### 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 W RAP directs its Technical Oversight Committee (TOC) to track emissions and to de scribe the tracking process in such a way that can be included in state and tr ibal implementation pl ans. At a minimum, using the most recent state emission invento ries available, the TOC should produce a report for each five- year implementation pl an 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 W RAP identifies one clean air corridor as

shown in Figure 1 . The counties within the corridor are listed in Table 1 . For ease of administration, th e corridor’s boundary follows county lines.

3. Pursuant to 40 CFR 51.309(d)(3)(ii), the W RAP has examined patterns of growth in the corridor and finds that they are not c ausing 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 emissi ons 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 th e 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 furt her visibility impact analys is or additional emission

reduction measures until at least the next SIP revision (2008). However, the WRAP encourages its appropriate techni cal 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 specifi cally require that visibility transport commissions, including the Gr and 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, th e 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 wind s, hence defining a corridor to the northwest that must be protected to meet the broa der visibility goal of the Clean Air Act.

In its regional haze rule, the EPA provides more sp ecificity 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 w ill 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 de scribe and provide for implementation of comprehensive emission tracking strategies fo r clean-air corridors to ensure that the visibility does not degrade on the least-impair ed days at any of th e 16 Class I areas. The strategy must include:

(i) An identification of clean-air corri dors. The EPA will evaluate the State’s identification of such corridors based upon the reports of the Co mmission’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, si gnificant emissions increases that could have, or are having, visibility impa irment at one or more of the 16 Class I

areas.

(iii) In areas outside of cl ean-air corridors, an identific ation of significant emissions growth that could begin, or is beginning, to impair the quali ty of air in the corridor and thereby lead to visibility degr adation for the least-impaired da ys in one or more of the 16

Class I areas.

(iv) If impairment of air quality in cl ean air corridors is identified pursuant to §§51.309(d)(3)(ii) and (iii), an anal ysis 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 pr ovide the data necessary for other states to comply and should make a good faith effort to pr otect the integrity of clean air corridors.

III. The Commission’s Findings and Recommendations

The Commission found that clean air corridors exis t and that, generally, cl ean air comes to the

Colorado Plateau from the northwest. 2 The Commission determined that one such corridor covers southern Utah, eastern Oregon, southweste rn Idaho, and major porti ons 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. 4 identifies three factors that expl ain why air from the northwest is

clean when it arrives at the Colorado Plateau : low emissions of ai r pollutants, enhanced dispersion of the air pollutants due to highe r 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 fre quently 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 em issions 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 Plat eau. Specifically, a working group within the Meteorology Subcommittee used results from the IA S 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 de crease in visibility on the Plateau. 5 The working group found that increasing emissions by 25% within the co rridor 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 co rridor would result in a change of 2.5 dv. 6 This

2 Grand Canyon Visibility Transport Commission. Reco mmendations 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 Regi ons that Influence Clean Air on the Colora do 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 fo r the corridor but ra ther on the general understanding of a corridor to the northwest of the Plateau. The im plication, nonetheless, is that

a 25% increase in emissions within the corridor c ould be considered a leve l 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 co rridor to determine whether emissions would increase 25% by 2040. The assessment found that emissions are not exp ected to increase 25% by 2040. 7 Specifically, BBC used a weighting scheme defined in the IAS model to account for the varying effects of di fferent pollutants on visibility. Tota l weighted emissions of elemental carbon, nitrogen oxides, organic car bon, particulate matter, reactiv e 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 Commissi on recommended that no targeted policies or regulatory programs to control emi ssions 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 r ecommended that, within areas that are sources of clean air, the tracking and accounting system s hould 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. E MISSIONS T RACKING

– §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 repor t 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 emission s to a 1996 baseline for purposes of this

section.

The tracking described above is intended to ensu re that any unexpected changes are identified. This tracking would coincide with the peri odic SIP revisions re quired in 2008, 2013, and 2018. States and tribes already prep are 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 inventorie s annually for sources of sulfur dioxide of 100 tons per year or greater for compliance with the stationary sour ce 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 regi onal haze SIPs every five years. Further information on tracking point sources a nd area sources is provided below.

P OINT S OURCES

. 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 ampl e 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 le ss likely that a trend in that direction would go unnoticed.

A REA A ND M OBILE S OURCES

. Population and economic growth is expected to be slow in the corridor, holding down emissions from area and m obile sources within th e corridor. Federal standards recently promulgated for on-road so urces and additional ones pending for non-road sources are expected to reduce emissions from bo th of these source cate gories during the first planning period of the implementation plans (20 18). However, emissions from prescribed burning are expected to increas e 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 se vere. 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. Noneth eless, careful fire emissions tr acking is warranted and is being developed under separate WRAP policy and technical efforts.

B. B OUNDARY O F T HE C LEAN A IR C ORRIDOR – §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 ba lancing of demographic, economic, and air quality impact analyses performed on this corr idor 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 Washi ngton and southwestern Montana fr om the corridor. These small areas are far from the Colorado Plateau and unlikely to affect the Class I ar eas on the Plateau. In contrast, counties have been added to the corrid or 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 Ore gon, 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. I DENTIFICATION O F E MISSIONS I NCREASES – §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 impac ts at one or more of the 16 Class I areas. Nor, at this time, are such emission incr eases expected during the fi rst planning period (2003-2018). Analyses performed by the Grand Canyon Vi sibility Transport Commission found that an increase of 25% in weighted emissions woul d result in a 0.7 dv reduction in visibility, whereas the weighted emission incr ease expected by 2018 is only 4%.

Patterns of growth in th e corridor are first examined by comp aring 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 we re only slightly higher than in 1990. In the clean air corridor 73,637 tons of SO 2 were emitted in 1990 and 73,756 were emitted in 1996; 232,704 tons of NO

x 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 de crease in visibility on the clean est 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 increase s. 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 addi tion, the most recent and compre hensive projection of emissions (discussed below) is based on the 19 96 inventory, not th e 1990 inventory.

The WRAP also examined emission projections. Th ese 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. PM 10 and PM 2.5 emissions are expected to increase about 7% and 18%, respectively. NOx and VOC emissions, however, are expected to decrease about 15% and 26%, respectively. SO 2 emissions are expected to increa se about 5% within the corridor, even with the declining milestones of the back stop emissions trading program. Overall, SO 2 emissions are expected to decline by 17% in the 13-state contiguous WRAP region by 2018, 12
and the fact that the projectio ns show a 5% increase in SO 2 within the clean air corridor is a result of non-road mobile sources using high-sulfur diesel fuel. This sour ce 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 cer tification and fuel standards for non-road vehicles and equipment. Thus, 5% should be viewed as an upper bound on the possible increase of SO 2

.

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 impact s on visibility, the WRAP estimated a weighted emissions increase according to the VEEU syst em 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 qual ity of the air in the corridor and thereby lead to visibility degradation for the least im paired days in one or more of the 16 Class I

areas.

The WRAP sees two purposes for emissions tracking in areas outside the corri dor: first, to determine if such emissions are degrading visibil ity in the corridor, which may potentially affect one or more of the 16 Class I areas; and sec ond, to compensate for any uncertainties in establishing the boundary of the corr idor, such as those relating to computed airmass trajectories or introduced by aligning the corridor with county boundaries. Again, SO 2 emissions are expected to decline throughout the WRAP region. Emissions of other pollutants are also expected to decline. All visi bility-impairing pollutants from on -road mobile sources, with the exception of some minor ammonia emissions, are expected to decline su bstantially. 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 ot her Class I areas by 2008. Such measures are likely to “overlap” the clean air corridor and area s outside the corridor in such a way that provide further protection to the 16 Class I areas on the 20% cleanest days.

D. I F I MPAIRMENT O F A IR Q UALITY I N T HE C ORRIDOR I S I DENTIFIED – §309(d)(3)(iv)

Since impairment of air quality in clean air co rridors has not been identified pursuant to 40 CFR 51.309(d)(3)(ii) and (iii), the W RAP 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 th e Causes of Haze report – to take into account the assessment and protec tion 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, incl uding provisions for the identification of the need for additional emission reduction measures, and implementation of theWRAP 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 em issions increases. Should any effects be identified, the WRAP will conduct an an alysis to determine the sources of impairment within six months of completion of the invent ory indicating the increas e. Additi onal control measures that may be warranted would be develope d within another six mont hs. The criteria the states and tribes would follow in making this dete rmination 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) base d on feasibility, cost, and anticipated visibility benefits. Any necessary additional control measur es would be added in the next five-year SIP

revision.

E. D O O THER C ORRIDORS E XIST ?

– §309(d)(3)(v)

The WRAP finds no other clean air corrido rs beyond the corridor identified in Figure 1

.

The regional haze rule requires that implementa tion 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 sli ghtly different set of 20% cleanest days and a slightly different set of back tr ajectories on those days, especially at sites furthest to the north and east. This may result in a broader or separa te corridor. Such anal ysis 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 WR AP as part of its Causes of Haze report.

V. Conclusion

The bottom line is that, while the area to the north west 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 qua lity 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 cl ean air corridors play in protec ting 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 improv es 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 GilliamWRAP Policy on Clean Air Corridors Submitted for Board Approval, 11/13/02 11

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

Point Area On Road Non Road Paved Unpaved Total

SO 2 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

PM 10 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

PM 2.5 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 an d 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 t he 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 cr eate the VEEU-weighted inventory.