridded air quality modeling results to assess the 

amount of visibility improvement expected from emissions reductions across all sources by 2018. 

Using the revised IMPROVE light extinction equation, applying these metrics for all WRAP region Class 

I areas allows a regionally consistent approach between the individual regional haze implementation plans 

based on the analysis of the impact of individual light extinction species (SO4, NO3, organic carbon, 

elemental carbon, fine soil, and coarse material) and the sources contributing to their formation. These 

metrics also allow the most up-to-date scientific analysis of the total visibility impact in units of light 

extinction or the haze index (the deciview) required in the Regional Haze Rule. 

The 2000-04 baseline period monitoring data have been reviewed by the AoH WG and found to be 

adequate as the basis of regional haze plans. Specific monitoring sites (5) with limited data from this 5-

year period have additional site-specific recommendations for their limited data record. The alternative 

natural conditions estimates also incorporate the revised IMPROVE equation so that progress in reducing 

emissions and increasing visibility across the nominal 60-year period defined in the Regional Haze Rule 

to achieve natural conditions in Class I areas can be better assessed in terms of natural and anthropogenic 

sources, as well as the degree of controllability. 

Finally, the WRAP RRF projection method for future visibility conditions incorporates the default EPA 

method modified with a standard approach to select monitoring data for projecting regional modeling 

results. This allows for a better, accounting of both source strength and temporal variation to produce a 

consistent estimate of future visibility conditions based on modeling comprehensive emissions 

inventories. The revised IMPROVE equation and alternative Natural Conditions Estimates are not 

endorsed for individual state air program evaluation of BART modeling results for technical and process 

reasons discussed in the Monitoring Metrics document since these have not been incorporated into the 

EPA or Federal Land Managers' Air Quality Related Values Work Group (FLAG). 
2

Technical Recommendations for Haze Planning in the WRAP region

1) Apply the revised IMPROVE light extinction equation as developed and approved in 2005 

by the IMPROVE Steering Committee to convert from mass concentration measurements 

to light extinction for visibility analysis and regional haze planning at each WRAP region 

Class I area. 

2) Use the 2000-04 Best and Worst Days’ metrics as calculated and reported by VIEWS and 

TSS. Individual states should review the data completeness and any data substitutions for 

their CIAs. 

3) Use the alterative Natural Conditions Estimates in combination with the 2000-04 Best and 

Worst Days’ metrics as calculated and reported by VIEWS and TSS, utilizing the revised 

IMPROVE equation, for visibility analysis and regional haze planning at each WRAP 

region Class I area. 

4) Use the alternative WRAP RRF Projection Method for projecting 2018 visibility conditions 

in combination with the 2000-04 Best and Worst Days’ metrics as calculated and reported 

by VIEWS and TSS, utilizing the revised IMPROVE equation and alternative Natural 

Conditions Estimates, for visibility analysis and regional haze planning at each WRAP 

region Class I area. Study the effect of this method for Fine Soil and Coarse Material 

visibility modeling results before applying it to those IMPROVE light extinction species. 
3

Attribution of Haze Workgroup’s 

Technical Recommendations on Monitoring Metrics for Regional Haze Planning 

9/22/06 DRAFT 

Background

This document summarizes the recommended monitoring metrics for application by states and tribes for 

regional haze planning, for the December 2007 plans’ due date. The intent of this document is to identify 

the currently available best technical monitoring metrics and reasons for using them. The motivation 

behind this consensus product from the Attribution of Haze Workgroup is to lay out a regionally 

consistent approach for applying the following monitoring metrics at each WRAP region Class I area: 

• The light extinction equation to convert from mass concentration measurements to visibility; 

• The 2000-04 baseline period visibility monitoring data; 

• Natural visibility conditions estimates – the 2064 goal to be defined in the 2007 haze plans; and 

• The 2018 planning milestone projected visibility value (using the Relative Reduction Factor or RRF), 

a scaling factor from monitoring data to be applied to regional gridded air quality modeling results to 

assess the amount of visibility improvement expected from emissions reductions across all sources by 

2018. 

This document provides a protocol for applying the monitoring metrics, and can serve as a reference 

document for individual haze implementation plans. 

Use of Monitoring Data in Support of Regional Haze Planning 

The need for a consistent regional methodology for evaluating the WRAP region results from studying 

the nature and causes of light extinction and fundamental requirement to apply the best metrics to the 

regional haze planning process. Conceptually this process is shown in Figure 1. 

Figure 1. Conceptual Diagram of Monitoring Metrics for Planning under the Regional Haze Rule

Monitoring Metrics for Haze Planning

IMPROVE Equation

used to determine light

extinction and associated

deciview values 

Natural

Conditions Estimates

(species)

2000-04

Baseline Monitoring

Period Data

(species)

Future Year Projections

of Modeling Results scaled

against 2000-04 species

monitoring data

Measured & modeled

values used in Glide Path

Analysis for each

Class I area

(dv, species)
4

The Interagency Monitoring of PROtected Visual Environments (IMPROVE) national monitoring 

program has been designated by EPA to collect visibility impairment data representing the 156 Class I 

Federal areas (CIAs) with visibility protection under the Clean Air Act. More information about the 

IMPROVE monitoring program, the 110 monitoring sites across the nation selected to represent the 156 

CIAs, and the Steering Committee managing the monitoring program in support of regional haze plans 

required of all 50 states under the EPA Regional Haze Rule (RHR) can be found at: 

http://vista.cira.colostate.edu/improve/Overview/Overview.htm. EPA has also published a broader 

guidance document on Visibility Monitoring, see: http://www.epa.gov/ttn/amtic/files/ambient/visible/r99-003.pdf. 

Light extinction theory 

Based on aerosol research, the technical method to determine visibility impairment is to calculate the light 

extinction coefficient (bext). This is defined as the loss of image-forming light per unit distance due to 

scattering and absorption by particles and gases in the atmosphere. The light extinction coefficient is the 

sum of the scattering coefficient (bscat) and absorption coefficient (babs), which are similarly defined as 

the loss of light per unit distance by scattering and absorption mechanisms respectively. The light 

extinction coefficient can be represented mathematically as: 

; 

where s, a, g, and p refer to scattering and absorption by gases and particles, respectively. 

Speciated Monitoring Data 

To determine visibility impairment under the RHR, filter sampling data from IMPROVE aerosol monitors 

are collected and analyzed following Standard Operating Procedures, see: 

(http://vista.cira.colostate.edu/improve/Publications/SOPs/UCDavis_SOPs/IMPROVE_SOPs.htm). 

These resulting mass concentration data are then converted to light extinction using an algorithm referred 

to as the IMPROVE equation; this equation has been reviewed and changed during the last 2 years, (see: 

http://vista.cira.colostate.edu/improve/Publications/GrayLit/019_RevisedIMPROVEeq/RevisedIMPROV

EAlgorithm3.doc), creating a ripple effect for the monitoring metrics required for use in regional haze 

planning. 

 

Haze Index- The Deciview (dv) 

The RHR also requires that the light extinction data from the IMPROVE equation be analyzed and 

presented in terms of a “haze index” value called the deciview (dv); the dv index is related to total light 

extinction, described at: http://vista.cira.colostate.edu/improve/Publications/NewsLetters/apr_93.pdf. As 

such, light extinction for regional haze planning could be analyzed in terms of its component contributors 

to light extinction as defined in the old and revised IMPROVE equations, and/or as the associated 

deciview values. 

 

IMPROVE Light Extinction Equation – Original vs. Revised

The original IMPROVE light extinction equation was adopted by the Steering Committee and used in 

their principle publications (see: http://vista.cira.colostate.edu/improve/Publications/Principle_pubs.htm) 

since the early 1990s. The equation has also been widely evaluated and used in peer-reviewed journal 

articles as well as urban visibility studies. For those reasons, the EPA adopted this equation in their 2003 
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guidance document on Tracking Progress Under the Regional Haze Rule. See: 

http://www.epa.gov/ttn/oarpg/t1/memoranda/rh_tpurhr_gd.pdf. 

The equation uses additive extinction by chemical species as measured by the IMPROVE aerosol 

monitor, combined with the effect of Relative Humidity (RH), to estimate the scattering of light by fine 

and coarse particles. The original IMPROVE equation, as adopted by EPA in their guidance document, is 

used to estimate total light extinction for the purposes of planning under the RHR: 

The brackets in this equation indicate the species concentration. The factors 3, 4, 1, and 0.6 are the m2

/g 

dry specific scattering efficiency for each of the respective species. . Thus, a sulfate particle is three 

times more effective in scattering light than a particle of soil. 

To account and control for relative humidity effects in the light extinction data to be used in regional haze 

planning, EPA 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 climatological mean humidity 

correction factors designed to represent each CIA. The hourly RH measurements from each site were 

converted to f(RH) values using a nonlinear weighting factor curve. Values above 95% RH were set equal 

to the f(RH) corresponding to 95% RH. 

Appendix A of: http://www.epa.gov/ttn/oarpg/t1/memoranda/rh_tpurhr_gd.pdf presents these values. 

Determination of the humidity factors is described in section 3.6 of that document. Using over 370 

humidity monitoring locations across the country, monthly f(RH) values were calculated. In most regions 

there is a seasonal cycle of relative humidity, which is accounted for by generating the appropriate 

monthly f (RH) values, as in Appendix A. The 12 monthly averaged f (RH) values are listed for each 

IMPROVE or IMPROVE protocol site and their corresponding Class I areas. The site specific values are 

listed for each CIA and are recommended to be used for all visibility and tracking progress calculations 

for that CIA. These values are provided in Table A-2. A table of 12 monthly-averaged f(RH) values for 

each CIA is also provided in Table A-3 for informational purposes. 

Overview of revised IMPROVE equation

The IMPROVE light extinction equation was analyzed, revised, and approved by the Steering Committee 

during 2005. A detailed discussion of the revised equation and the reasons for changing it can be found 

at: http://vista.cira.colostate.edu/improve/Publications/NewsLetters/IMPNews4thQtr2005.pdf. A 

summary of the differences between the original and revised equations follows. 

The original IMPROVE equation produces reasonable estimates of light scattering over a broad range of 

conditions. However, it tends to underestimate the highest extinction values and overestimate the lowest 

extinction values. Since EPA adopted the equation for use in RHR planning, the original IMPROVE 

equation has been scrutinized carefully to assess deficiencies that could bias the implementation of the 

RHR. With those concerns identified, the IMPROVE Steering Committee initiated an internal review 

resulting in recommendations for revisions of the light extinction equation. The review team of scientists 

from the National Park Service and the Cooperative Institute for Research in the Atmosphere developed a 

revised algorithm that reduces biases in light extinction estimates and is as consistent as possible with the 
6

current scientific literature. Review of the original equation and suggested revisions are presented at: 

http://vista.cira.colostate.edu/improve/Publications/NewsLetters/IMPNews2ndQtr2005.pdf. 

In July 2005, the equation review results and proposed revisions were presented to the IMPROVE 

Steering Committee. A subcommittee was formed to further investigate the proposed equation. The 

subcommittee included scientists who worked on the initial review, as well as scientists who have been 

critical of the original IMPROVE equation. Their work resulted in the final version of the equation, 

which was again presented to the Steering Committee. In December 2005, the IMPROVE Steering 

Committee voted to adopt this revised equation for use by IMPROVE as an alternative to the current 

approach. 

The new equation splits ammonium sulfate, ammonium nitrate, and organic carbon compound 

concentrations into two size fractions: small and large. The equation for estimating the light extinction 

for the RHR is:

bext≈ 2.2 x fs(RH) x [small sulfate] + 4.8 x fL(RH) x [large sulfate] 

+ 2.4 x fs(RH) x [small nitrate] + 5.1 x fL(RH) x [large nitrate] 

+ 2.8 x [small organic mass] + 6.1 x [large organic mass] 

+ 10 x [elemental carbon] 

+ 1 x [fine soil] 

+ 1.7 x fss(RH) x [sea salt] 

+ 0.6 x [coarse mass] 

+ Rayleigh scattering (site-specific) 

+ 0.33 x [NO2 (ppb)] 

Though not explicitly shown, the organic mass concentration used is 1.8 times the organic carbon mass 

concentration, (changed from 1.4 times carbon mass the original equation uses). New terms have also 

been added for sea salt and for absorption by NO2. The apportionment of the total concentration of 

sulfate compounds into the concentrations of small and large size fractions is accomplished using the 

following equations: 

[large sulfate] = [total sulfate] x [total sulfate], for [total sulfate] < 20 μg/m3 

[large sulfate] = [total sulfate], for [total sulfate] ≥ 20 μg/m3 

[small sulfate] = [total sulfate] - [large sulfate] 

The same equations are used to apportion total nitrate and total organic mass into small and large size 

fractions. Sea salt is calculated as 1.8 x [chloride], or 1.8 x [chlorine] if the chloride measurement is 

below detection limits, missing, or invalid. The new equation contains three distinct water growth terms, 

designated fS, fL, and fSS for the small and large sulfate and nitrate fractions, and for sea salt, respectively. 

Technical justification for revisions

The new IMPROVE equation for estimating light extinction for the RHR contains five major revisions 

from the original equation: 

1) A sea salt term has been added. Sea salt is a particular concern for coastal locations where the 

sum of the major components of light extinction and mass has been deficient. 

2) The assumed organic mass to organic carbon ratio has been changed from 1.4 to 1.8, to reflect 

more recent peer-reviewed literature on the subject. 

3) The Rayleigh scattering factor has been changed from a network-wide constant to a site-specific 

value. This factor is based on the elevation and annual average temperature of individual 

monitoring sites. 

4) A split component extinction efficiency model for sulfate, nitrate, and organic carbon components 

has been developed. The model includes new water growth terms for sulfate and nitrate to better 

estimate light extinction at the high and low extremes of the range of extinction. 
7

5) An NO2 light absorption term has been added. This term can only be used at sites with available 

NO2 concentration data. 

Comparison of original versus revised equation

One of the most compelling reasons for developing a revised equation was to reduce the biases in light 

scattering estimates at the extremes, when compared to nephelometer measurements a direct measure 

particle scattering. To assess the performance of the new equation, the fractional bias for each sample 

period was calculated as the difference in estimated aerosol light scattering divided by the measured light 

scattering using collocated nephelometers. These biases were then averaged into quintiles to indicate the 

bias in each of those five subsets of data. Analysis shows that the revised equation has lower fractional 

bias than the original equation, in all but the haziest conditions.

Scatter plots (Figures 2 and 3) of light scattering estimates from the original and revised equations versus 

nephelometer data for all available data at 21 monitoring sites were used to view the overall performance 

differences. These figures show bias at the extremes is reduced using the revised, compared to the 

original, equation (i.e., the points tend to be better centered on the one-to-one line). However, they also 

show the somewhat reduced precision of the revised equation compared to the original (i.e., points are 

more broadly scattered). 

Figure 2. Scatter plot of the original IMPROVE equation-estimated particle light scattering versus 

measured particle light scattering.

Figure 3. Scatter plot of the revised IMPROVE equation-estimated particle light scattering versus 

measured particle light scattering.
8

Directly measured light scattering data from collocated Optec NGN-2 nephelometers were used to 

evaluating the performance of the original IMPROVE equation, as well as for development and 

performance evaluation of several possible versions of the revised equation, leading to the final revised 

equation approved by the IMPROVE Steering Committee. The 21 nephelometer sites used in the 

evaluation were:

Acadia National Park, Maine Lye Brook Wilderness, Vermont 

Big Bend National Park, Texas Mammoth Cave National Park, Kentucky 

Boundary Waters Canoe Area Wilderness, Minnesota Mount Rainier National Park, Washington 

Columbia River Gorge National Scenic Area, Oregon Mount Zirkel Wilderness, Colorado 

Dolly Sods Wilderness, West Virginia Okefenokee National Wildlife Refuge, Georgia 

Gila Wilderness, New Mexico Shenandoah National Park, Virginia 

Grand Canyon National Park, Arizona Shining Rock Wilderness, North Carolina 

Great Gulf Wilderness, New Hampshire Snoqualmie Pass, Washington 

Great Smoky Mountains National Park, Tennessee/North Carolina Three Sisters Wilderness, Oregon 

Jarbidge Wilderness, Nevada Upper Buffalo Wilderness, Arkansas 

Lone Peak Wilderness, Utah 

The revised IMPROVE equation reduces the biases compared to measurements at the high and low 

extremes, and is most apparent for the hazier eastern sites. The composition of “best and worst days” is 

very similar by the original and new equations. Most of the reduction of bias associated with the revised 

equation is attributed to the use of the split component extinction efficiency method for sulfate, nitrate, 

and organic components that permitted variable extinction efficiency depending on the component mass 

concentration. The revised equation also contains specific changes incorporating a better understanding of 

the atmosphere based on recent scientific literature. It reflects a more complete accounting for 

contributors to haze (e.g., sea salt and NO2 terms), and uses site-specific Rayleigh scattering terms to 

reduce elevation-related bias. EPA has prepared monthly average f(RH) terms for all IMPROVE 

monitoring sites for the revised equation. The revised equation has been added to the suite of data 

analysis tools on the Visibility Information Exchange (VIEWS - http://vista.cira.colostate.edu/views/) and 

the WRAP Technical Support System (TSS - http://vista.cira.colostate.edu/tss/Results/Monitoring.aspx) 

web sites. A complete discussion and report is available on the IMPROVE Web site at 

http://vista.cira.colostate.edu/IMPROVE/Publications/GrayLit/Gray_literature.htm. 

Effects on Regional Haze Planning in the WRAP Region: Original vs. Revised IMPROVE Equation

The effects of the revised IMPROVE equation on the data used for regional haze planning under the RHR 

have been evaluated by the AoH Workgroup on a December 9, 2005 call and in more detail at a 

Workgroup Meeting on January 24-25, 2006. The detailed presentation at: 

http://wrapair.org/forums/aoh/meetings/060124m/Review_of_New_IMPROVE_Equ_012406_ARS.pdf, 

was the basis for the following observations from the AoH Workgroup. Analysis of the nature and causes 

of visibility impairment at the more than 100 CIAs in the WRAP region strongly suggests that control 

strategies for regional haze planning be evaluated using the revised IMPROVE equation, and that results 

be presented in units of both bext and dv. 

For the purposes of regional haze planning, the revised IMPROVE equation has benefits, as it: 

• incorporates new terms to more completely account for haze; 

• uses updated research information; 

• was developed by comparing to directly-measured optical light scattering data at collocated sites; and 

• reduces known biases. 

For regional haze planning purposes, using the revised IMPROVE equation has some tradeoffs, as it: 
9

• is a national data analysis and addresses data distribution for the whole country, with the associated 

large sulfate impact dominating Eastern US visibility, and does not as directly address the mix of light 

extinction causes at Western Class I area monitoring sites; 

• requires that in using the revised equation to the 2000-04 baseline monitoring period would also 

require that it be applied to as well to natural conditions estimates, to insure a consistent calculation 

of the glide path for each Class I area; 

• has a somewhat greater uncertainty that causes it to mis-select the 20 % worst visibility days for the 

RHR a little more frequently, although little difference was observed with respect to the composition 

on those 20 % worst visibility days; 

• chooses a “better fit” for all data across the distribution at the sites tested over the better precision of 

the original IMPROVE equation for individual data points in the middle of the distribution; and 

• has little consequence for SO4 and NO3 in the WRAP region in terms of the “split component 

extinction efficiency method”, as Figures 4 and 5 below show – Figure 6 shows that this new method 

for Organic Mass would have a more profound impact in the WRAP region, as expected from the 

episodic impacts of wildland fire emissions, as shown on the following page.

While the consistent use of the revised IMPROVE equation is recommended for regional haze planning in 

the WRAP region as the most useful and robust technical method to support reasonable progress analysis, 

the revised IMPROVE equation is not endorsed for use in evaluation of BART modeling results. The 

interpretation of BART modeling results for screening and subsequent determination of BART controls 

involves consideration of a range of various modeled values, and is a state-by-state regulatory decision; a 

number of WRAP region states have used the original equation in their BART determinations and are 

quite far along in regulatory rulemaking to determine BART controls at this time. As the BART analyses 

and determinations are not a comprehensive analysis of all sources contributing to or controllable for 

reasonable progress, and the BART regulatory findings of individual states are based on multiple factors, 

no recommendation or endorsement of a particular light extinction equation is needed. 

Recommendation:

Apply the revised IMPROVE light extinction equation as developed and approved in 2005 by the 

IMPROVE Steering Committee to convert from mass concentration measurements to light 

extinction for visibility analysis and regional haze planning at each WRAP region Class I area.
10

Figures 4, 5, 6. Histograms of Sulfate, Nitrate, and Organic Mass concentration data from all IMPROVE 

sites for 2000-04 in the WRAP (same data in paired charts, lower charts in logarithmic 

scale). 
11

2000-04 Baseline Visibility Period Monitoring Data

The RHR requires that data from the IMPROVE monitoring sites representing CIAs for the 2000-04 

baseline monitoring period is to be used to: 

 

• Determine the current level of visibility impairment under the RHR; 

• Quantify sources and pollutant species contributing to impairment; 

• Assist in classifying natural and anthropogenic sources’ contributions to impairment; and 

• Develop regional haze implementation plans demonstrating reasonable progress across a 60-year 

timeline toward the Clean Air Act goal of no manmade visibility impairment in CIAs. 

A short overview of the IMPROVE monitoring program operations is presented earlier in this document; 

additional information is at: http://vista.cira.colostate.edu/improve/. 

Quality Assurance of IMPROVE Monitoring Data

The IMPROVE monitoring program has a Quality Assurance Project Plan (QAPP), see: 

http://vista.cira.colostate.edu/improve/Publications/QA_QC/IMPROVE_QAPP_R0.pdf, and a Quality 

Management Plan (QMP), see: 

http://vista.cira.colostate.edu/improve/Publications/QA_QC/IMPROVEAerosolQMP_May2002.PDF; 

both were published in 2002. The QMP and QAPP were prepared and the QA activities identified in that 

QMP and QAPP are executed by the laboratory and field operations contractor team for the IMPROVE 

aerosol sampler network, institutional members of the contractor team and locations are: 

• Crocker Nuclear Laboratory – University of California, Davis, NC; 

• Research Triangle Institute – Research Triangle Park, NC; 

• Desert Research Institute – Reno, NV; and 

• Environmental Protection Agency – coordinated by OAQPS, Research Triangle Park, NC. 

Staff of the National Park Service, the U.S. Forest Service, and the Cooperative Institute for Research in 

the Atmosphere (CIRA) at Colorado State University all provide additional quality assurance checks for, 

and analyses of, the IMPROVE data from the contractor team. The continuity of contractor team 

members over time, the centralized QA efforts of the contractor team, as well as the additional QA and 

analytical efforts coordinated by CIRA all combine to provide a very high level of confidence in the 

IMPROVE data for regulatory planning purposes. 

Quality Assurance of the 2000-04 Baseline Monitoring Period WRAP Region IMPROVE Data 

Batches of IMPROVE aerosol sampler data are published through the VIEWS website, see: 

http://vista.cira.colostate.edu/views/. The publication goal for these data is quarterly, sometimes the data 

are published twice-yearly or an annual update is made depending when updated data are provided by 

Crocker Nuclear Laboratory. In October 2005, an update specific to the 2000-04 data was made, see: 

http://vista.cira.colostate.edu/improve/Publications/GrayLit/018_IMPROVEDataResubmission/DataRede

liverySummary2005.pdf. 

Crocker Nuclear Laboratory resubmitted all of the IMPROVE aerosol data to VIEWS for the 2000-04 

monitoring period in October 2005. The data were resubmitted to correct several errors and discrepancies 
12

in the data in order to provide the RHR analysts with the best available data set. There were four 

systematic changes that affected large blocks of data: 

 

• New flow rate validation flags were assigned; 

• Flow rates were recalculated to correct an error in the calculation that existed prior to January 2004; 

• Spectral corrections were applied to sulfur and aluminum data collected beginning in December 2001, 

when Crocker changed the elemental analysis technique; and 

• Carbon analysis data were resubmitted to correct a bias in the data. 

In addition to these systematic changes, a number of site-specific data problems were resolved and the 

data were resubmitted as well. Examples included inadvertently swapped samples, backdated flow rate 

calibrations, samples requiring reanalysis, and equipment problems that were resolved after the original 

data had been submitted. 

Attribution of Haze Workgroup Review of 2000-04 Baseline Period Monitoring Data

Data completeness was reviewed at the AoH Workgroup meeting of November 16-17, 2005, see: 

http://wrapair.org/forums/aoh/meetings/051116m/Summary_of_Regional_Haze_Baseline_Data_111605_

ARS.pdf. This was a preliminary assessment, and the IMPROVE data have been updated since that time. 

The RHR Tracking Progress guidance document published by EPA in 2003, see: 

http://www.epa.gov/ttn/oarpg/t1/memoranda/rh_tpurhr_gd.pdf, prescribes the method for calculating the 

Worst 20% and Best 20% Visibility Days’ metrics to determine the baseline period values to be used in 

regional haze planning. The following steps to calculate these metrics are already complete in VIEWS 

and the TSS, using both the original and revised IMPROVE light extinction equations; specific steps are: 

• Assemble daily speciated data and monthly f(RH) values from each IMPROVE site for each CIA; 

• Perform allowed data substitutions as prescribed in Tracking Progress guidance, if warranted; 

• Sites must have at least 3 of 5 years of “complete” data; 

• Calculate daily extinction, convert to Haze Index (deciviews); 

• Determine the average Haze Index of the 20% worst and best visibility days for each complete year, 

average these annual values for baseline period statistic; and 

• Determine Glide Path by comparing the 2000-04 baseline value with natural conditions. 

Five (5) WRAP region IMPROVE sites did not meet 2000-04 baseline monitoring period data 

completeness criteria of at least 3 years of data: 

• Arizona - (BALD) Mount Baldy 

• California - (KAIS) Kaiser and (RAFA) San Rafael 

• Utah: - (CAPI) Capitol Reef 

• Washington - (NOCA) North Cascades 

A site-specific Baseline period value will be calculated for these ClAs on VIEWS and TSS, in 

consultation with the appropriate state. Methods may include data prior to 2000, data from nearby, 

representative sites, et cetera. 

Recommendation:

Use the 2000-04 Best and Worst Days’ metrics as calculated and reported by VIEWS and TSS. 

Individual states should review the data completeness and any data substitutions for their CIAs. 
13

2064 Natural Visibility Conditions Estimates

Default natural visibility conditions estimates and the associated methodology were published by EPA in 

2003 at: http://www.epa.gov/ttn/oarpg/t1/memoranda/rh_envcurhr_gd.pdf. This Estimating Natural 

Visibility Conditions Under the Regional Haze Rule guidance document was prepared prior to the review 

of the IMPROVE light extinction equation during 2005 and provided estimates of natural conditions in 

haze index units (deciviews) only. Numerous criticisms of these estimates have been noted, see the 

collection of documents at: http://www.wrapair.org/forums/aamrf/projects/NCB/index.html. These 

estimates are in terms of the original IMPROVE light extinction equation, and are not comparable to 

baseline period monitoring metrics calculated with the revised IMPROVE equation. 

Regional haze planning in the WRAP region should separately assess reasonable progress toward the 

national visibility goal by IMPROVE light extinction species, due to the mixture of sulfate, nitrate, 

carbonaceous aerosols, and soil materials causing Western visibility impairment, along with the large 

amount of natural fire and dust emissions contributing to visibility impairment. For those reasons, 

Alternative Natural Conditions Estimates, in term of deciviews, total light extinction, and IMPROVE 

species light extinction have been developed, reviewed, and proposed for use in regional haze planning; 

see: http://vista.cira.colostate.edu/improve/Publications/NewsLetters/IMPNews2ndQtr2006.pdf. 

Alternative Natural Visibility Conditions 

EPA 2003 RHR guidance 2003 for tracking progress and estimating natural conditions were based on the 

original IMPROVE equation, providing a consistent set of instructions. see: 

http://vista.cira.colostate.edu/improve/Publications/GuidanceDocs/guidancedocs.htm. As noted earlier, a 

revised IMPROVE equation was developed and approved that mitigated some of the technical criticism of 

the original IMPROVE equation, especially as it applied to implementing the RHR through the regional 

haze implementation plans due in December 2007. Many of the regional planning organizations (RPOs) 

and states indicated their preference to use the revised equation, but to do so they need natural haze 

condition estimates for their CIAs determined in a consistent manner (i.e., by the new IMPROVE 

equation). The revised IMPROVE equation is described earlier in this document. 

Estimates of natural haze levels using either equation involve applying the equation to estimates of 

natural species concentrations. The natural species concentration estimates used for this purpose come 

from the 1990 NAPAP State of Science Report 24 by John Trijonis, see: 

http://vista.cira.colostate.edu/improve/Publications/Principle/NAPAP_SOS/High%20Res/napap_(high).ht

m and are typical values for the eastern and western U.S. Some methodology is needed to adjust these 

typical values to estimate the 20% best and 20% worst values. A goal in developing the new values is to 

avoid problems identified in the EPA default approach. The Natural Haze Levels II Committee was 

established by the Inter-RPO Monitoring & Data Analysis Workgroup in Spring 2006 to review and 

refine, as appropriate, a methodology developed by Roger Ames (CIRA) for applying the new IMPROVE 

equation for estimating light extinction from aerosol concentrations to natural species concentration 

estimates. Ultimately this would provide natural haze estimates for the 20% best and 20% worst day for 

each of the CIAs covered by the RHR. The committee was composed of Marc Pitchford, NOAA; Bill 

Malm, NPS; Bruce Polkowsky, NPS; Pat Brewer, VISTAS; Tom Moore, WRAP; Ivar Tombach, 

consultant; John Vimont, NPS; Rich Poirot, Vermont; Roger Ames, CIRA; and Naresh Kumar, EPRI. 

The committee work has been summarized in an annotated presentation that is available at: 

http://vista.cira.colostate.edu/datawarehouse/improve/docs/naturalhazelevelsIIreport.ppt. This 

information was presented July 27, 2006 at the Attribution of Haze Workgroup meeting, see: 

http://wrapair.org/forums/aoh/meetings/060726den/index.html and on August 14 at the RPO 
14

Monitoring/Data Analysis Workgroup conference call. Comments received by August 25, 2006 were 

used to generate an approved approach that was forwarded to the individual RPOs for their consideration 

and use. 

The alternative natural conditions estimates offer a number of advantages for regional haze planning, over 

the default estimates in EPA guidance. The alternative conditions are divided into the 6 measured 

IMPROVE light extinction species, and can still be totaled to estimate total visibility in light extinction or 

deciviews, using the revised IMPROVE equation. The alternative estimates are based on analyzing and 

estimating the natural fraction of the 2000-04 baseline period monitoring data at each IMPROVE 

monitoring site. At more than a dozen sites, the natural fractions estimated by the alternative method for 

the 2000-04 data were compared to the period of record (>15 years) data for the same sites, and the 

alternative method for the 2000-04 period is quite similar to the long period of record since the late 1980s. 

Tables 1 through 4 below show example data summaries for regional haze planning use in documenting 

differences between EPA default and alternative methods of estimating natural conditions at each CIA. 

Both EPA default and alternative natural conditions estimates represent 2064 target values for regional 

haze planning purposes; the true values are not known at this time, and more analysis is needed to refine 

these estimates for future regional haze planning cycles, as anthropogenic emissions are reduced and 

natural visibility conditions are better measured. 

The alternative natural conditions estimates are not used in evaluation of BART modeling results, as their 

use would require the use of the revised IMPROVE equation, which has not been incorporated into EPA 

or the Federal Land Managers' Air Quality Related Values Work Group (FLAG) guidance for the 

evaluating modeling results for impacts from, and control of, individual stationary sources. The 

application of the original IMPROVE equation and specified natural conditions in evaluation of BART 

modeling results is for a fundamentally different purpose than regional haze planning, as defined in the 

EPA BART rule. 

While the consistent use of the revised IMPROVE equation is recommended for regional haze planning in 

the WRAP region as the most useful and robust technical method to support reasonable progress analysis, 

the revised IMPROVE equation is not endorsed for use in evaluation of BART modeling results. The 

interpretation of BART modeling results for screening and subsequent determination of BART controls 

involves consideration of a range of various modeled values, and is a state-by-state regulatory decision; a 

number of WRAP region states have used the original equation in their BART determinations and are 

quite far along in regulatory rulemaking to determine BART controls at this time. As the BART analyses 

and determinations are not a comprehensive analysis of all sources contributing to or controllable for 

reasonable progress, and the BART regulatory findings of individual states are based on multiple factors, 

no recommendation or endorsement of a particular light extinction equation is needed. 

As the revised IMPROVE equation and alternative natural conditions estimates recommended for analysis 

of reasonable progress in the WRAP region were developed specifically based on regional haze 2000-04 

baseline period monitoring data, the equation and the associated alternative natural conditions estimates 

were designed to be consistently integrated and used in species and total visibility impact glide paths to 

natural conditions; allowing assessment of the amount of visibility impairment by species to be reduced 

from the 2000-04 baseline period through to 2064 for each Class I area. Use of the revised IMPROVE 

equation and alternative natural conditions estimates in individual BART analyses process of each state is 

not necessary and may not be accurate, as BART controls are required for implementation by 2012, and 

the natural conditions already defined in the existing EPA or the Federal Land Managers' Air Quality 

Related Values Work Group (FLAG) guidance are sufficient for screening and evaluation purposes. 

Further, precise estimates of aerosol mass and size distributions at natural conditions is unknown for 
15

short-term planning purposes, and will likely by different than the alternative natural conditions current 

recommended, as over the next ~60 years, anthropogenic aerosol loadings decrease, more and better 

aerosol data are collected, and implementation of regional haze control plans move forward. 

Recommendation:

Use the alterative Natural Conditions Estimates in combination with the 2000-04 Best and Worst 

Days’ metrics as calculated and reported by VIEWS and TSS, utilizing the revised IMPROVE 

equation, for visibility analysis and regional haze planning at each WRAP region Class I area. 
16

Table 1 – Comparison of Natural Visibility Conditions Estimates using EPA Default and New Alternative Methods, 

in Regional Haze Rule Uniform Rate of Progress 2018 Target Values 

20% Best Days 

2064 Natural 

Conditions 

20% Worst Days 

2000-04 Baseline 

Period Monitoring 

Data 

20% Worst Days 

2064 Natural 

Conditions 

Uniform Rate of 

Progress: 2018 Haze 

Planning Milestone 

for 20% Worst Days 

Mandatory Federal Class I Area State Monitoring Site Code 

Default method 

(dv) 

Alt. 

method 

(dv) 

Default method 

(dv) 

Alt. 

method 

(dv) 

Default method 

(dv) 

Alt. 

method 

(dv) 

Default method 

(dv) 

Alt. 

method 

(dv) 

Agua Tibia 

Wilderness Area CA AGTI1 

Arches NP UT ARCH1 

Table 2 – Light Extinction Component s of 20% Worst Visibility Days for 2064 Natural Visibility Conditions Estima

IMPROVE light extinction equation & alternative method for estimating Natural Visibility Conditions) 

Light Extinction Components of 20% 

2064 Natural Visibility Condition

Mandatory Federal Class I Area State Monitoring Site Code 

20% Worst 

Days 2064 

Natural 

Conditions 

Estimates (dv) 

(from Table 1) 

20% Worst 

Days 2064 

Natural 

Conditions 

Estimates 

(1/Mm) 

Sulfate 

(1/Mm) 

Nitrate 

(1/Mm) 

Organic 

Material 

(1/Mm) 

Elemental 

Carbon 

(1/Mm) 

Agua Tibia Wild. Area CA AGTI1 

Arches NP UT ARCH1 

Table 3 – Light Extinction Components of 20% Best Visibility Days for 2064 Natural Visibility Conditions Estimate

(revised IMPROVE light extinction equation & alternative method for estimating Natural Visibility Con

Light Extinction Components of 20

2064 Natural Visibility Condition

Mandatory Federal Class I Area State Monitoring Site Code 

20% Best 

Days 2064 

Natural 

Conditions 

Estimates (dv) 

(from Table 1) 

20% Best 

Days 2064 

Natural 

Conditions 

Estimates 

(1/Mm) 

Sulfate 

(1/Mm) 

Nitrate 

(1/Mm) 

Organic 

Material 

(1/Mm) 

Elemental 

Carbon 

(1/Mm) 

Agua Tibia Wild. Area CA AGTI1 

Arches NP UT ARCH1 
17

Table 4 - Uniform Rate of Progress: 2018 Haze Planning Milestone for 20% Worst Days and 2018 Visibility Improv

20% Worst Days, by light extinction component, using Alternative Natural Conditions Estimates 

Uniform Rate of Progress: 

2018 Haze Planning Milestone: 

Light Extinction Components on 20% Worst Days (1/Mm) 

Uniform Rate 

2018 Visibility Impr

Mandatory Light Extinction Components Federal Class I Area 

State Monitoring Site Code 

Sulfate Nitrate Organic Material Elemental Carbon Fine Soil Coarse Material Sulfate Nitrate Organic Material 

Agua Tibia 

Wilderness Area CA AGTI1 

Arches NP UT ARCH1 
18

2018 Planning Milestone Visibility Projection Values

In order to prepare regional haze implementation plans for the initial RHR milestone date of 2018, air 

quality modeling results comparing the impacts of historic and projected emissions estimates using 

consistent meteorology have to be projected in terms of monitored visibility values, using monitoring data 

from the 2000-04 baseline period as a controlling term. The concept is that both historic and future 

modeling results contain errors, but by using the monitoring data to control the predictions of the model, 

the relative change in model results from 2000-04 to 2018 can be a reasonable assessment of likely air 

quality changes. Several questions about which monitoring are representative and the best to use are 

discussed next. Information about the air quality model is also reviewed. 

The EPA default guidance method is to use “2002 worst monitored days” to project future visibility 

conditions in “draft” 2001 EPA guidance, even though the RHR requires monitoring data from 2000-04 

baseline period to be used as the basis of the regional haze implementation plans. This guidance also 

applies to interpreting the modeling results for PM2.5 National Ambient Air Quality Standard (NAAQS) 

attainment demonstrations, and is also closely related to attainment demonstrations required for the 

Ozone NAAQS. The EPA guidance is found at: 

http://www.epa.gov/scram001/guidance/guide/draft_pm.pdf. The purpose of applying the EPA guidance 

was to develop a series of relative Reduction Factors (RRFs) using monitoring data to control and “fit” 

historic and projected model predictions, assuming that the air quality model is better at predicting 

relative changes in concentration than absolute concentrations. 

Basic steps for applying the EPA RRF guidance to project visibility conditions in 2018 at each CIA are: 

• Model species concentrations for 2002; 

• Model species concentrations for 2018 scenario; 

• Determine a species-specific RRF for the average of the 20% worst monitored days (selected from 

2002 IMPROVE data), where, for example: 

3⁄4 RRF sulfate = 2018 sulfate/2002 sulfate 

• Using the selected 20% worst days for 2002, apply the RRF values, such that: 

3⁄4 2018 concentration = Average [RRF x Baseline concentration] 

• Calculate projected 2018 visibility values for 20% worst days for each CIA. 

The 20% best visibility days are projected in the same manner, selecting the 20% best monitored days 

from 2002 IMPROVE data. Several issues with this approach are evident when analyzing the regional 

haze monitoring data and model performance. 

Figures 7 through 11, below show the monthly variation in the fractional occurrence of worst days at 5 

randomly-selected IMPROVE monitoring sites. These figures clearly show a fair amount of inter-annual 

variation in the distribution of 20% worst monitored visibility days. As the EPA guidance requires the 

use of specific days only from 2002, these figures demonstrate that 2002 is not similar to other years in 

the 2000-04 baseline period, likely due to variation in [mostly] fire and dust emissions, and differences 

in meteorology from year to year. For this reason, the use of only 2002 for projecting visibility in 2018 

is not in keeping with the requirements of the RHR. 
19

Figures 7, 8, 9, 10, 11. Time Series of Monthly Variation in the Fraction Variation of the 20% Worst 

Monitored Days at randomly-selected WRAP region Class I areas.

Agua Tibia, CA (AGTI1) Distribution of 20% Worst Days by Year (IMPROVE data)

0

0.1

0.2

0.3

0.4

1 2 3 4 5 6 7 8 9 10 11 12

Mont h

2001

2002

2003

2004

Salt Creek, NM (SACR1) Distribution of 20% Worst Days by Year (IMPROVE data)

0

0.05

0.1

0.15

0.2

0.25

0.3

1 2 3 4 5 6 7 8 9 10 11 12

Month

Fraction

 of Worst Days

2001

2002

2003

2004

Sawtooth, ID (SAWT1) Distribution of 20% Worst Days by Year (IMPROVE data)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 2 3 4 5 6 7 8 9 10 11 12

Month

Fraction

 of Worst Days

2001

2002

2003

2004

Badlands (BADL1) Distribution of 20% Worst Days by Year (IMPROVE data)

0

0.1

0.2

0.3

0.4

0.5

1 2 3 4 5 6 7 8 9 10 11 12

Month

Fraction

 of Worst Days

2000

2001

2002

2003

2004

Mount Rainier (MORA1) Distribution of 20% Worst Days by Year (IMPROVE data)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

1 2 3 4 5 6 7 8 9 10 11 12

Month

Fraction

of Worst Days

2000

2001

2002

2004
20

Air quality modeling in support of regional haze implementation planning is conducted by the WRAP 

Regional Modeling Center (RMC), see: http://pah.cert.ucr.edu/aqm/308/. The RMC uses state of the art, 

regional gridded photochemical models for aerosol modeling called CMAQ and CAMx. Extensive and 

numerous air quality modeling studies have been performed by the RMC over the past 6 years to support 

regional haze planning in the WRAP region. 

Performance of the RMC models for 2002-specific emissions and meteorology is done by comparing air 

quality modeling results to 2002 monitored results from the IMPROVE and other monitoring networks’ 

by species; see: http://pah.cert.ucr.edu/aqm/308/reports/final/2002_MPE_report_main_body_FINAL.pdf. 

Fire and dust air quality modeling results, and their projection using RRFs was considered at a WRAP 

workshop in May 2006, see: http://www.wrapair.org/forums/ioc/meetings/060523m/. Aerosol sampling 

at IMPROVE sites is 24 hours in duration, midnight to midnight, conducted every-3rd-day, and the 

hourly modeling results are summed and matched in time. The RMC models provide hourly estimates of 

visibility and aerosol species concentrations. The modeled gaseous and aerosol species are "mapped" to 

the IMPROVE and other networks’ observational species, to calculate mass and aerosol light extinction 

in comparable terms. More information on RMC species mapping is shown in Tables 5 and 6 below. 

Table 5 – Mapping of Gaseous RMC Model Species to Gaseous Observational Species 
21

Table 6 – Mapping of Particulate RMC Model Species to Particulate Observational Species 

Model results often do not precisely match monitoring data due to temporal, spatial, chemical, and 

physical limitations in the emissions estimates, the accuracy and precision of the meteorology data 

employed in the models, and/or the chemical and physics parameterizations in the CMAQ and CAMx 

models. This is especially true for Coarse Material, and to a lesser degree, Fine Soil, see Figure 12 

below. Most dust emissions contributing to modeled fine soil and coarse material have been held 

constant from 2002 to 2018 in RMC modeling, due to the lack of acceptable methods to project factors 

causing changes in dust emissions over time. In contrast, the RMC has applied average fire emissions of 

carbonaceous aerosols for the period 2000-04, and believe that those modeling simulations, called the 

“Plan02” series, provide reasonable modeling approximations of visibility impacts in the 2000-04 

baseline monitoring period. Other emissions in the Plan02 modeling series are from 2002 and are 

thought to be representative of the 5-year baseline period. Only 2002 meteorology is used in the Plan02 

series; for more information on the Plan02 series, see Plan02 references in Table 1 at: 

http://pah.cert.ucr.edu/aqm/308/cmaq.shtml. 
22

Figure 12. Example Model Performance “Bugle Plot” for 2002 RMC Model Results versus IMPROVE 

Monitoring Data for all WRAP region Class I areas in the RMC modeling domain.
23

WRAP RRF Projection Method

 

An alternative “WRAP RRF projection method” to use 2000-04 distribution of worst days at each 

IMPROVE site is proposed for use in lieu of the EPA default RRF method. Due to model performance 

issues discussed above, the fine soil and coarse material projection values will need to be subjected to 

additional analysis before using this approach. For sulfate, nitrate, and carbonaceous aerosols, the 

basic steps for applying the WRAP RRF projection method for visibility conditions in 2018 at each CIA 

are: 

• Determine daily modeled species concentrations for 2000-04 baseline period using Plan02c 

modeling results; 

• Determine daily modeled species concentrations for 2018 using Base18b [and subsequent scenarios] 

modeling results; 

• Determine the distribution and average monthly number of 20% worst monitored visibility days in 

deciviews for each CIA monitoring site [examples shown in Figures 7-11 above, by converting the 

fraction of days by month to the nearest integer value for each month, totaling 24 values across 12 

months] – this becomes the number of modeled days to select in the next step; 

• Starting with the highest modeled 20% worst days, choose values in order by month from Plan02c 

and Base18b model results for the associated 2000-04 average monthly number of worst monitored 

days as shown above; 

• Apply the modeled ratio by to each IMPROVE light extinction species, for example: 

3⁄4 WRAP RRF January highest day modeled sulfate concentration = (2018 January modeled 

sulfate/2002 January modeled sulfate) 

• Report projected 2018 average 20% worst visibility values for each CIA; and 

• For the appropriate number of worst modeled days by month, also report the deciview, total light 

extinction, and species extinction and species mass concentration values for each CIA. 

The 20% best visibility days would also be projected for 2018 in the same manner, using modeled best 

days’ values for the Plan02c and Base18b modeling simulation results’, using the distribution and 

average monthly number of 20% best monitored visibility days. 

The concept of selecting ratios of visibility modeling results to project future visibility conditions, fitted to 

the distribution of 20% worst and best monitored days by month for the 2000-04 baseline monitoring 

period better represents the "20% Worst and Best" visibility days for the RHR visibility metric than the 

EPA default guidance approach of selecting specific days in 2002. The worst and best days are not likely 

to happen on exactly the same days in 2018, and the fractional monthly variation over the 2000-04 time 

period is substantial. The WRAP RRF Projection Method works around data completeness issues: 

• Missing data and/or incomplete data for worst and best days’ monitored distributions from 2002 – a 

number of WRAP region sites are affected; 

• The systematic bias of the 1 in 3-day sampling frequency of the monitored observations from the 1 in 

1-day [actually hourly] frequency of the modeling results; and 

• The episodic and substantial nature of dust and fire impacts on individual IMPROVE samplers and 

CIAs in a particular year. 

The WRAP RRF Projection Method provides additional benefits for regional haze analysis: 

• Determines a common monthly distribution of 20% Worst and Best visibility days to allow a 

standardized method to compare modeling and monitoring results. 
24

• Allows regional haze planning to be based on the most complete estimates of historic and projected 

air quality conditions, with longer-term (5 years) of monitoring data used to ground the modeled 

values. 

• Allows consideration of the quality of emissions and meteorological inputs to the air quality model in 

conjunction and in balance with limitations of monitoring data for regional haze planning for 

complex air quality issues such as the impact of fire and dust emissions. 

While the consistent use of the revised IMPROVE equation is recommended for regional haze planning in 

the WRAP region as the most useful and robust technical method to support reasonable progress 

analysis, the revised IMPROVE equation is not endorsed for use in evaluation of BART modeling results. 

The interpretation of BART modeling results for screening and subsequent determination of BART 

controls involves consideration of a range of various modeled values, and is a state-by-state regulatory 

decision; a number of WRAP region states have used the original equation in their BART determinations 

and are quite far along in regulatory rulemaking to determine BART controls at this time. As the BART 

analyses and determinations are not a comprehensive analysis of all sources contributing to or 

controllable for reasonable progress, and the BART regulatory findings of individual states are based on 

multiple factors, no recommendation or endorsement of a particular light extinction equation is needed. 

As the revised IMPROVE equation and alternative natural conditions estimates recommended for 

analysis of reasonable progress in the WRAP region were developed specifically based on regional haze 

2000-04 baseline period monitoring data, the equation and the associated alternative natural conditions 

estimates were designed to be consistently integrated and used in species and total visibility impact glide 

paths to natural conditions; allowing assessment of the amount of visibility impairment by species to be 

reduced from the 2000-04 baseline period through to 2064 for each Class I area. Use of the revised 

IMPROVE equation and alternative natural conditions estimates in individual BART analyses process of 

each state is not necessary and may not be accurate, as BART controls are required for implementation 

by 2012, and the natural conditions already defined in the existing EPA or the Federal Land Managers' 

Air Quality Related Values Work Group (FLAG) guidance are sufficient for screening and evaluation 

purposes. Further, precise estimates of aerosol mass and size distributions at natural conditions is 

unknown for short-term planning purposes, and will likely by different than the alternative natural 

conditions current recommended, as over the next ~60 years, anthropogenic aerosol loadings decrease, 

more and better aerosol data are collected, and implementation of regional haze control plans move 

forward. 

Recommendation:

Use the alternative WRAP RRF Projection Method for projecting 2018 visibility conditions in 

combination with the 2000-04 Best and Worst Days’ metrics as calculated and reported by VIEWS and 

TSS, utilizing the revised IMPROVE equation and alternative Natural Conditions Estimates, for 

visibility analysis and regional haze planning at each WRAP region Class I area. Study the effect of 

this method for Fine Soil and Coarse Material visibility modeling results before applying it to those 

IMPROVE light extinction species.