### UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION IX AIR DIVISION



## Technical Support Document for EPA Evaluation of Interpollutant Trading Ratios in the 2015 San Joaquin PM 2.5 

## Plan

Prepared by Scott Bohning, Region 9 Air Division

January 20162



### Table of Contents

I. Introduction ......................................

................................................... ..................................... 3

A. IPT Terminology Used ..............................

................................................... ..................... 3

II. Considerations in Estimating Trading Ratios .......

................................................... ............. 5

A. EPA Guidance ......................................

................................................... .......................... 5

B. General IPT considerations ........................

................................................... .................... 7

III. IPT Ratio Estimation Methods in the SJV 2015 PM 2.5 Plan ............................................. .... 9

IV. Use of IPT ratios in the SJV 2015 PM 2.5 Plan ............................................. ....................... 14

A. IPT for Reasonable Further Progress ...............

................................................... ............ 14

1. RFP considerations ................................

................................................... ................... 14

2. RFP in the SJV Plan ...............................

................................................... .................. 17

B. IPT for Motor Vehicle Emission Budgets for Transpor tation Conformity ..................... 20

C. IPT for Aggregate Commitments .....................

................................................... ............ 24

V. Conclusions for Interpollutant Trading in the SJV 2 015 PM 2.5 Plan .................................. 273 

### I. Introduction

In several contexts, the 2015 PM 2.5 Plan 1 for the San Joaquin Valley (SJV) meets the emissio n reduction requirement for one pollutant by substitu ting reductions of another, a form of interpollutan t trading (IPT). (Here “pollutant” means directly emi tted PM 2.5 or a PM 2.5 precursor.) The contexts include the statutory requirements for Reasonable F urther Progress (RFP), Motor Vehicle Emissions Budgets (MVEBs) for transportation conformity, and compliance with the emission reduction commitments in the 2008 PM 2.5 Plan. The substitution or trading depends on such trading being allowed in a given context, and on determining a trading ra tio, that is, how many tons per day (tpd) emissions of one pollutant is equivalent in some sense to 1 tpd of the other in that context. It is assumed that so me form of air quality modeling is necessary to justif y a trading ratio, since modeling is the only way t o assess the different chemical roles and geographic and temporal distribution of the pollutants in assessing the amounts that are equivalent. Determin ation of the ratio requires choices on what emissio ns scenarios should be modeled, which ambient output f rom the model is used, and where geographically the equivalence should be assessed in the area. Thi s Technical Support Document (TSD) evaluates the use of IPT in the 2015 PM 2.5 Plan and accompanies EPA’s Notice of Proposed Rule making (NPR) on that plan.

A. IPT Terminology Used As an example to illustrate the IPT terminology use d here, the 2015 PM 2.5 Plan provides for a trading mechanism for the MVEBs, with a 9:1 NO x for direct PM 2.5 trading ratio. That is, 9 tpd of NO x reductions may be used for meeting a 1 tpd PM 2.5 (DPM) reduction requirement (“NO x for DPM”

1 The 2015 PM2.5 Plan for the SJV comprises two Cali fornia State Implementation Plan (SIP) submittals. The first submittal is the “2015 Plan for the PM 2.5 Standard,” which the State submitted to EPA on Jun e 25, 2015. The second submittal is the “2018 Transportation Conformity Bu dgets for the San Joaquin Valley PM 2.5 SIP Plan Supplement,” which the State submitted to EPA on August 13, 2015.4

trading). (Stated in terms of budgets, the mechanis m allows for expanding the DPM budget by 1 tpd via “transferring” 9 tpd of NO x emissions from the NO x budget.) A trade in the other direction would be DPM for NO x , with a 1:9 ratio by supposition. For all of the 2 015 PM 2.5 Plan IPT contexts, the general approach was the same: model an emission reduction of each pollutant to estimate its ambient sensitivi ty in μg/m 3 per tpd, and use ratios between these sensitivitie s as trading ratios. Details for the particular contexts are provided below, following discussion o f regulatory requirements and some methodology considerations. The terms “trading ratio” and “effectiveness ratio” are both used in the trading context, and care must be taken to specify which is meant and also th e direction of the trade. (The ratio is not a priori independent of the direction, so “equivalence ratio ” is generally not used here.) A trading ratio is the relative amount of two pollutants’ emissions needed for the same ambient effect; an effectiveness ratio is the relative ambient effect for the same emissions of each pollutant; these are reciprocals. A trading ratio can be derived from sensitivity to the pollutants’ emission changes (assuming sensitivity is estimated with an appropriate method). Let S A be the sensitivity of ambient PM 2.5 to emissions reductions of pollutant A in μg/m 3 per tpd (a decrease of S A μg/m 3 results from 1 tpd reduction of A); and similarly for S B and pollutant B. The ambient effect of emissions i s the product of the sensitivity in μg/m 3 per tpd and emissions in tpd Let R:1 be the trading ratio o f A for B: R tpd of pollutant A reductions are need ed for an ambient effect equivalent to 1 tpd of pollut ant B reductions (B is R times as effective as A). Then the two pollutants have an equal effect in μg/m 3 when

ambient effect of A = ambient effect of B

or: 

### S A

× 

### R = 

### S B × 

### 1 (ambient effect = sensitivity in μg/m 3 per tpd × emissions in tpd)5

and so, solving for R, the A for B trading ratio is R = S B /S A . Note that this is the same as the B-A effectiveness ratio , the effect of 1 tpd of B per effect of 1 tpd of A , that is, B is R times as effective as A, or A is only 1/R as effective as B. Altogether the following are equivalent: A for B trading ratio of R:1 = S B /S A B for A trading ratio of 1/R = S A /S B B-A effectiveness ratio of R = S B /S A A-B effectiveness ratio of S A /S B = 1/R For the DPM:NO x example above, there is a 1:9 DPM for NO x trading ratio; the DPM-NO x effectiveness ratio is 9 and the NO x -DPM effectiveness ratio is 1/9; reducing DPM is 9 times as effective as reducing NO x , on a tpd basis.


### II. Considerations in Estimating Trading Ratios

A. EPA Guidance EPA has allowed IPT in several regulatory contexts, including Reasonable Further Progress demonstrations under CAA section 172(c)(2), Motor V ehicle Emissions Budgets for transportation conformity purposes under CAA section 176(c), and C ontingency Measures under CAA section 172(c)(9). 2 Each of these contexts involves an accounting of emissions totals or targets, independent of the geographic and temporal distribution of those e missions, and independent of atmospheric chemistry. The required totals are derived from the modeled at tainment demonstration, which indeed does consider distribution and chemistry, but in assessing whethe r the required emissions targets are met, by defaul t distribution and chemistry are not relevant or cons idered. However, those very considerations are intrinsic to IPT: how much of one pollutant is “equ ivalent” to another depends on their ambient effect ,

2 See, e.g., 76 FR 69896 (November 9, 2011) and 79 FR 29327 (May 22, 2014).6

and the ambient effect of each pollutant depends on when and where it is emitted, the atmospheric chemistry it experiences, and location and time for which the ambient effect is estimated. The equivalency will also depend on what particular amb ient metric is being used ( e.g. , average or 98th percentile), and the specifics of the method used t o estimate changes in the metric ( e.g. , size and distribution of emission changes to model). Thus th ere is an inherent tension between emissions-based accounting, as in a typical RFP analysis, and the i nclusion of model-based equivalent ambient effects; one assumes emissions distribution and timing do no t matter, the other fundamentally relies on them. (This is similar to the difference between “carryin g capacity,” the total emissions consistent with attainment, and a modeled attainment demonstration, which accounts for the distribution and chemistry of the emissions.) The EPA provided guidance on IPT ratios in a 2011 I PT memo 3 concerning trading for offsets in nonattainment New Source Review (NNSR) programs. Th e memo outlines a general procedure involving modeling to estimate pollutant sensitivit ies, with calculation of offset ratios “in a manner similar to the EPA’s 2007 technical assessment.” 4 The latter used photochemical modeling to assess sensitivity to 25%, 50%, and 75% emissions reductio ns of several subsets of anthropogenic emissions of direct PM 2.5 , NO x , and SO 2 . It computed ratios of sensitivities at many times and places, and identified a
NO x for DPM ratio and a SO 2 for DPM ratio as the 75th and 90th percentiles, re spectively, among the ratios computed. Criteria for this selection includ ed environmental protectiveness, stability among locations, and consistency across the emission redu ction scenarios. These are desirable criteria for a ll IPT contexts, not just NNSR. The 2011 IPT memo also recommended a step to “Conduct quality assurance of the resulting ratios and evaluate thei r interpretation and appropriateness given the natu re of

3 Memorandum, dated July 21, 2011, Gina McCarthy, As sistant Administrator, to Regional Air Division Dir ectors, Regions 1–10, Subject: Revised Policy to Address Re consideration of Interpollutant Trading Provisions for Fine Particles (PM 2.5 ) (“2011 IPT memo”).

4 “Details on Technical Assessment to Develop Interp ollutant Trading Ratios for PM 2.5 Offsets”, Memorandum from Tyler Fox, EPA Air Quality Modeling Group, to EPA d ocket # EPA-HQ-OAR-2003-0062, July 23, 2007.7

PM 2.5 sources and formation in the area of interest. Thi s evaluation will likely require emissions inventory data and observed ambient data for PM 2.5 and its component species.” 5 We consider the recommendations and criteria provided in the 2011 I PT memo as part of our evaluation of the IPT ratios in the 2015 PM 2.5 Plan.

B. General IPT considerations Several general methodological considerations may a pply to the regulatory contexts discussed below. First, the ambient response to emissions in μg/m 3 per tpd (sensitivity) will in general be different for the annual and 24-hour average concentrations; trading ratios between pollutants may also be different for the annual and the 24-hour NAAQS. Second, when μg/m 3 per tpd sensitivity estimates are based on modelin g, the tpd used for modeling may not match that for the entire nonattai nment area: the modeling inventory covers the modeling domain and varies by day and so does not m atch the single tpd values in the planning emission inventory for the nonattainment area (NAA). For pur poses of a trading ratio, the differences underlyin g the modeling and NAA tpd tend to cancel; the same d ense urban and sparse rural areas and their emissions are included (or excluded) in both the nu merator and denominator. Still, this could be a concern when the modeling domain and the NAA cover significantly different areas, and especially when the pattern of differences between the modelin g domain and the NAA geographic distributions is in turn different for the two pollutants being trad ed. As an extreme example, if pollutant A is mainly rural and has large emissions in the modeling domai n but little within the NAA, whereas pollutant B is mainly urban and present within both the modeling d omain and the NAA, then the sensitivities and the trading ratio based on the modeling inventory will not match those that could be estimated on the basi s of the NAA inventory, which makes no distinction ba sed on location. Thus, caution should be used

5 2011 IPT Memo, p. 5.8

when applying model-based sensitivities directly to emission reductions for the full NAA, for example to estimate the μg/m 3 effect of a control measure. A third general consideration is that the trading d irection matters. A trading ratio may be “conservative” for trading pollutant A for pollutan t B, that is, it tends to require more substitute reductions than are needed for equivalency to B. De pending on how it is derived, that ratio may not be conservative for trading in the other direction, B for A, as it may tend to require fewer reductions t han are needed for equivalency to A. Fourth, since ratios are involved, when the polluta nt sensitivity in the denominator is small relative to the numerator, the result will be sensi tive to the details of the approach, including the number of decimal places retained, so numerical stability can be a concern. It is desirable to have an approa ch and ratio that are robust in the face of the degree of precision available. A related issue is that wh en sensitivity is computed based on a geographic distr ibution of emissions reflecting particular control measures, it may be that one of the pollutants has negligible sensitivity at a relevant location, so t hat a ratio cannot be computed there. A case by case appr oach may be necessary in such situations. Finally, for contexts in which it is appropriate to estimate ambient sensitivity at multiple locations, it is desirable to have a rational and c onservative approach for handling multiple ratios, whether by using different ratios for different geo graphic zones (by using, for example, the average o r maximum of the ratios), or by some other approach. The variability over the locations may be so great that separate ratios are warranted for different ge ographic zones. If an average over multiple locatio ns is used, one must choose between averaging over the lo cations’ ambient effects before computing a single ratio, and computing a ratio for each location and then averaging. If a trading ratio in the other dir ection is needed, one must choose between just using the r eciprocal of the already-computed ratio for the fir st direction, or computing individual location (recipr ocal) ratios and then averaging; the reciprocal of an9

average is not equal to the average of the reciproc als. In any case, the rationale for any selected ap proach should be consistent with the underlying purpose of the relevant statutory requirement.


### III. IPT Ratio Estimation Methods in the SJV 2015 PM 2.5 

### Plan

As mentioned above, the 2015 PM 2.5 Plan used interpollutant trading ratios derived fr om modeled precursor sensitivities, to help address em ission reduction requirements in several regulatory contexts. This section discusses how the State and/ or District derived these IPT ratios.

The 2015 PM 2.5 Plan does not specifically discuss the development of interpollutant trading ratios itself, but cites work done for the 2008 PM 2.5 Plan 6 and 2012 PM 2.5 Plan. The IPT ratio approaches in those plans are described next, followed by a di scussion of how the ratios were used in particular regulatory contexts in the 2015 PM 2.5 Plan, principally in Chapter 6 (“Demonstration of Federal Requirements”) and in the CARB Staff Report. A summ ary of the ratios and the regulatory contexts they were used in appears in the table below.

Method 1. The 2008 PM 2.5 Plan modeling simulated the full year 2000 base ca se, and incorporated measurements made during the 2000 CRPA QS study. 7 Since that plan was focused on the 1997 annual PM 2.5 NAAQS, it included IPT sensitivity simulations onl y for the annual PM 2.5 design value. 8 Starting from the 2014 endpoint of the 2008 PM 2.5 Plan, CARB separately modeled 10% reductions in total NO x emissions and direct PM 2.5 emissions, presumably by applying a constant facto r to emissions at all times and places, i.e. , area-wide and proportionally uniform, or across-t he-board. The resulting changes to the PM 2.5 annual design value concentrations at three monito r sites in Bakersfield

6 The 2008 PM 2.5 Plan put forward IPT ratios for Nonattainment area New Source Review (NNSR), but EPA did not approve them; since NSR ratios are not put forward for approval in the 2015 SJV Plan, they are not dis cussed in this TSD.

7 CRPAQS is the California Regional Particulate Air Quality Study. More information is available about CRPAQS at http://www.arb.ca.gov/airways/ccaqs.htm.

8 CARB, “Precursor Effectiveness” document, May 13, 2011, part of the CARB 2011 Progress Report. See also “Contingency precursor effectiveness ratio using ad ditional information,” Memorandum from Scott Bohnin g, EPA Region 9 to File for docket EPA-R09-OAR-2013-0534, August 12 , 2013.10

(California Ave., Planz Rd., and Golden State Hwy.) were computed. When combined with the tpd emissions corresponding to the 10%, the changes yie lded sensitivities for each pollutant in μg/m 3 per tpd. The ratios of DPM to NO x sensitivities gave DPM-NO x effectiveness ratios — or NO x :DPM trading ratios — of 8.7, 9.3, and 8.9 for the respective si tes, with an average of 8.9.

The 2008 PM 2.5 Plan stated that the emissions used were the total in the modeling domain, rather than just the anthropogenic emissions or just the m otor vehicle emissions. The documentation stated th at the “method... provides a uniform metric for tracki ng progress relative to the attainment emissions targets,” presumably including for RFP. In subseque nt CARB submittals, including the 2015 PM 2.5 Plan as well as in EPA actions, the value of 9 was used. The 2008 PM 2.5 Plan and its updates ( e.g. , CARB 2011 Progress Report, and the SJVAPCD 2008 PM 2.5 Plan Progress Report), 9 the 2012 PM 2.5 Plan, and the current 2015 PM 2.5 Plan do not specifically document the rationale fo r the overall approach but each use the ratio of sensitivities as a trading ratio, and the design value as the metric of ambient equiv alency.

Method 2a. The 2012 PM 2.5 Plan’s approach for developing IPT ratios was para llel to that in the
2008 PM 2.5 Plan. The 2012 PM 2.5 Plan was focused on the 2006 24-hour NAAQS, and si nce high 24- hour values occur in the SJV principally in winter, only the first and fourth quarters of the 2007 bas e case year were modeled, and only 24-hour IPT sensit ivity simulations were performed. Starting from the 2019 endpoint of the 2012 PM 2.5 Plan, CARB separately modeled 25% reductions in an thropogenic emissions of direct PM 2.5 emissions and in each precursor: NO x , SOx, ammonia, and VOC. The 2012
PM 2.5 Plan included tables with the μg/m 3 response to each 25% reduction, and also the μg/m 3 per tpd

9 “2011 Progress Report”: “Progress Report on Implem entation of PM 2.5 State Implementation Plans (SIP) for the South Coast and San Joaquin Valley Air Basins and P roposed SIP Revisions,” adopted on April 28, 2011 b y CARB, submitted with the adopting resolution and other su pporting documentation by CARB on May 18, 2011; “SJ V PM 2.5 Progress Report”: “2008 PM 2.5 Plan Progress Report”, SJVUAPCD, Draft March 2011. Cited in the TSD for our re-proposal on the 2008 PM 2.5 Plan. 76 FR 41338 (July 13, 2011).11

sensibility of each pollutant; results are given fo r each of 10 monitoring sites, not just those in Bakersfield.

The NO x :DPM ratios implied by the sensitivities listed (th at is, ratios derived by dividing the sensitivities) ranged from 2.8 to 4.8, with an aver age of 4.1; the Plan text states that the NO x for DPM ratio is about 4. The SOx:DPM implied trading ratio s range from 1.6 to 7.8, with an average of 4.8; th e text states that the SOx for DPM ratio is about 5. Finally, the NH 3 :NO x implied trading ratios range from 4.3 to 14.0, with an average of 8.8; the Plan text states that the NH 3 -NO x effectiveness ratio is 1/9, which is consistent with the 8.8, though in the reverse d irection. 10 A table of these 24-hr sensitivity results also appears in the current 2015 PM 2.5 Plan. 11 Since each ratio in the Plan text is fairly close to the average over individual site ratios implied by the Plan’s s ensitivity table, presumably this was the basis for the ratios. Method 2b. Two site-specific sensitivity results ar e also referred to in the text of 2012 PM 2.5 Plan, in the context of an assessment of which prec ursors are significant. Based on sensitivity calcul ated using the area-wide method for the particular Baker sfield-California, the Plan stated: “Relative to th e other pollutants, ammonia reductions at the Bakersf ield-California site are only 2.3% as effective as direct PM 2.5 reductions, and only 10% as effective as NO x reductions.” 12 These imply a DPM for NH 3 trading ratio of 0.23:1, and a NO x for ammonia ratio of 0.10:1 or 1:10, though the 20 12 PM 2.5 Plan did not state these explicitly as trading ratios. The 2 015 PM 2.5 Plan makes an identical statement in its Chapter 2. 13

10 2012 PM 2.5 Plan, Appendix G (“Weight of Evidence”), p.G-64 an d G-65, including Table 7. 11 2015 PM 2.5 Plan, CARB Staff Report, Appendix A, Table 2, p.A- 27. 12 2012 PM 2.5 Plan, Appendix G (“Weight of Evidence”), p. G-65, and Chapter 4 (“Scientific Foundation and PM 2.5 Modeling Results”), p. 4-11.

13 2015 PM 2.5 Plan, Chapter 2 (“PM 2.5 Challenges and Trends in the San Joaquin Valley”), p.2-2712

Method 3. In addition, the 2012 PM 2.5 Plan evaluated the effect of DPM and the precursor emissions reductions occurring in Kern County only, to assess the effect of localized reductions on th e Bakersfield-California Ave. design value. Presumabl y the same general approach of sensitivity ratios was used; the Plan did not state the magnitude of t he reductions modeled, nor whether they were the total or just the anthropogenic emissions. 14 The reported effectiveness values implied an 8.3:1 NO x for DPM trading ratio; 8.0 is used elsewhere in the 201 2 PM 2.5 Plan. On the basis of μg/m 3 per ton evaluated at Bakersfield-California Ave., localized reductions of NO x are 1.5 times as effective as areawide reductions; for DPM and SOx, local reductions are nearer 3 times as effective. These are consiste nt with the expected greater ambient effect of emissio n reductions occurring nearer the monitor, as opposed to spread throughout the modeling domain. A NO x for NH 3 trading ratio was not computed in the 2012 PM 2.5 Plan; it reported respective sensitivities for NH 3 and NO x of 0.12 μg/m3 and 0.02 μg/m 3 per tpd, implying a NO x for NH 3 trading ratio of 1:6 or 0.167. However, any numbe r in the range of 0.12 to 0.21 is consistent with these sensitivity v alues, accounting for possible rounding. The corresponding implied NH 3 for NO x trading ratio is 6:1, with a possible range of 4.8 to 8.3 accounting for rounding.

The 2015 PM 2.5 Plan’s attainment demonstration relies on scaling of the modeling used for the
2008 PM 2.5 Plan. Accordingly, the modeling and trading ratios derived from the 2008 PM 2.5 Plan are more directly useful for the technical conclusions of the 2015 PM 2.5 Plan. However, results from the
2012 PM 2.5 Plan can also be useful and appropriate, especiall y when they are the only ones available, such as the ammonia sensitivity analyses used for t rading ratios involving ammonia.

14 2012 PM 2.5 Plan, Appendix G (“Weight of Evidence”), p.G-68.13 

### Table 1. Interpollutant Trading Ratios in SJV PM2.5 Plans **2008 plan for 1997 annual PM2.5 NAAQS** Method 1: Based on modeled Bakersfield design value sensitivity to 10% reductions in total areawide em issions **Trade Ratio**  **2008 Plan contexts 2015 Plan contexts**  NOx for DPM 9:1 MVEB Conformity MVEB Conformity; Aggregate Commitments DPM for NOx 1:9 Contingency Contingency **2012 Plan for 2006 24-hr PM2.5 NAAQS** Method 2a: Based on modeled multiple site design va lue sensitivities to 25% in reductions in anthropog enic areawide emissions **Trade Ratio**  **2012 Plan contexts 2015 Plan contexts**  NOx for DPM 4:1 Precursor significance not applicable

SOx for DPM 5:1 Precursor significance Aggregate Commitments -- SJV District † NOx for NH3 1:9 Precursor significance not applicable Method 2b: Based on modeled Bakersfield site design value sensitivities to 25% in reductions in anthro pogenic areawide emissions

**Trade Ratio**  **2012 Plan contexts 2015 Plan contexts**  NOx for NH3 1:10* Precursor significance in Bakersfield Precursor significance in Bakersfield; RFP DPM for NH3 0.23:1* Precursor significance in Bakersfield Precursor significance in Bakersfield Method 3: Based on modeled Bakersfield design value sensitivity to unspecified reductions in Kern Coun ty emissions **Trade Ratio**  **2012 Plan contexts 2015 Plan contexts**  NOx for DPM 8:1 MVEB conformity not applicable DPM for NOx 1:8 Contingency not applicable DPM for SOx 1:1 Contingency not applicable NOx for NH3 1:6** Not used not applicable **Note** : This table merely reports the IPT ratios and cont exts as they appear in the SJV plans; for EPA evalu ation of the ratios, see the main text. * The 1:10 or 10% is based on sensitivity using the areawide method, but calculated for just the Baker sfieldCalifornia monitoring site; the other values are av erages over multiple site ratios -- Method “2a”. **A NOx for NH3 trading ratio was not explicitly co mputed in the 2012 Plan; 1.6 is implied by NH3 and NOx sensitivities it reported. (2012 Plan, App. Weight of Evidence, p.G-68) † The District used a SOx for DPM trading raCo in a ccounCng for its own commitments, but the CARB acco unCng of District and ARB commitments as a whole used onl y the 9:1 NOx for DPM trading ratio.14 

### IV. Use of IPT ratios in the SJV 2015 PM 2.5 

### Plan

This section discusses where and how the IPT ratios were used, with an evaluation against the underlying requirements.

A. IPT for Reasonable Further Progress

1. RFP considerations The projected concentration at the design value mon itor ( i.e., the monitor with the highest design value in the nonattainment area) is a reasonable st arting point for an evaluation of air quality impac ts. As noted in the Federal Register Notice that this TSD accompanies, use of the design value monitor is required under 40 CFR section 51.1009(h) for RFP. T he Notice also cites 51.1009(g)(2) on the acceptability of alternative emissions scenarios fo r RFP that are “projected to result in a generally equivalent improvement in air quality by the milest one year as would be achieved under the benchmark RFP plan.” For purposes of computing a sensitivity ratio, some other concentration at the design valu e monitor might be acceptable, for example the concen tration from the single base case year typically available from a modeling exercise. In reality diff erent years could respond differently. But for proj ected design values, the same modeled response (Relative Reduction Factor or RRF, the fractional change in design value between the base or reference year and the future year) is applied to each year composing a three-year design value (or to an average of design values), essentially assuming that all years respo nd in the same way. This inclusion of monitored years oth er than the one modeled (base case) may therefore be obscuring the model response to a degree, and so might be less desirable for purposes of computing a sensitivity ratio. Of course, for estimating the μg /m 3 effect of an emissions change on the design value itself, it is more appropriate to use the standard RRF procedure and include all the design value year s in estimating the sensitivity. These considerations of varying yearly responses are moot if the baseline design value period used as the starting point for the plan does not include the base case year used t o15

evaluate the model, since there is then no choice b ut to apply the modeled response to monitored years other than the base case year. When modeling is used as the basis for estimating c oncentration changes, choices must be made for a) which model year to make the estimate for, b ) the magnitude of emissions changes modeled, and c) the geographic and temporal distribution of thos e changes. For a), it would be reasonable to make t he sensitivity estimate using modeling of the attainme nt year, as opposed to using the base year or using a separate sensitivity estimate for each milestone ye ar, since both RFP and the attainment demonstration ultimately target the attainment year, the year for which the effect of any interpollutant trading wou ld be reflected in the atmosphere. As a practical matter, modeling is typically available only for the base year and the attainment year. Emissions effects at the e nd of the planning period are appropriate to use as the starting point for sensitivity calculations, since they provide consistency with the modeled attainmen t year and with the end point of RFP, and reflect the cumulative effect of the emissions changes during the planning period. For b), for the greatest compatibility the magnitud e of emission changes modeled in estimating sensitivity would be comparable to those expected d uring the full planning period, or else those expec ted during the last RFP interval(s). Emissions changes that are much larger or smaller may have different sensitivity, since the ambient response is generall y nonlinear, rather than just proportional; they co uld result in different trading ratios and different eq uivalence demonstrations. Similarly, for c), for th e greatest compatibility the distribution of emission changes modeled in estimating sensitivity would be comparable to those expected during the planning pe riod. The timing of emissions during the day and the physical location of emissions can make a large difference in their impact at the design value monitor, so the distribution of emissions changes c ould have a substantial effect on the equivalence demonstration. For both b) and c), it would be more consistent with RFP to model the effect of changes16

in anthropogenic emissions only, keeping biogenic e missions constant since they remain present and affect the atmospheric chemistry while the anthropo genic emissions decline. In the absence of a gridded modeling inventory deve loped for one of the last RFP years, one way of addressing b) and c) would be to model a selecte d RFP year inventory interpolated between the base year and attainment year (which are spatially and t emporally resolved), and compute sensitivity by comparing its concentrations to those from the atta inment year modeling. Alternatively, since RFP concerns the sum of reductions occurring area-wide and for all sources in the area, modeling the effec t of uniform (“across the board”) reductions for all sources may give an adequate approximation of the sensitivity of the geographically variable reductio ns that are projected to occur. Using “across the b oard” reductions effectively interpolates between zero an d the attainment year emissions, rather than betwee n the base year and the attainment year, which would be more compatible with sensitivity used for an IPT ratio. Still, for small fractional changes, the dif ference between these two approaches would not be large. In any case, since the CFR requirements for RFP do not address the this topic, there is no stro ng recommendation for the magnitude or distribution of emissions to model, though they should have a rational basis and be tied to the attainment demons tration modeling. In practice, in order to implement IPT and arrive a t a single trading ratio number for use in an emissions accounting context like RFP, a more or le ss uniform distribution of emissions changes (“across the board” reductions) may be used to appr oximate the projected actual changes in the emissio n inventory. The RFP requirement does not specify tr acking the geographic distribution of emissions or concentrations at each milestone year. The attainme nt demonstration is not affected by interpollutant trades within these emissions accounting exercises (though actual emission trades in Motor Vehicle Emissions Budgets could affect it), and regardless of the method used to derive a trading ratio, the e nd point of RFP remains the attainment year emissions levels. This implies that the particular method use d is not crucial, as long as it is generally consiste nt with the attainment demonstration modeling. (Thi s is17

especially so when the amounts trading are small re lative to the total emissions.) No matter what rati os are used in assessing RFP for the milestone years, the attainment year emissions for each pollutant converge to the attainment year values used in the attainment demonstration. This means that there is some flexibility in choosing the method for determi ning IPT equivalent targets and its associated trad ing ratio.

2. RFP in the SJV Plan

The 2015 PM 2.5 Plan relies on interpollutant trading in the regul atory context of Reasonable Further Progress (RFP) towards attainment. While th e Plan does not concede that ammonia is a significant precursor under CAA section 189(e), for which RFP and other control requirements apply, it nevertheless contains an RFP demonstration includin g ammonia. Since ammonia emissions increase during the period of the plan, 2012 to 2018 and 201 2 to 2020, it is not possible to demonstrate annual incremental reductions as required for RFP if ammon ia is considered by itself. However, as discussed above, it is permissible to use an alternative emis sion scenario with an equivalent ambient effect at the design value monitor, and that scenario may include increases in the emissions of a particular precurs or. The equivalency of the Plan’s emission scenario is demonstrated in the CARB Staff Report in a
NO x -Ammonia RFP table that relies on the amount of NO x reductions equivalent to the required ammonia reductions; that equivalence is based on a NO x for ammonia trading ratio of 0.1:1 (or 10%). 15 No citation for that ratio is provided in this port ion of the CARB Staff Report, but in discussing precursor significance, Chapter 2 states that “ammo nia reductions at the Bakersfield-California site a re ... only 10% as effective as NO x reductions” 16 Also a 0.1:1 (or 10%) NO x for ammonia trading ratio matches the site-specific Bakersfield-California se nsitivity implied by a table elsewhere in CARB Staf f

15 2015 PM 2.5 Plan, CARB Staff Report, p. 26, including Table 12 (“NO x and Ammonia Emissions Levels Between 2012 and 2017 and NO x Equivalence (tpd)”).

16 2015 PM 2.5 Plan, Chapter 2, p.2-27.18

Report (“2012 PM 2.5 Plan Sensitivity Table”, based on modeling from th e 2012 Plan), 17 which cites modeling performed for the 2012 PM 2.5 Plan. A nearly identical statement and table appea r in the 2012
PM 2.5 Plan. 18
The procedure followed in the NO x -Ammonia RFP table is mathematically equivalent to the procedure laid out in the preamble for the 2007 PM 2.5 Implementation Rule (PMIR) proposal 19 ; the difference is that it uses concentration-equivalent emissions (“NO x equivalent emissions”) instead of concentrations per se to show the ambient equivalence of the alternative emission scenario. The 2007 PMIR preamble focused on equivalent concentrations, estimating the μg/m 3 ambient impact of each precursor as the product of its emissions in tpd an d its sensitivity in μg/m 3 per tpd, e.g. E NOx × S NOx . Ambient equivalence would require that the Plan’s e mission scenario have a concentration equivalent to that from an emissions scenario that meets RFP. In this case, it would require that the effect of the ammonia increase be equivalent to that of a NO x increase, but such that the NO x total still meets RFP. In terms of the concentration due to just the added em issions, this requirement is


### E NOx × 

### S NOx = 

### E NH3 × 

### S NH3

and also that the total NO x after adding E NOx must meet RFP. Dividing both sides by the NO x sensitivity
S NOx gives


### E NOx = 

### E NH3 × ( 

### S NH3 / 

### S NOx ) = 

### E NH3 × 

### R NOx:NH3

where R NOx:NH3 is the NO x for NH 3 trading ratio, in this case 0.10. The product E NH3 × R NOx:NH3 is the
“NO x equivalent” of E NH3 tpd of ammonia increase, to be added to the existi ng NO x emissions, as is

17 2015 PM 2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Tabl e 2. 18 2012 PM 2.5 Plan, Chapter 4, p. 4-11; 2012 PM 2.5 Plan, Appendix G (“Weight of Evidence”), p. G-65, Table 7. 19 72 FR 20586 at 20639 (April 25, 2007). As noted in the Federal Register Notice for this action, the “ Clean Air Fine Particle Implementation Rule” or “PM 2.5 Implementation Rule” was remanded in 2013, though not specifically because of the issues addressed here.19

done in the 2015 PM 2.5 Plan, CARB Staff Report, Table 12, p. 26. This sho ws that the 2007 PMIR preamble procedure is essentially the same as that used in the 2015 PM 2.5 Plan. In numbers, the 2012 to 2017 increase in ammonia emissions is 17.5 tpd; usi ng the 0.10 trading ratio gives a NO x equivalent of 1.8 tpd to be added in for a total of 237.5 tpd to compare to the RFP target. However, several issues could be raised over the pa rticular NO x for ammonia trading ratio of 0.1 or 1:10 that was used. It was based on 24-hour aver age PM 2.5 modeling from the 2012 PM 2.5 Plan; however the 2015 PM 2.5 Plan is based on scaling of annual modeling from t he 2008 Plan, and the emissions levels in the RFP table are tpd for the a nnual average. Ratios for 24-hour and annual averag e PM 2.5 can be substantially different; as noted above the 24-hour NO x for PM ratio was 4:1 in the 2012
PM 2.5 Plan, but the annual ratio was 9:1 in the 2008 PM 2.5 Plan. Annual ammonia sensitivity modeling was not conducted for the 2008 PM 2.5 Plan, so the 24-hour ratios provide the best avail able technical basis for the NOx for ammonia ratio for the 2015 PM 2.5 Plan.

Another issue is that the ratio was evaluated at th e Bakersfield-California site, but the annual NAAQS design value site is Madera. As discussed in the Federal Register Notice that this TSD accompanies, EPA agrees with the State that the 201 1 Madera data is anomalously high, and so also the 2013 design value. Both Hanford and Bakersfield-Pla nz also have higher 2013 annual design values than Bakersfield-California. Information on Hanford sensitivity is not available, but for BakersfieldPlanz the ratio implied by the 2012 PM 2.5 Plan Sensitivity Table is 1:8.9 or 0.113 (the aver age over all sites gives 1:8.8 or 0.124). There is also the Bake rsfield-California ratio based on sensitivity to lo calized Kern County emission reductions, 1:6 or 0.167. Use of a localized ratio may or may not be appropriate, depending on the particular distribution of emissio ns in question, but an area-wide-based ratio is generally more appropriate for the area-wide emissi ons accounting that RFP is based on.

In the current situation the exact value of the rat io is not crucial, since the projected NO x emissions are well below the required RFP target; i n 2017 there is room for an 18 tpd increase. The 20 1720

ammonia increase from 2012 is 17.5 tpd, so even a r atio as high as 1:1 would still result in RFP being met. Since the Plan provides evidence that ambient PM 2.5 is relatively insensitive to ammonia reductions, the issue of 24-hour vs. annual ratio l ikewise does not affect the conclusion of the RFP demonstration.

B. IPT for Motor Vehicle Emission Budgets for Transpor tation Conformity EPA explicitly allows IPT via regulation for Motor Vehicle Emissions Budgets (MVEBs) for conformity: 40 CFR 93.124(b) “trade emissions among budgets... for different pollutants or precursors” as long as there are “appropriate mechanisms for su ch trades.” Criteria for assessing the adequacy of a mechanism are not provided. Unlike for the other regulatory contexts considered here, trades under conformity are not just emissions accountings, but have the potential to sh ift the actual distribution of emissions before the attainment year, and so could affect the validity o f the attainment demonstration, possibly at multipl e locations. In practice, the amount of trading may b e small compared to total vehicle emissions, so thi s may not be an issue; the history of trades in an ar ea could be useful in evaluating this. And unlike f or RFP, there is no explicit requirement to tie the me thod to the design value site. Both of these sugges t that the equivalency demonstration may need to have a broader geographic scope. Different trading ratios might apply in different geographic zones; o r if there is to be a single ratio, a choice must b e made on the appropriate degree of conservatism, e.g. , average or maximum among the ratios computed for different locations. On the issue of how to model for ambient equivalenc y, the best fit to MVEB trading is to estimate sensitivity to emissions changes using mod eled changes to the motor vehicle inventory, as opposed to the full inventory. Vehicle emissions ca n sometimes be conveniently grouped for model sensitivity runs, so this would not be difficult. T he temporal distribution of vehicle emissions may b e21

significantly different than for other source types , which may affect the trading ratio estimate. Nevertheless, a uniform distribution of emissions c hanges (“across the board” reductions) may still be appropriate, especially if the vehicle emissions ar e spread over a large area or have a distribution generally similar to the overall inventory. Also, s ince the ratio is part of a mechanism for unknown particular future trades that are hypothetical at t he time the ratio is needed, any assumption other t han a uniform geographic distribution may be hard to supp ort, though the distribution of emissions from the current transportation system and of upcoming large projects may be well known. Finally, despite these concerns, the MVEB remains a n accounting of total emissions like RFP, in that the very idea of a “budget” is independent of the distribution of the emissions. There is no stro ng recommendation for the magnitude and distribution o f emissions to model, but using changes to the vehicle emissions themselves and using multiple loc ations should be considered. To inform future trading ratio development, a backward look at the c umulative effect of particular trades may be warranted if they resulted in large changes to the amount or distribution of motor vehicle emissions.

The 2015 PM 2.5 Plan relies on interpollutant trading in the regul atory context of the Motor Vehicle Emissions Budget (MVEB) for conformity. Thi s would allow a NO x for DPM trading ratio of 9:1. The Plan provides two citations for this ratio , the EPA approval of the same ratio for the 2008 P M 2.5 Plan, as revised in 2011, and also the “staff repor t taken to the ARB Governing Board for adoption and included in the full 2015 PM 2.5 Plan package that will be submitted to EPA.” 20 However, the CARB Staff report does not mention trading ratios for co nformity; it does mention a 1:9 DPM for NO x ratio for contingency purposes. This appears to be an oversig ht, so the previous EPA approval is the main rationale provided.

20 2015 PM 2.5 Plan, Chapter 6, p. 6-17.22

EPA approved the 2008 Plan IPT method as used in th e “Contingency Measure SIP” for establishing a 1:9 DPM for NO x ratio, in that trading direction only, for countin g excess reductions of direct PM 2.5 for NO x to meet the requirement contingency measures in CA A section 172(c)(9). 21 In that case, EPA considered the likely conservativeness of the ratio given the difference in the geographical distribution of NO x and DPM. Trading in the other direction, 9:1 NO x for DPM would not necessarily be conservative for contingency, given the particular emission sources involved. However, it is not necessary to rely on that previous EPA finding of c onservativeness, which was specific to contingency. For the more area-wide motor vehicle emissions, the ratio would be more neutral, rather than conservative in one direction or the other. The ultimate source of the 9:1 ratio is a CARB supp lemental document submitted in an update to the 2008 PM 2.5 Plan. It relied on the modeling platform used for that plan to estimate sensitivities of ambient PM 2.5 concentrations to NO x and direct PM 2.5 emission reductions, and a trading ratio calculate d from those. 22 The 2015 PM 2.5 Plan relies upon the same modeling, so that ratio is most compatible with this Plan. EPA approved the 2008 PM 2.5 Plan method for establishing a 9:1 NO x for DPM trading ratio, in that trading direction only, for MVEB conformity for the 2008 PM 2.5 Plan and its updates; and only using vehicle NO x emission reductions in excess of those needed to m eet the NO x budget. 23 This approval was partly based on EPA finding the method “adequate for assessing the effect of area-wide emissions changes, such as are used in conformity b udgets. The method modeled ‘across the board’ emissions changes over the entire modeling domain; emissions considered in transportation conformity

21 CARB, “Quantifying Contingencies for the 2008 PM 2.5 Plan”, dated June 20, 2013, submitted July 3, 2013

. Proposed rule: 78 FR 53113 at 53122 (August 28, 201 3); Final rule: 79 FR 29327 (May 22, 2014). Note th at EPA has proposed to withdraw its approval of the contingenc y measures for reasons unrelated to IPT. 80 FR 4919 0 (August 17, 2015).

22 CARB, “Precursor Effectiveness” document, May 13, 2011, part of the CARB 2011 Progress Report. 23 76 FR 69896 at 69923 (November 9, 2011).23

are also domain-wide.” 24 We continue to believe the State’s method for est ablishing a 9:1 NO x for DPM trading ratio is acceptable for transportation conf ormity purposes. One issue of possible concern here is that conformi ty budgets are established separately for the 24-hour and annual PM 2.5 standards, and the 9:1 ratio was based on only ann ual average modeling performed for the 2008 PM 2.5 Plan. Ratios applicable to the 24-hour standard ar e available based on modeling performed for the 2012 PM 2.5 Plan: an 8:1 ratio based on Bakersfield ambient PM 2.5 sensitivity to Kern County emission reductions, and a 4:1 ratio based on average sensitivity to area-wide reductions. These were discussed above in the secti on III of this TSD (“IPT Ratio Estimation Methods in the SJV 2015 PM 2.5 Plan“).We find the area-wide approach more appropr iate for the context of the area-wide emissions and reductions considered in an emissions accounting such as the MVEB budget.

In the 2012 PM 2.5 Plan, which focused on the 2006 24-hour PM 2.5 NAAQS, an 8:1 NO x for DPM was asserted to be the correct one for purposes of the MVEB, 25 citing the Plan’s section on contingency measures, which asserted a 1:8 NO x for DPM ratio was appropriate there. 26 That section in turn cited the Plan’s Appendix G (“Weight of Evidence”), discussed above. No rationale was provided for using the localized 8:1 or 1:8 ratios for MVEB conformity and contingency, respectively. This usage is not consistent with the EPA approval area-wide emission s-based ratios in the 2008 PM 2.5 Plan, which was partly on the basis that conformity is an area-wide program. Also, there is no discussion of conservativeness or numerical stability of the resu lting ratios; 8.3 rounded to 8.0 is less conservati ve for the 8:1 conformity ratio, while 1/8.3 = 0.120 repla ced by 1/8.0 = 0.125 is more conservative for the 1 :8 contingency ratio.

24 Id. at 69902. 25 2012 PM 2.5 Plan, Appendix C (“Mobile Sources”), p. C-32. 26 2012 PM 2.5 Plan, Chapter 9 (“Attainment Demonstration, RACM, RFP, and Contingency Measures”), p. 9-14.24

In any case, EPA did propose approval of an 8:1 NO x for DPM trading ratio, in that trading direction only, for MVEB conformity for the 2012 PM 2.5 Plan. 27 Part of the basis for the approval was the conservativeness of the localized ratio of 8:1 compared to ratios from modeling area-wide reductions, which were 5 or less for every site, wi th an average of 4.1. These ratios were based on th e modeled sensitivity of 24-hour design values to emi ssion reductions. 28

Since the 4:1 and 8:1 NO x for DPM trading ratios available based on 24-hour average modeling are both less than the 9:1 ratio from annual averag e modeling and proposed in the Plan for MVEB budget trading, they both would require less reduct ion of NO x emission to offset each ton per day of direct PM 2.5 emissions. EPA therefore proposes to approve the 2 015 PM 2.5 Plan MVEB budget trading mechanism that uses the 9:1 NO x for direct PM 2.5 trading ratio. EPA expects that the implications of different rati os for 24-hour and annual PM 2.5 conformity budgets will be addressed more systematically in fu ture plans, since the outcome could be different th an for the current Plan.

C. IPT for Aggregate Commitments

The 2015 PM 2.5 Plan relies on interpollutant trading in the regul atory context of Aggregate Commitments, the emissions reductions committed to in the previous plan. Demonstrating that these commitments were met is a precondition for EPA gran ting a request to extend the attainment deadline under CAA section 188(e). For direct PM 2.5 , NO x , and SOx, the Plan includes such demonstration in

27 80 FR 1816 at 1841 (January 13, 2015). 28 The 2012 PM 2.5 Plan also relied on a 1:8 DPM for NO x ratio and a 1:1 DPM for SOx ratio in meeting the contingency measure requirements. See 2012 PM 2.5 Plan, Chapter 9, pp. 9-8 and 9-14. However, EPA di d not act on those ratios. Since EPA agreed the Plan demonstrated that attainment by the Moderate area attainment date of December 31, 2015 was impracticable, contingency measures were not re quired. 80 FR 1816 at 1837 (“January 13, 2015).25

Chapter 6 for SJV District commitments 29 and in the CARB Staff Report for the combined SJV District and CARB commitments. 30
In a manner very similar to the RFP measure account ing discussed above, when there is a shortfall in one pollutant, equivalent reductions o f a different pollutant were used to make up for it ; equivalence was determined via an interpollutant tr ading ratio derived from modeled sensitivity to emission reductions. An apparent shortfall in the c ommitted reductions for direct PM 2.5 emissions was addressed by using NO x and SOx reductions that were in excess relative to the commitments for those pollutants. The NO x for DPM trading ratio used was 9:1, and the SO x for PM trading ratio used was 4:1. These are referred to in the Plan as a “1:9 trading ratio” and a “1:4 trading ratio”. This notation is the reverse of the convention used in this TSD, or else the Plan is just assuming that the equivalent poll utant amounts are independent of the trading direction. H ad pollutant geographic distribution and conservativeness been considered in the calculation s, the ratios for the two directions might not be numerical reciprocals as assumed in the Plan.

For the NO x :DPM ratio, Plan Chapter 6 (p.6-3) cites Appendix H (“Weight of Evidence”) of the
2008 PM 2.5 Plan, but this appears to be in error as that docu ment does not provide any trading ratios. The CARB Staff Report refers to a DPM for NO x trading ratio of 1:9 in the context of Aggregate Commitments, referring to modeling performed in the 2008 PM 2.5 Plan, and EPA’s approval of use of the ratio for contingency. 31 As was explained above for conformity, the ultimat e source for the 9:1 NO x for DPM trading ratio used was a supplemental docum ent included in the 2011 update to the 2008 PM 2.5 plan; it was derived from sensitivity to area-wide reductions.

29 2015 PM 2.5 Plan, Chapter 6, pp. 6-3 and p.6-4. 30 2015 PM 2.5 Plan, CARB Staff Report, pp. 21 and 22. 31 2015 PM 2.5 Plan, CARB Staff Report, p. 22.26

The citation given for the SOx for DPM 4:1 ratio at Chapter 6 p.6-3 is Appendix G (“Weight of Evidence”) of the 2012 PM 2.5 Plan. This must be the same 2012 PM 2.5 Plan material 32 cited above under Method 2a in section III on “IPT Ratio Estimation M ethods in the SJV 2015 PM 2.5 Plan,” which stated a value of 5, apparently from averaging the ratios ca lculated for multiple sites in the SJV, derived fro m sensitivity to area-wide reductions. It is not expl icitly stated where the 2015 PM 2.5 Plan’s value of 4 came from. However, the site-specific ratios for th e Bakersfield-California and Bakersfield-Planz site s are 4.3 and 3.6, based on the relative sensitivity of ambient PM 2.5 concentrations to a ton of emission reduction of DPM or SO x ; 33 these average to 3.9, or about 4. Note that the SO x for DPM ratio appears only in the Plan portion (Chapter 6, sec. 6.1.1) de scribing the SJV District’s accounting of its own commitments, and not in the CARB accounting of the SJV District and CARB control measures combined (CARB Staff Report p.22). Thus, the SOx fo r DPM ratio is not relied upon for the State’s demonstration that aggregate commitments have been met, nor in this EPA action on the Plan. It is arguable whether a ratio based on area-wide r eductions is the most appropriate for trading reductions from specific identified measures, as in contingency and commitments. To remain close to the modeled attainment demonstration it is desirabl e to consider the geographic distribution of the emission reductions. Still, the regulatory requirem ent is an accounting of a certain tpd of emission reductions, without regard for their distribution. The commitments are not tied to particular NAAQS av eraging times, so it is not clear whether ratios based on 24-hour or annual PM 2.5 modeling would be more appropriate.. Nor is it cle ar whether it is appropriate in this regulatory context to use ra tios that are averages over those for individual si tes, or ratios for the design value site. The former arguab ly represents the area as a whole, which would be reasonable for an area-wide emissions accounting li ke this; but for RFP there is a preference for

32 2012 PM 2.5 Plan, Appendix G, pp. G-64 and G-65. 33 2015 PM 2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Tabl e 2.27

evaluation of ratios at the design value site. For the Bakersfield-Planz site, which is the 2013 desig n value site for the 24-hour NAAQS, the site-specific NO x for DPM trading ratio is 3.6, again based on the relative sensitivities of ambient PM 2.5 concentrations to emission reductions of DPM. 34 Since this is lower than the ratio used in the 2015 PM 2.5 Plan, the Plan arguably requires conservatively la rge NO x reductions to replace the DPM. In the absence of EPA guidance on the appropriate b asis for a ratio, and the conservativeness of the approach that was used, EPA finds that the area -wide approach and the resulting interpollutant use d are acceptable for the aggregate commitments in thi s Plan.



### V. Conclusions for Interpollutant Trading in the SJV 2 015 PM 2.5 

### Plan

Neither the 2008 PM 2.5 Plan or the 2012 PM 2.5 Plan, nor the 2015 PM 2.5 Plan that relies upon the prior plans, addressed the various methodology cons iderations mentioned in the introductory discussion above, such as: appropriateness for any particular regulatory context, suitability for the 24-hour as opposed to the annual standard, differences in the sizes and geographic distribution between the inventories for the modeling domain and for the NAA , numerical stability, conservativeness of the rati o, conservativeness for trading in the other direction , and robustness for different tpd trading amounts. For example, in the 2008 PM 2.5 Plan there was no discussion of why the average of the three computed annual Bakersfield trading ratios should be used, a s opposed to say, the maximum (or minimum, for trades in the other direction). Nor was robustness in the face of the roughly 5% variability among nea rby sites within a single city explored. It is not unre asonable to use the average, but how reasonable it is may depend on the regulatory context in which the ratio is used.

34 2015 PM 2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Tabl e 2.28

Plan’s citations for the interpollutant trading rat ios used sometimes involve a chain through multiple other documents, and it mixes together rat ios from different previous plans for various locations for different PM 2.5 averaging times, without regard to the direction o f the trade or the regulatory context for which the ratio was develope d or was previously approved by EPA. Nevertheless, this was not a case of “cherry picking” ratios conv enient for the particular places they were applied. Rather, on the whole the 2015 PM 2.5 Plan used ratios developed for the 2008 PM 2.5 Plan when they were available (NO x for DPM and DPM for NO x ), and when they were not, relied on 2012 PM 2.5 Plan ratios (SOx for DPM and NO x for NH 3 ). This mixture is partly a function of the time an d resource constraints faced by the State and District for this Plan, whic h had to be assembled in a short time, and which wa s not able to include new modeling to address all the regulatory contexts in a uniform manner. The Plan’s method involving uniform area-wide reduc tions is a plausible one for assessing the effect of area-wide control programs or budgets, si nce they are also relatively uniform as a first approximation — though that is a rebuttable presump tion — and because the particular distribution is less important for requirements focusing on account ing of emissions totals. Yet the details of the met hod can be significant for the specific emissions numbe rs in that accounting, and the outcome of the regulatory decision based upon it, and so warrants care. EPA has given relatively little guidance on the met hodology considerations, and some of the issues raised may prove to be of minor importance o nce they are explored. In the absence of previous guidance, the State has some leeway in using a gene ric approach to IPT ratios for the various regulato ry context. But EPA expects that, for each regulatory context, some of the methodology considerations wil l be addressed in modeling and documentation for upco ming plans for the 2006 and 2012 PM 2.5 NAAQS. It is not sufficient to state that so many tons of one pollutant “is equivalent” to one ton of another without regard for the regulatory context or the di rection of the trade.