WRAP Policy on Clean Air Corridors

Approved by WRAP Board

November 13, 2002

I. Summary of WRAP Policy

1. Pursuant to 40 CFR 51.309(d)(3), the WRAP directs its Technical Oversight 

Committee (TOC) to track emissions and to describe the tracking process in such a 

way that can be included in state and tribal implementation plans. At a minimum, 

using the most recent state emission inventories available, the TOC should produce a 

report for each five-year implementation plan revision on the current and projected

emissions in the clean air corridor and in areas outside the corridor and compare 

these emissions to a 1996 baseline for purposes of this section.

2. Pursuant to 40 CFR 51.309(d)(3)(i), the WRAP identifies one clean air corridor as 

shown in Figure 1. The counties within the corridor are listed in Table 1. For ease 

of administration, the corridor’s boundary follows county lines. 

3. Pursuant to 40 CFR 51.309(d)(3)(ii), the WRAP has examined patterns of growth in 

the corridor and finds that they are not causing significant emission increases that 

could have or are having visibility impacts at one or more of the 16 Class I areas. 

Nor, at this time, are such emission increases expected during the first planning 

period (2003-2018). Analyses performed by the Grand Canyon Visibility Transport 

Commission found that an increase of 25% in weighted emissions would result in a 

0.7 dv reduction in visibility, whereas the weighted emission increase expected by 

2018 is only 4%.

4. Pursuant to 40 CFR 51.309(d)(3)(iii), the WRAP has examined emissions growth in 

areas outside the corridor and finds that significant emissions growth is not 

occurring that could begin or is beginning to impair the quality of the air in the 

corridor and thereby lead to visibility degradation for the least impaired days in one 

or more of the 16 Class I areas.

5. Since impairment of air quality in clean air corridors has not been identified 

pursuant to 40 CFR 51.309(d)(3)(ii) and (iii), the WRAP finds no requirement under 

40 CFR 51.309(d)(3)(iv) for further visibility impact analysis or additional emission 

reduction measures until at least the next SIP revision (2008). However, the WRAP 

encourages its appropriate technical activities – such as the Causes of Haze report –

to take into account the assessment and protection of clean air corridors.

6. Pursuant to 40 CFR 51.309(d)(3)(v), the WRAP finds no other clean air corridors 

beyond the corridor identified in Figure 1.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 2

II. Clean Air Corridors, The Clean Air Act, And The Regional Haze Rule

The Clean Air Act Amendments of 1990 specifically require that visibility transport 

commissions, including the Grand Canyon Visibility Transport Commission (“Commission”), 

address “the establishment of clean air corridors, in which additional restrictions on increases in 

emissions may be appropriate to protect visibility in affected class I areas.”1 The Clean Air Act 

also requires protection of clean air corridors in a less direct way. The Act establishes as a 

national goal the prevention of any future impairment of visibility in mandatory Class I areas. 

As a measure of progress towards this goal, the U.S. Environmental Protection Agency (EPA) 

has established a criteria of no degradation on the 20% cleanest days. Such days on the Colorado 

Plateau are usually dominated by northwest winds, hence defining a corridor to the northwest 

that must be protected to meet the broader visibility goal of the Clean Air Act.

In its regional haze rule, the EPA provides more specificity on the requirements to protect clean 

air corridors, based largely on the recommendations of the Commission. The preamble of the 

rule defines a clean air corridor as “a region that generally brings clean air to a receptor region” 

The preamble also says, “the requirement to track emissions will enable states to quickly

determine if changes in patterns of emissions will reduce the number of clean air days (defined 

as the average of the 20% clearest days) in any of the 16 Class I areas.” The actual requirements 

of the rule are found in 40 CFR 51.309(d)(3):

The [state implementation] plan must describe and provide for implementation of 

comprehensive emission tracking strategies for clean-air corridors to ensure that the 

visibility does not degrade on the least-impaired days at any of the 16 Class I areas. The 

strategy must include:

(i) An identification of clean-air corridors. The EPA will evaluate the State’s 

identification of such corridors based upon the reports of the Commission’s Meteorology

Subcommittee and any future updates by a successor organization.

(ii) Within areas that are clean-air corridors, an identification of patterns of growth or 

specific sites of growth that could cause, or are causing, significant emissions increases 

that could have, or are having, visibility impairment at one or more of the 16 Class I

areas.

(iii) In areas outside of clean-air corridors, an identification of significant emissions 

growth that could begin, or is beginning, to impair the quality of air in the corridor and 

thereby lead to visibility degradation for the least-impaired days in one or more of the 16 

Class I areas.

(iv) If impairment of air quality in clean air corridors is identified pursuant to 

§§51.309(d)(3)(ii) and (iii), an analysis of the effects of increased emissions, including 

provisions for the identification of the need for additional emission reductions measures, 

and implementation of the additional measures where necessary.

 

1

42 U.S.C. 2169B(d)(2)(A).
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 3

(v) A determination of whether other clean air corridors exist for any of the 16 Class I

areas. For any such clean air corridors, an identification of the necessary measures to 

protect against future degradation of air quality in any of the 16 Class I areas.

These requirements do not apply to states submitting state implementation plans (SIPs) under 

§308 of the rule. However, such states should provide the data necessary for other states to 

comply and should make a good faith effort to protect the integrity of clean air corridors.

III. The Commission’s Findings and Recommendations

The Commission found that clean air corridors exist and that, generally, clean air comes to the 

Colorado Plateau from the northwest.2 The Commission determined that one such corridor 

covers southern Utah, eastern Oregon, southwestern Idaho, and major portions of Nevada. This 

corridor was identified by the Commission’s Meteorology Subcommittee, which examined the 

size and boundaries of the corridor under varying assumptions about the number of days defined 

as clean and the amount of protection to be afforded.

3

Related work by Green et. al. 4identifies three factors that explain why air from the northwest is 

clean when it arrives at the Colorado Plateau: low emissions of air pollutants, enhanced 

dispersion of the air pollutants due to higher average ventilation (wind speed multiplied by

mixing depth), and increased removal of pollutants due to precipitation. Although the corridor is 

mostly arid, the cleanest days occur most frequently in the winter, when there is more 

precipitation than average. Green et al., nonetheless, conclude that the most important factor at 

the south rim of the Grand Canyon for most weather conditions is the low emissions of pollutants 

in the area to the northwest.

In addition to identifying a clean air corridor, the Commission projected emissions growth within 

the corridor through 2040 and found that growth is not expected to have a perceptible negative 

impact on the cleanest days on the Colorado Plateau. Specifically, a working group within the 

Meteorology Subcommittee used results from the IAS model (the model used to project visibility

impacts in other Commission work) to estimate the emissions increase from 1990 that would be 

necessary to cause a perceptible decrease in visibility on the Plateau.5 The working group found 

that increasing emissions by 25% within the corridor would result in an average change of 

0.7 deciviews (dv), which would be imperceptible to most people under most conditions, while a 

100% increase in emissions within the corridor would result in a change of 2.5 dv.6 This 

 

2 Grand Canyon Visibility Transport Commission. Recommendations for Improving Western Vistas. Western 

Governors’ Association. Denver, CO. June 1996.

3 Meteorological Subcommittee, Grand Canyon Visibility Transport Commission. Clean Air Corridors: A 

Framework for Identifying Regions that Influence Clean Air on the Colorado Plateau. Denver, CO. August 1995.

4 Green, M. C.; Pitchford, M. L.; and Ashbaugh, L.L. Identification of Candidate Clean Air Corridors for the 

Colorado Plateau. J. Air & Waste Manage. Assoc. 1996. 46(5), 446.

5 Marc Pitchford. Oral communication. October 3, 2002. Participants on the working group included Dr. Pitchford, 

Dr. William Malm, and Dr. Ivar Tombach.

6 BBC Research & Consulting, Inc., for the Operations Committee of the Grand Canyon Visibility Transport 

Commission. Clean Air Corridor: An Economic Perspective. Denver, CO. November 1995. Page III-2:6.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 4

estimate was not based on a specific boundary for the corridor but rather on the general 

understanding of a corridor to the northwest of the Plateau. The implication, nonetheless, is that 

a 25% increase in emissions within the corridor could be considered a level of growth that would 

not impact visibility. 

Using one of the proposed corridor alignments examined by the Meteorology Subcommittee – a 

corridor that would protect the 30% cleanest days on the Colorado Plateau, adjusted to account 

for emissions density and IAS region boundaries – BBC Research & Consulting conducted an 

economic and demographic assessment of the corridor to determine whether emissions would 

increase 25% by 2040. The assessment found that emissions are not expected to increase 25% 

by 2040.7 Specifically, BBC used a weighting scheme defined in the IAS model to account for 

the varying effects of different pollutants on visibility. Total weighted emissions of elemental 

carbon, nitrogen oxides, organic carbon, particulate matter, reactive organic gases, and sulfur 

oxides in 1990 were 52,073 VEEU tons.8 A 25% increase would yield 65,092 VEEU tons. BBC 

projected that emissions in the corridor would increase to 55,047 VEEU tons by 2040, thus 

leaving an ample margin of safety of 10,054 VEEU tons.9

As a result of these analyses, the Commission recommended that no targeted policies or 

regulatory programs to control emissions growth were needed at that time, but that a regional 

tracking and accounting system be implemented to make sure that the frequency of clear days 

does not decrease at the 16 Class I areas and that the Commission’s assumptions about increased 

emissions are proven reliable. The Commission recommended that, within areas that are sources 

of clean air, the tracking and accounting system should identify patterns of growth that have a 

negative impact on visibility and that, in areas outside the clean air corridors, the tracking and 

accounting system should identify significant emissions growth that begins to impair the quality

of air in the corridor.

IV. WRAP Policy

A. EMISSIONS TRACKING – §309(d)(3)

The WRAP directs its Technical Oversight Committee (TOC) to track emissions and to 

describe the tracking process in such a way that can be included in state and tribal 

implementation plans. At a minimum, using the most recent state emission inventories 

available, the TOC should produce a report for each five-year implementation plan 

revision on the current and projected emissions in the clean air corridor and in areas 

outside the corridor and compare these emissions to a 1996 baseline for purposes of this 

section.

The tracking described above is intended to ensure that any unexpected changes are identified. 

This tracking would coincide with the periodic SIP revisions required in 2008, 2013, and 2018. 

States and tribes already prepare inventories at least every three years to meet federal 

 

7 BBC report, page III-5

8 Visibility Equivalency Emission Units

9 BBC report, page III-6.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 5

requirements and will prepare detailed inventories annually for sources of sulfur dioxide of 100 

tons per year or greater for compliance with the stationary source provisions of §309.10 The 

WRAP will use these state and tribal data for tracking emissions in general and can summarize 

emissions for the counties and tribal lands within the corridor and for areas outside the corridor 

for use by states and tribes as they revise their regional haze SIPs every five years. Further 

information on tracking point sources and area sources is provided below.

POINT SOURCES. Any new, large source will be required to undergo a Prevention of Significant 

Deterioration review and an Air Quality Related Values analysis before receiving an air quality

permit and will also be subject to New Source Performance Standards and other requirements, 

giving the public, states, tribes, and federal land managers ample opportunity to evaluate any

possible visibility impacts on the 16 Class I areas. Thus, it is unlikely that point sources will lead 

to a 25% increase and even less likely that a trend in that direction would go unnoticed.

AREA AND MOBILE SOURCES. Population and economic growth is expected to be slow in the 

corridor, holding down emissions from area and mobile sources within the corridor. Federal 

standards recently promulgated for on-road sources and additional ones pending for non-road 

sources are expected to reduce emissions from both of these source categories during the first 

planning period of the implementation plans (2018). However, emissions from prescribed 

burning are expected to increase and, depending on the location of the burns, could affect 

visibility in the 16 Class I areas. It is hard to predict how great the effect will be on clean days, 

but it is not expected to be severe. For one, prescribed fires generally occur in the spring and 

fall, whereas most clear days occur in the winter. In addition, prescribed fires are much less 

intense than wild fires. Nonetheless, careful fire emissions tracking is warranted and is being 

developed under separate WRAP policy and technical efforts.

B. BOUNDARY OF THE CLEAN AIR CORRIDOR – §309(d)(3)(i)

The WRAP identifies one clean air corridor as shown in Figure 1. The counties within 

the corridor are listed in Table 1. For ease of administration, the corridor’s boundary 

follows county lines. 

The WRAP adopts this boundary based on a balancing of demographic, economic, and air 

quality impact analyses performed on this corridor and their subsequent review and consensusbased approval by the Commission. The boundary identified is a slight modification of the 

boundary defined in the BBC report described above. The grid cells in the air quality analyses 

did not follow state or county boundaries, and for ease of administration the WRAP has removed 

small areas of southern Washington and southwestern Montana from the corridor. These small 

areas are far from the Colorado Plateau and unlikely to affect the Class I areas on the Plateau. In 

contrast, counties have been added to the corridor that were not originally included in the 

boundary defined in the BBC report. These include Box Elder, Tooele, and Grand Counties in 

Utah, Wasco and Sherman Counties in Oregon, and Cassia and Lemhi Counties in Idaho.

 

10 Also see Western Regional Air Partnership. Voluntary Emissions Reduction Program for Major Industrial 

Sources of Sulfur Dioxide in Nine Western States and a Backstop Market Trading Program, An Annex to the 

Report of the Grand Canyon Visibility Transport Commission. Denver, CO. September 29, 2000.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 6

C. IDENTIFICATION OF EMISSIONS INCREASES – §309(d)(3)(ii) and (iii)

Pursuant to 40 CFR 51.309(d)(3)(ii), the WRAP has examined patterns of growth in the 

corridor and finds that they are not causing significant emission increases that could 

have or are having visibility impacts at one or more of the 16 Class I areas. Nor, at this 

time, are such emission increases expected during the first planning period (2003-2018). 

Analyses performed by the Grand Canyon Visibility Transport Commission found that an 

increase of 25% in weighted emissions would result in a 0.7 dv reduction in visibility, 

whereas the weighted emission increase expected by 2018 is only 4%.

Patterns of growth in the corridor are first examined by comparing 1990 emissions (those used in 

the Commission’s final report) to 1996 emissions (the most recent comprehensive data set). This 

comparison is not easily made because emissions were aggregated into different categories. 

Nonetheless, it appears that emissions in 1996 were only slightly higher than in 1990. In the 

clean air corridor 73,637 tons of SO2 were emitted in 1990 and 73,756 were emitted in 1996; 

232,704 tons of NOx were emitted in 1990 and 256,762 were emitted in 1996. In addition, the 

WRAP examined data from IMPROVE monitors and found that none of the seven long-term 

sites showed any significant decrease in visibility on the cleanest days for the period from 1988 

through 1998.11

The WRAP is recommending, as part of this policy, that future clean air corridor analyses use a 

baseline year of 1996 to quantify emission increases. The first reason for this recommendation is 

that the 1996 inventory has been more carefully assembled than the 1990 inventory. The second 

reason is that future inventories are more likely to be structured like the 1996 inventory, thereby 

facilitating comparison. In addition, the most recent and comprehensive projection of emissions 

(discussed below) is based on the 1996 inventory, not the 1990 inventory. 

The WRAP also examined emission projections. These are used as a means to identify potential 

future increases that should be more carefully tracked and to identify preventive measures that 

could be implemented in a timely fashion. Table 2 summarizes the projected change in 

emissions between 1996 and 2018. PM10 and PM2.5 emissions are expected to increase about 7% 

and 18%, respectively. NOx and VOC emissions, however, are expected to decrease about 15% 

and 26%, respectively. SO2 emissions are expected to increase about 5% within the corridor, 

even with the declining milestones of the backstop emissions trading program. Overall, SO2

emissions are expected to decline by 17% in the 13-state contiguous WRAP region by 2018, 12

and the fact that the projections show a 5% increase in SO2 within the clean air corridor is a 

result of non-road mobile sources using high-sulfur diesel fuel. This source of sulfur dioxide is 

expected to be drastically reduced (e.g., from a fuel sulfur content of 3,000 ppm to 15 ppm) 

before 2018 according to announcements by EPA to develop new engine certification and fuel 

standards for non-road vehicles and equipment. Thus, 5% should be viewed as an upper bound 

on the possible increase of SO2.

 

11 EPA. Visibility in Mandatory Federal Class I Areas (1994-1998), A Report to Congress. EPA-452/R-01-008.

12 WRAP Emissions Inventory Forum. 2018-1996 Difference: Actual to Control Spreadsheet. WRAP Web Site. 

September 25, 2002.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 7

Since different pollutants have different impacts on visibility, the WRAP estimated a weighted 

emissions increase according to the VEEU system used by the Commission. As shown in 

Table 3, the weighted increase is expected to be 4%, substantially less than the 25% increase 

thought to be necessary to achieve an impact that may be perceptible. It is also worth noting the 

safety margins included within this analysis – the fact that the BBC corridor protects 30% of the 

clean days, not 20%; the benefits of new non-road mobile source standards; and the uncertainty

in where additional electricity generating capacity will be located.

Pursuant to 40 CFR 51.309(d)(3)(iii), the WRAP has examined emissions growth in areas 

outside the corridor and finds that significant emissions growth is not occurring that 

could begin or is beginning to impair the quality of the air in the corridor and thereby 

lead to visibility degradation for the least impaired days in one or more of the 16 Class I 

areas.

The WRAP sees two purposes for emissions tracking in areas outside the corridor: first, to 

determine if such emissions are degrading visibility in the corridor, which may potentially affect 

one or more of the 16 Class I areas; and second, to compensate for any uncertainties in 

establishing the boundary of the corridor, such as those relating to computed airmass trajectories 

or introduced by aligning the corridor with county boundaries. Again, SO2 emissions are 

expected to decline throughout the WRAP region. Emissions of other pollutants are also 

expected to decline. All visibility-impairing pollutants from on-road mobile sources, with the 

exception of some minor ammonia emissions, are expected to decline substantially. And all 

visibility impairing pollutants from non-road mobile sources are expected to decline, especially 

in areas upwind of the corridor. This decline would be greatly enhanced if the EPA promulgates 

stricter standards for non-road engines and fuel, as it has announced to do. Also, NOx and PM 

from existing stationary sources remains to be addressed in future implementation plans by 2008 

under Sections 308 and 309 of the regional haze rule. Finally, all states will have to implement 

measures to achieve reasonable progress in other Class I areas by 2008. Such measures are 

likely to “overlap” the clean air corridor and areas outside the corridor in such a way that provide 

further protection to the 16 Class I areas on the 20% cleanest days. 

D. IF IMPAIRMENT OF AIR QUALITY IN THE CORRIDOR IS IDENTIFIED – §309(d)(3)(iv)

Since impairment of air quality in clean air corridors has not been identified pursuant to 

40 CFR 51.309(d)(3)(ii) and (iii), the WRAP finds no requirement under 40 CFR 

51.309(d)(3)(iv) for further visibility impact analysis or additional emission reduction 

measures until at least the next SIP revision (2008). However, the WRAP encourages its 

appropriate technical activities – such as the Causes of Haze report – to take into 

account the assessment and protection of clean air corridors.

The rule specifies that if impairment of air quality in the clean air corridor is identified, the plan 

must include "an analysis of the effects of increased emissions, including provisions for the 

identification of the need for additional emission reduction measures, and implementation of the
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 8

additional measures if necessary." For reasons stated above, the WRAP finds no need at this 

time for additional emission reduction measures.

The periodic WRAP inventories to be produced by the TOC, as instructed above, will identify 

growth in emissions, and the periodic updates to the WRAP Causes of Haze report will help 

identify any effect on visibility that may result from such emissions increases. Should any

effects be identified, the WRAP will conduct an analysis to determine the sources of impairment 

within six months of completion of the inventory indicating the increase. Additional control 

measures that may be warranted would be developed within another six months. The criteria the 

states and tribes would follow in making this determination are (a) the location of the significant 

emissions growth, (b) type of source activity causing the emissions growth, and (c) the 

appropriate control measure for the source(s) based on feasibility, cost, and anticipated visibility

benefits. Any necessary additional control measures would be added in the next five-year SIP 

revision. 

E. DO OTHER CORRIDORS EXIST? – §309(d)(3)(v)

The WRAP finds no other clean air corridors beyond the corridor identified in Figure 1.

The regional haze rule requires that implementation plans identify whether any other clean air 

corridors exist for any of the 16 Class I areas. The WRAP finds no such areas other than the 

corridor to the northwest of the Colorado Plateau identified in Figure 1. The WRAP recognizes, 

however, that additional work to identify clean air corridors may be needed. For example, 

several monitors have recently been installed at Class I areas on the Plateau which were not 

previously monitored. These may generate a slightly different set of 20% cleanest days and a 

slightly different set of back trajectories on those days, especially at sites furthest to the north 

and east. This may result in a broader or separate corridor. Such analysis should be performed 

when sufficient data are available. Adequate monitoring data could be available by 2004, and 

analysis of those data could be published by the WRAP as part of its Causes of Haze report. 

V. Conclusion

The bottom line is that, while the area to the northwest of the Colorado Plateau delivers clean air 

to the Plateau on the cleanest days, emissions from throughout much of the region affect the 

Class I areas on the Plateau. Thus, emissions throughout the WRAP region will be tracked 

carefully. Ongoing WRAP efforts to improve the quality of inventories and the models used to 

make projections, and to produce a periodic Causes of Haze report, will bring increased 

understanding of the role that clean air corridors play in protecting the cleanest days. In the 

final analysis, the indicator of success or failure will be whether the measured light extinction at 

the Class I areas on the Colorado Plateau improves or declines on the cleanest days. Any 

indication of deterioration on the cleanest days should trigger an immediate investigation of the 

cause, as well as efforts to correct the problem.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 9

Figure 1. Clean Air Corridor Endorsed by the WRAP.
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 10

Table 1. Counties Within the Clean Air Corridor Endorsed by the WRAP.

State County State County

Idaho Ada Oregon Grant

Idaho Adams Oregon Harney

Idaho Blaine Oregon Jefferson

Idaho Boise Oregon Lake

Idaho Butte Oregon Malheur

Idaho Camas Oregon Morrow

Idaho Canyon Oregon Sherman

Idaho Cassia Oregon Umatilla

Idaho Custer Oregon Union

Idaho Elmore Oregon Wallowa

Idaho Gem Oregon Wasco

Idaho Gooding Oregon Wheeler

Idaho Idaho Utah Beaver

Idaho Jerome Utah Box Elder

Idaho Lemhi Utah Carbon

Idaho Lincoln Utah Emery

Idaho Minidoka Utah Garfield

Idaho Owyhee Utah Grand

Idaho Payette Utah Iron

Idaho Twin Falls Utah Juab

Idaho Valley Utah Kane

Idaho Washington Utah Millard

Nevada Churchill Utah Piute

Nevada Douglas Utah San Juan

Nevada Elko Utah Sanpete

Nevada Esmeralda Utah Sevier

Nevada Eureka Utah Tooele

Nevada Humboldt Utah Washington

Nevada Lander Utah Wayne

Nevada Lincoln

Nevada Lyon

Nevada Mineral

Nevada Nye

Nevada Pershing

Nevada Storey

Nevada Washoe

Nevada White Pine

Nevada Carson City

Oregon Baker

Oregon Crook

Oregon Deschutes

Oregon Gilliam
WRAP Policy on Clean Air Corridors

Submitted for Board Approval, 11/13/02 11

Table 2. Changes in Clean Air Corridor Emissions (Assuming SO2 Milestones Are Met).

Point Area On Road Non Road Paved Unpaved Total

SO2 1996 51,413 9,260 2,065 10,838 0 0 73,576

2018 45,330 10,614 413 21,596 0 0 77,954

2018-1996 -6,082 1,354 -1,652 10,758 0 0 4,378

NOx 1996 85,782 12,935 93,581 64,462 0 0 256,762

2018 109,863 17,576 28,692 62,557 0 0 218,689

2018-1996 24,080 4,641 -64,889 -1,905 0 0 -38,072

PM10 1996 27,055 142,776 3,872 5,952 5,740 47,733 233,128

2018 32,748 154,966 2,640 6,763 12,402 38,828 248,347

2018-1996 5,692 12,190 -1,232 811 6,662 -8,904 15,219

PM2.5 1996 11,987 41,595 3,495 5,487 1,435 7,160 71,160

2018 14,583 52,069 2,058 6,228 3,101 5,824 83,863

2018-1996 2,595 10,474 -1,438 740 1,665 -1,336 12,702

VOC 1996 5,993 95,921 69,899 38,535 0 0 210,349

2018 7,921 95,515 22,651 29,233 0 0 155,321

2018-1996 1,927 -406 -47,248 -9,301 0 0 -55,029

Table 3. Total Change in Emissions Weighted to Reflect Relative Impact on Visibility.

SO2 NOx PM10 PM2.5 VOC EC* OC* Total Change

1996 VEEU 5,445 1,746 1,958 932 294 902 856 12,133 --

2018 VEEU 5,769 1,487 2,086 1,099 217 985 935 12,578 4%

* Estimates of elemental and organic carbon, EC and OC, were not available to the CAC Work Group 

for the 1996 and 2018 emission inventories. Values for this analysis were derived from the 

estimates of EC and OC for the 1990 inventory of the 9 GCVTC states. The method used was to take 

the proportion of EC to fine and coarse particulates (PM2.5 + PM10) in the 1990 inventory and use that 

same proportion to calculate an EC value for the 1996, 2018, and 2018 milestone inventories. The

same method was used for OC.

** VEEU – Visibility Equivalency Emission Units (Used in the GCVTC IAS Model.)

VEEU weights

PM2.5 PM10 NOx VOC SO2 EC OC

0.0131 0.0084 0.0068 0.0014 0.0740 0.6497 0.2466

Each category in the inventory is multiplied by these factors to create the VEEU-weighted inventory.