UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

REGION IX AIR DIVISION

Technical Support Document 

for EPA Evaluation of 

Interpollutant Trading Ratios 

in the 2015 San Joaquin PM2.5 Plan 

Prepared by 

Scott Bohning, Region 9 Air Division 

January 2016 

 
2 

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 PM2.5 Plan ................................................. 9

IV. Use of IPT ratios in the SJV 2015 PM2.5 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 Transportation Conformity ..................... 20

C. IPT for Aggregate Commitments .................................................................................... 24

V. Conclusions for Interpollutant Trading in the SJV 2015 PM2.5 Plan .................................. 27
3 

I. Introduction 

In several contexts, the 2015 PM2.5 Plan1 for the San Joaquin Valley (SJV) meets the emission 

reduction requirement for one pollutant by substituting reductions of another, a form of interpollutant 

trading (IPT). (Here “pollutant” means directly emitted PM2.5 or a PM2.5 precursor.) The contexts 

include the statutory requirements for Reasonable Further Progress (RFP), Motor Vehicle Emissions 

Budgets (MVEBs) for transportation conformity, and compliance with the emission reduction 

commitments in the 2008 PM2.5 Plan. The substitution or trading depends on such trading being allowed 

in a given context, and on determining a trading ratio, 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 some 

form of air quality modeling is necessary to justify a trading ratio, since modeling is the only way to 

assess the different chemical roles and geographic and temporal distribution of the pollutants in 

assessing the amounts that are equivalent. Determination of the ratio requires choices on what emissions 

scenarios should be modeled, which ambient output from the model is used, and where geographically 

the equivalence should be assessed in the area. This Technical Support Document (TSD) evaluates the 

use of IPT in the 2015 PM2.5 Plan and accompanies EPA’s Notice of Proposed Rulemaking (NPR) on 

that plan. 

A. IPT Terminology Used 

As an example to illustrate the IPT terminology used here, the 2015 PM2.5 Plan provides for a 

trading mechanism for the MVEBs, with a 9:1 NOx for direct PM2.5 trading ratio. That is, 9 tpd of NOx

reductions may be used for meeting a 1 tpd PM2.5 (DPM) reduction requirement (“NOx for DPM” 

 

1

 The 2015 PM2.5 Plan for the SJV comprises two California State Implementation Plan (SIP) submittals. The first 

submittal is the “2015 Plan for the PM2.5 Standard,” which the State submitted to EPA on June 25, 2015. The second 

submittal is the “2018 Transportation Conformity Budgets for the San Joaquin Valley PM2.5 SIP Plan Supplement,” which 

the State submitted to EPA on August 13, 2015. 
4 

trading). (Stated in terms of budgets, the mechanism allows for expanding the DPM budget by 1 tpd via 

“transferring” 9 tpd of NOx emissions from the NOx budget.) A trade in the other direction would be 

DPM for NOx, with a 1:9 ratio by supposition. For all of the 2015 PM2.5 Plan IPT contexts, the general 

approach was the same: model an emission reduction of each pollutant to estimate its ambient sensitivity 

in μg/m3 per tpd, and use ratios between these sensitivities as trading ratios. Details for the particular 

contexts are provided below, following discussion of 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 the 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 SA be the sensitivity of ambient PM2.5 to emissions reductions of 

pollutant A in μg/m3 per tpd (a decrease of SA μg/m3 results from 1 tpd reduction of A); and similarly 

for SB and pollutant B. The ambient effect of emissions is the product of the sensitivity in μg/m3

 per tpd 

and emissions in tpd Let R:1 be the trading ratio of A for B: R tpd of pollutant A reductions are needed 

for an ambient effect equivalent to 1 tpd of pollutant B reductions (B is R times as effective as A). Then 

the two pollutants have an equal effect in μg/m3 when 

ambient effect of A = ambient effect of B 

or: SA × R = SB × 1 (ambient effect = sensitivity in μg/m3 per tpd × emissions in tpd) 
5 

and so, solving for R, the A for B trading ratio is R = SB/SA. 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 = SB/SA 

B for A trading ratio of 1/R = SA/SB

B-A effectiveness ratio of R = SB/SA 

A-B effectiveness ratio of SA/SB = 1/R 

For the DPM:NOx example above, there is a 1:9 DPM for NOx trading ratio; the DPM-NOx

effectiveness ratio is 9 and the NOx-DPM effectiveness ratio is 1/9; reducing DPM is 9 times as 

effective as reducing NOx, 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 Vehicle Emissions Budgets for transportation 

conformity purposes under CAA section 176(c), and Contingency 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 emissions, and independent of atmospheric chemistry.

The required totals are derived from the modeled attainment demonstration, which indeed does consider 

distribution and chemistry, but in assessing whether the required emissions targets are met, by default 

distribution and chemistry are not relevant or considered. However, those very considerations are 

intrinsic to IPT: how much of one pollutant is “equivalent” 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 ambient metric is being used (e.g., average or 98th 

percentile), and the specifics of the method used to estimate changes in the metric (e.g., size and 

distribution of emission changes to model). Thus there is an inherent tension between emissions-based 

accounting, as in a typical RFP analysis, and the inclusion of model-based equivalent ambient effects;

one assumes emissions distribution and timing do not matter, the other fundamentally relies on them. 

(This is similar to the difference between “carrying 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 IPT memo3

 concerning trading for offsets in 

nonattainment New Source Review (NNSR) programs. The memo outlines a general procedure 

involving modeling to estimate pollutant sensitivities, 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 reductions of several subsets of anthropogenic emissions of

direct PM2.5, NOx, and SO2. It computed ratios of sensitivities at many times and places, and identified a 

NOx for DPM ratio and a SO2 for DPM ratio as the 75th and 90th percentiles, respectively, among the 

ratios computed. Criteria for this selection included environmental protectiveness, stability among 

locations, and consistency across the emission reduction scenarios. These are desirable criteria for all 

IPT contexts, not just NNSR. The 2011 IPT memo also recommended a step to “Conduct quality 

assurance of the resulting ratios and evaluate their interpretation and appropriateness given the nature of 

 

3

 Memorandum, dated July 21, 2011, Gina McCarthy, Assistant Administrator, to Regional Air Division Directors, 

Regions 1–10, Subject: Revised Policy to Address Reconsideration of Interpollutant Trading Provisions for Fine Particles 

(PM2.5) (“2011 IPT memo”). 

4

 “Details on Technical Assessment to Develop Interpollutant Trading Ratios for PM2.5 Offsets”, Memorandum from 

Tyler Fox, EPA Air Quality Modeling Group, to EPA docket # EPA-HQ-OAR-2003-0062, July 23, 2007. 
7 

PM2.5 sources and formation in the area of interest. This evaluation will likely require emissions 

inventory data and observed ambient data for PM2.5 and its component species.”5

 We consider the 

recommendations and criteria provided in the 2011 IPT memo as part of our evaluation of the IPT ratios

in the 2015 PM2.5 Plan. 

B. General IPT considerations 

Several general methodological considerations may apply to the regulatory contexts discussed 

below. First, the ambient response to emissions in μg/m3

 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/m3

 per tpd sensitivity estimates are based on modeling, the tpd used for 

modeling may not match that for the entire nonattainment area: the modeling inventory covers the 

modeling domain and varies by day and so does not match the single tpd values in the planning emission

inventory for the nonattainment area (NAA). For purposes of a trading ratio, the differences underlying 

the modeling and NAA tpd tend to cancel; the same dense urban and sparse rural areas and their 

emissions are included (or excluded) in both the numerator 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 modeling domain and the NAA geographic distributions is 

in turn different for the two pollutants being traded. As an extreme example, if pollutant A is mainly

rural and has large emissions in the modeling domain but little within the NAA, whereas pollutant B is

mainly urban and present within both the modeling domain 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 basis 

of the NAA inventory, which makes no distinction based 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/m3

 effect of a control measure. 

A third general consideration is that the trading direction matters. A trading ratio may be 

“conservative” for trading pollutant A for pollutant B, that is, it tends to require more substitute 

reductions than are needed for equivalency to B. Depending 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 than 

are needed for equivalency to A. 

Fourth, since ratios are involved, when the pollutant sensitivity in the denominator is small 

relative to the numerator, the result will be sensitive 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 approach 

and ratio that are robust in the face of the degree of precision available. A related issue is that when 

sensitivity is computed based on a geographic distribution of emissions reflecting particular control 

measures, it may be that one of the pollutants has negligible sensitivity at a relevant location, so that a 

ratio cannot be computed there. A case by case approach 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 conservative approach for handling multiple ratios, 

whether by using different ratios for different geographic zones (by using, for example, the average or 

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 geographic zones. If an average over multiple locations is 

used, one must choose between averaging over the locations’ ambient effects before computing a single 

ratio, and computing a ratio for each location and then averaging. If a trading ratio in the other direction 

is needed, one must choose between just using the reciprocal of the already-computed ratio for the first 

direction, or computing individual location (reciprocal) ratios and then averaging; the reciprocal of an 
9 

average is not equal to the average of the reciprocals. In any case, the rationale for any selected approach 

should be consistent with the underlying purpose of the relevant statutory requirement. 

III. IPT Ratio Estimation Methods in the SJV 2015 PM2.5 Plan 

As mentioned above, the 2015 PM2.5 Plan used interpollutant trading ratios derived from 

modeled precursor sensitivities, to help address emission reduction requirements in several regulatory

contexts. This section discusses how the State and/or District derived these IPT ratios. 

The 2015 PM2.5 Plan does not specifically discuss the development of interpollutant trading 

ratios itself, but cites work done for the 2008 PM2.5 Plan6

 and 2012 PM2.5 Plan. The IPT ratio approaches 

in those plans are described next, followed by a discussion of how the ratios were used in particular 

regulatory contexts in the 2015 PM2.5 Plan, principally in Chapter 6 (“Demonstration of Federal 

Requirements”) and in the CARB Staff Report. A summary of the ratios and the regulatory contexts they 

were used in appears in the table below. 

Method 1. The 2008 PM2.5 Plan modeling simulated the full year 2000 base case, and 

incorporated measurements made during the 2000 CRPAQS study.7 Since that plan was focused on the 

1997 annual PM2.5 NAAQS, it included IPT sensitivity simulations only for the annual PM2.5 design 

value.8 Starting from the 2014 endpoint of the 2008 PM2.5 Plan, CARB separately modeled 10% 

reductions in total NOx emissions and direct PM2.5 emissions, presumably by applying a constant factor 

to emissions at all times and places, i.e., area-wide and proportionally uniform, or across-the-board. The 

resulting changes to the PM2.5 annual design value concentrations at three monitor sites in Bakersfield 

 

6

 The 2008 PM2.5 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 discussed 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 additional information,” Memorandum from Scott Bohning, 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 yielded sensitivities for each pollutant in μg/m3

 per 

tpd. The ratios of DPM to NOx sensitivities gave DPM-NOx effectiveness ratios — or NOx:DPM trading 

ratios — of 8.7, 9.3, and 8.9 for the respective sites, with an average of 8.9. 

The 2008 PM2.5 Plan stated that the emissions used were the total in the modeling domain, rather 

than just the anthropogenic emissions or just the motor vehicle emissions. The documentation stated that 

the “method... provides a uniform metric for tracking progress relative to the attainment emissions 

targets,” presumably including for RFP. In subsequent CARB submittals, including the 2015 PM2.5 Plan 

as well as in EPA actions, the value of 9 was used. The 2008 PM2.5 Plan and its updates (e.g., CARB 

2011 Progress Report, and the SJVAPCD 2008 PM2.5 Plan Progress Report),9 the 2012 PM2.5 Plan, and 

the current 2015 PM2.5 Plan do not specifically document the rationale for the overall approach but each 

use the ratio of sensitivities as a trading ratio, and the design value as the metric of ambient equivalency. 

Method 2a. The 2012 PM2.5 Plan’s approach for developing IPT ratios was parallel to that in the 

2008 PM2.5 Plan. The 2012 PM2.5 Plan was focused on the 2006 24-hour NAAQS, and since high 24-

hour values occur in the SJV principally in winter, only the first and fourth quarters of the 2007 base 

case year were modeled, and only 24-hour IPT sensitivity simulations were performed. Starting from the

2019 endpoint of the 2012 PM2.5 Plan, CARB separately modeled 25% reductions in anthropogenic 

emissions of direct PM2.5 emissions and in each precursor: NOx, SOx, ammonia, and VOC. The 2012 

PM2.5 Plan included tables with the μg/m3 response to each 25% reduction, and also the μg/m3 per tpd 

 

9

 “2011 Progress Report”: “Progress Report on Implementation of PM2.5 State Implementation Plans (SIP) for the 

South Coast and San Joaquin Valley Air Basins and Proposed SIP Revisions,” adopted on April 28, 2011 by CARB, 

submitted with the adopting resolution and other supporting documentation by CARB on May 18, 2011; “SJV PM2.5 Progress 

Report”: “2008 PM2.5 Plan Progress Report”, SJVUAPCD, Draft March 2011. Cited in the TSD for our re-proposal on the 

2008 PM2.5 Plan. 76 FR 41338 (July 13, 2011). 
11 

sensibility of each pollutant; results are given for each of 10 monitoring sites, not just those in 

Bakersfield. 

The NOx:DPM ratios implied by the sensitivities listed (that is, ratios derived by dividing the 

sensitivities) ranged from 2.8 to 4.8, with an average of 4.1; the Plan text states that the NOx for DPM 

ratio is about 4. The SOx:DPM implied trading ratios range from 1.6 to 7.8, with an average of 4.8; the 

text states that the SOx for DPM ratio is about 5. Finally, the NH3:NOx implied trading ratios range from 

4.3 to 14.0, with an average of 8.8; the Plan text states that the NH3-NOx effectiveness ratio is 1/9, which 

is consistent with the 8.8, though in the reverse direction.10 A table of these 24-hr sensitivity results also 

appears in the current 2015 PM2.5 Plan.11 Since each ratio in the Plan text is fairly close to the average 

over individual site ratios implied by the Plan’s sensitivity table, presumably this was the basis for the 

ratios. 

Method 2b. Two site-specific sensitivity results are also referred to in the text of 2012 PM2.5

Plan, in the context of an assessment of which precursors are significant. Based on sensitivity calculated 

using the area-wide method for the particular Bakersfield-California, the Plan stated: “Relative to the 

other pollutants, ammonia reductions at the Bakersfield-California site are only 2.3% as effective as 

direct PM2.5 reductions, and only 10% as effective as NOx reductions.”12 These imply a DPM for NH3

trading ratio of 0.23:1, and a NOx for ammonia ratio of 0.10:1 or 1:10, though the 2012 PM2.5 Plan did 

not state these explicitly as trading ratios. The 2015 PM2.5 Plan makes an identical statement in its 

Chapter 2.13

 

10 2012 PM2.5 Plan, Appendix G (“Weight of Evidence”), p.G-64 and G-65, including Table 7. 

11 2015 PM2.5 Plan, CARB Staff Report, Appendix A, Table 2, p.A-27. 

12 2012 PM2.5 Plan, Appendix G (“Weight of Evidence”), p. G-65, and Chapter 4 (“Scientific Foundation and PM2.5

Modeling Results”), p. 4-11. 

13 2015 PM2.5 Plan, Chapter 2 (“PM2.5 Challenges and Trends in the San Joaquin Valley”), p.2-27 
12 

Method 3. In addition, the 2012 PM2.5 Plan evaluated the effect of DPM and the precursor

emissions reductions occurring in Kern County only, to assess the effect of localized reductions on the 

Bakersfield-California Ave. design value. Presumably the same general approach of sensitivity ratios 

was used; the Plan did not state the magnitude of the reductions modeled, nor whether they were the 

total or just the anthropogenic emissions.14 The reported effectiveness values implied an 8.3:1 NOx for 

DPM trading ratio; 8.0 is used elsewhere in the 2012 PM2.5 Plan. On the basis of μg/m3 per ton 

evaluated at Bakersfield-California Ave., localized reductions of NOx are 1.5 times as effective as areawide reductions; for DPM and SOx, local reductions are nearer 3 times as effective. These are consistent 

with the expected greater ambient effect of emission reductions occurring nearer the monitor, as 

opposed to spread throughout the modeling domain. A NOx for NH3 trading ratio was not computed in 

the 2012 PM2.5 Plan; it reported respective sensitivities for NH3 and NOx of 0.12 μg/m3 and 0.02 μg/m3

per tpd, implying a NOx for NH3 trading ratio of 1:6 or 0.167. However, any number in the range of 

0.12 to 0.21 is consistent with these sensitivity values, accounting for possible rounding. The 

corresponding implied NH3 for NOx trading ratio is 6:1, with a possible range of 4.8 to 8.3 accounting 

for rounding. 

The 2015 PM2.5 Plan’s attainment demonstration relies on scaling of the modeling used for the 

2008 PM2.5 Plan. Accordingly, the modeling and trading ratios derived from the 2008 PM2.5 Plan are 

more directly useful for the technical conclusions of the 2015 PM2.5 Plan. However, results from the 

2012 PM2.5 Plan can also be useful and appropriate, especially when they are the only ones available, 

such as the ammonia sensitivity analyses used for trading ratios involving ammonia. 

 

14 2012 PM2.5 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 emissions

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 value sensitivities to 25% in reductions in anthropogenic 

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 anthropogenic 

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 County 

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 contexts as they appear in the SJV plans; for EPA evaluation 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 BakersfieldCalifornia monitoring site; the other values are averages over multiple site ratios -- Method “2a”. 

**A NOx for NH3 trading ratio was not explicitly computed 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 accounCng for its own commitments, but the CARB accounCng 

of District and ARB commitments as a whole used only the 9:1 NOx for DPM trading ratio. 

 
14 

IV. Use of IPT ratios in the SJV 2015 PM2.5 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 monitor (i.e., the monitor with the highest design 

value in the nonattainment area) is a reasonable starting point for an evaluation of air quality impacts. 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. The Notice also cites 51.1009(g)(2) on the 

acceptability of alternative emissions scenarios for RFP that are “projected to result in a generally 

equivalent improvement in air quality by the milestone year as would be achieved under the benchmark 

RFP plan.” For purposes of computing a sensitivity ratio, some other concentration at the design value 

monitor might be acceptable, for example the concentration from the single base case year typically 

available from a modeling exercise. In reality different years could respond differently. But for projected 

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 respond in 

the same way. This inclusion of monitored years other 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/m3

 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 years 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 to 
15 

evaluate the model, since there is then no choice but to apply the modeled response to monitored years

other than the base case year. 

When modeling is used as the basis for estimating concentration 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 those changes. For a), it would be reasonable to make the 

sensitivity estimate using modeling of the attainment year, as opposed to using the base year or using a 

separate sensitivity estimate for each milestone year, since both RFP and the attainment demonstration

ultimately target the attainment year, the year for which the effect of any interpollutant trading would 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 end of the planning period are appropriate to use as the 

starting point for sensitivity calculations, since they provide consistency with the modeled attainment 

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 magnitude of emission changes modeled in estimating 

sensitivity would be comparable to those expected during the full planning period, or else those expected 

during the last RFP interval(s). Emissions changes that are much larger or smaller may have different 

sensitivity, since the ambient response is generally nonlinear, rather than just proportional; they could 

result in different trading ratios and different equivalence demonstrations. Similarly, for c), for the 

greatest compatibility the distribution of emission changes modeled in estimating sensitivity would be

comparable to those expected during the planning period. 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 could 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 changes
16 

in anthropogenic emissions only, keeping biogenic emissions constant since they remain present and 

affect the atmospheric chemistry while the anthropogenic emissions decline. 

In the absence of a gridded modeling inventory developed for one of the last RFP years, one way 

of addressing b) and c) would be to model a selected RFP year inventory interpolated between the base 

year and attainment year (which are spatially and temporally resolved), and compute sensitivity by 

comparing its concentrations to those from the attainment year modeling. Alternatively, since RFP 

concerns the sum of reductions occurring area-wide and for all sources in the area, modeling the effect 

of uniform (“across the board”) reductions for all sources may give an adequate approximation of the 

sensitivity of the geographically variable reductions that are projected to occur. Using “across the board” 

reductions effectively interpolates between zero and the attainment year emissions, rather than between 

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 difference 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 strong 

recommendation for the magnitude or distribution of emissions to model, though they should have a 

rational basis and be tied to the attainment demonstration modeling. 

In practice, in order to implement IPT and arrive at a single trading ratio number for use in an 

emissions accounting context like RFP, a more or less uniform distribution of emissions changes 

(“across the board” reductions) may be used to approximate the projected actual changes in the emission 

inventory. The RFP requirement does not specify tracking the geographic distribution of emissions or 

concentrations at each milestone year. The attainment 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 end 

point of RFP remains the attainment year emissions levels. This implies that the particular method used 

is not crucial, as long as it is generally consistent with the attainment demonstration modeling. (This is 
17 

especially so when the amounts trading are small relative to the total emissions.) No matter what ratios 

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 determining IPT equivalent targets and its associated trading 

ratio. 

2. RFP in the SJV Plan 

The 2015 PM2.5 Plan relies on interpollutant trading in the regulatory context of Reasonable 

Further Progress (RFP) towards attainment. While the 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 including ammonia. Since ammonia emissions increase 

during the period of the plan, 2012 to 2018 and 2012 to 2020, it is not possible to demonstrate annual

incremental reductions as required for RFP if ammonia is considered by itself. However, as discussed 

above, it is permissible to use an alternative emission scenario with an equivalent ambient effect at the 

design value monitor, and that scenario may include increases in the emissions of a particular precursor. 

The equivalency of the Plan’s emission scenario is demonstrated in the CARB Staff Report in a 

NOx-Ammonia RFP table that relies on the amount of NOx reductions equivalent to the required 

ammonia reductions; that equivalence is based on a NOx for ammonia trading ratio of 0.1:1 (or 10%).15

No citation for that ratio is provided in this portion of the CARB Staff Report, but in discussing 

precursor significance, Chapter 2 states that “ammonia reductions at the Bakersfield-California site are 

... only 10% as effective as NOx reductions”16 Also a 0.1:1 (or 10%) NOx for ammonia trading ratio 

matches the site-specific Bakersfield-California sensitivity implied by a table elsewhere in CARB Staff 

 

15 2015 PM2.5 Plan, CARB Staff Report, p. 26, including Table 12 (“NOx and Ammonia Emissions Levels Between 

2012 and 2017 and NOx Equivalence (tpd)”). 

16 2015 PM2.5 Plan, Chapter 2, p.2-27. 
18 

Report (“2012 PM2.5 Plan Sensitivity Table”, based on modeling from the 2012 Plan),17 which cites 

modeling performed for the 2012 PM2.5 Plan. A nearly identical statement and table appear in the 2012 

PM2.5 Plan.18

The procedure followed in the NOx-Ammonia RFP table is mathematically equivalent to the 

procedure laid out in the preamble for the 2007 PM2.5 Implementation Rule (PMIR) proposal19; the 

difference is that it uses concentration-equivalent emissions (“NOx 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/m3

 ambient impact of each 

precursor as the product of its emissions in tpd and its sensitivity in μg/m3

 per tpd, e.g. ENOx × SNOx. 

Ambient equivalence would require that the Plan’s emission 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 NOx increase, but such that the NOx total still meets RFP. In 

terms of the concentration due to just the added emissions, this requirement is 

ENOx × SNOx = ENH3 × SNH3

and also that the total NOx after adding ENOx must meet RFP. Dividing both sides by the NOx sensitivity 

SNOx gives 

ENOx = ENH3 × (SNH3/SNOx) = ENH3 × RNOx:NH3 

where RNOx:NH3 is the NOx for NH3 trading ratio, in this case 0.10. The product ENH3 × RNOx:NH3 is the 

“NOx equivalent” of ENH3 tpd of ammonia increase, to be added to the existing NOx emissions, as is 

 

17 2015 PM2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Table 2. 

18 2012 PM2.5 Plan, Chapter 4, p. 4-11; 2012 PM2.5 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 “PM2.5 Implementation Rule” was remanded in 2013, though not specifically because 

of the issues addressed here. 
19 

done in the 2015 PM2.5 Plan, CARB Staff Report, Table 12, p. 26. This shows that the 2007 PMIR 

preamble procedure is essentially the same as that used in the 2015 PM2.5 Plan. In numbers, the 2012 to 

2017 increase in ammonia emissions is 17.5 tpd; using the 0.10 trading ratio gives a NOx 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 particular NOx for ammonia trading ratio of 0.1 

or 1:10 that was used. It was based on 24-hour average PM2.5 modeling from the 2012 PM2.5 Plan; 

however the 2015 PM2.5 Plan is based on scaling of annual modeling from the 2008 Plan, and the 

emissions levels in the RFP table are tpd for the annual average. Ratios for 24-hour and annual average 

PM2.5 can be substantially different; as noted above the 24-hour NOx for PM ratio was 4:1 in the 2012 

PM2.5 Plan, but the annual ratio was 9:1 in the 2008 PM2.5 Plan. Annual ammonia sensitivity modeling 

was not conducted for the 2008 PM2.5 Plan, so the 24-hour ratios provide the best available technical 

basis for the NOx for ammonia ratio for the 2015 PM2.5 Plan. 

Another issue is that the ratio was evaluated at the 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 2011 Madera data is anomalously high, and so also the 

2013 design value. Both Hanford and Bakersfield-Planz 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 PM2.5 Plan Sensitivity Table is 1:8.9 or 0.113 (the average over all 

sites gives 1:8.8 or 0.124). There is also the Bakersfield-California ratio based on sensitivity to localized 

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 emissions in question, but an area-wide-based ratio is 

generally more appropriate for the area-wide emissions accounting that RFP is based on. 

In the current situation the exact value of the ratio is not crucial, since the projected NOx

emissions are well below the required RFP target; in 2017 there is room for an 18 tpd increase. The 2017 
20 

ammonia increase from 2012 is 17.5 tpd, so even a ratio as high as 1:1 would still result in RFP being

met. Since the Plan provides evidence that ambient PM2.5 is relatively insensitive to ammonia 

reductions, the issue of 24-hour vs. annual ratio likewise does not affect the conclusion of the RFP 

demonstration. 

B. IPT for Motor Vehicle Emission Budgets for Transportation 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 such 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 shift the actual distribution of emissions before the

attainment year, and so could affect the validity of the attainment demonstration, possibly at multiple 

locations. In practice, the amount of trading may be small compared to total vehicle emissions, so this 

may not be an issue; the history of trades in an area could be useful in evaluating this. And unlike for 

RFP, there is no explicit requirement to tie the method to the design value site. Both of these suggest 

that the equivalency demonstration may need to have a broader geographic scope. Different trading 

ratios might apply in different geographic zones; or if there is to be a single ratio, a choice must be 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 equivalency, the best fit to MVEB trading is to 

estimate sensitivity to emissions changes using modeled changes to the motor vehicle inventory, as 

opposed to the full inventory. Vehicle emissions can sometimes be conveniently grouped for model 

sensitivity runs, so this would not be difficult. The temporal distribution of vehicle emissions may be 
21 

significantly different than for other source types, which may affect the trading ratio estimate. 

Nevertheless, a uniform distribution of emissions changes (“across the board” reductions) may still be

appropriate, especially if the vehicle emissions are spread over a large area or have a distribution 

generally similar to the overall inventory. Also, since the ratio is part of a mechanism for unknown 

particular future trades that are hypothetical at the time the ratio is needed, any assumption other than a 

uniform geographic distribution may be hard to support, 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 an 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 strong 

recommendation for the magnitude and distribution of emissions to model, but using changes to the 

vehicle emissions themselves and using multiple locations should be considered. To inform future 

trading ratio development, a backward look at the cumulative effect of particular trades may be 

warranted if they resulted in large changes to the amount or distribution of motor vehicle emissions. 

The 2015 PM2.5 Plan relies on interpollutant trading in the regulatory context of the Motor 

Vehicle Emissions Budget (MVEB) for conformity. This would allow a NOx 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 PM2.5

Plan, as revised in 2011, and also the “staff report taken to the ARB Governing Board for adoption and

included in the full 2015 PM2.5 Plan package that will be submitted to EPA.”20 However, the CARB 

Staff report does not mention trading ratios for conformity; it does mention a 1:9 DPM for NOx ratio for 

contingency purposes. This appears to be an oversight, so the previous EPA approval is the main 

rationale provided. 

 

20 2015 PM2.5 Plan, Chapter 6, p. 6-17. 
22 

EPA approved the 2008 Plan IPT method as used in the “Contingency Measure SIP” for 

establishing a 1:9 DPM for NOx ratio, in that trading direction only, for counting excess reductions of 

direct PM2.5 for NOx to meet the requirement contingency measures in CAA section 172(c)(9).21 In that 

case, EPA considered the likely conservativeness of the ratio given the difference in the geographical

distribution of NOx and DPM. Trading in the other direction, 9:1 NOx 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 conservativeness, 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 supplemental document submitted in an update to 

the 2008 PM2.5 Plan. It relied on the modeling platform used for that plan to estimate sensitivities of 

ambient PM2.5 concentrations to NOx and direct PM2.5 emission reductions, and a trading ratio calculated 

from those.22 The 2015 PM2.5 Plan relies upon the same modeling, so that ratio is most compatible with 

this Plan. EPA approved the 2008 PM2.5 Plan method for establishing a 9:1 NOx for DPM trading ratio, 

in that trading direction only, for MVEB conformity for the 2008 PM2.5 Plan and its updates; and only 

using vehicle NOx emission reductions in excess of those needed to meet the NOx 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 budgets. 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 PM2.5 Plan”, dated June 20, 2013, submitted July 3, 2013. 

Proposed rule: 78 FR 53113 at 53122 (August 28, 2013); Final rule: 79 FR 29327 (May 22, 2014). Note that EPA has 

proposed to withdraw its approval of the contingency measures for reasons unrelated to IPT. 80 FR 49190 (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 establishing a 9:1 NOx for DPM 

trading ratio is acceptable for transportation conformity purposes. 

One issue of possible concern here is that conformity budgets are established separately for the 

24-hour and annual PM2.5 standards, and the 9:1 ratio was based on only annual average modeling 

performed for the 2008 PM2.5 Plan. Ratios applicable to the 24-hour standard are available based on 

modeling performed for the 2012 PM2.5 Plan: an 8:1 ratio based on Bakersfield ambient PM2.5 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 section III of this TSD (“IPT Ratio Estimation Methods 

in the SJV 2015 PM2.5 Plan“).We find the area-wide approach more appropriate for the context of the 

area-wide emissions and reductions considered in an emissions accounting such as the MVEB budget. 

In the 2012 PM2.5 Plan, which focused on the 2006 24-hour PM2.5 NAAQS, an 8:1 NOx 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 NOx 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 emissions-based ratios in the 2008 PM2.5 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 resulting ratios; 8.3 rounded to 8.0 is less conservative for 

the 8:1 conformity ratio, while 1/8.3 = 0.120 replaced by 1/8.0 = 0.125 is more conservative for the 1:8 

contingency ratio. 

 

24 Id. at 69902. 

25 2012 PM2.5 Plan, Appendix C (“Mobile Sources”), p. C-32. 

26 2012 PM2.5 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 NOx for DPM trading ratio, in that trading 

direction only, for MVEB conformity for the 2012 PM2.5 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, with an average of 4.1. These ratios were based on the 

modeled sensitivity of 24-hour design values to emission reductions.28

Since the 4:1 and 8:1 NOx for DPM trading ratios available based on 24-hour average modeling 

are both less than the 9:1 ratio from annual average modeling and proposed in the Plan for MVEB 

budget trading, they both would require less reduction of NOx emission to offset each ton per day of 

direct PM2.5 emissions. EPA therefore proposes to approve the 2015 PM2.5 Plan MVEB budget trading 

mechanism that uses the 9:1 NOx for direct PM2.5 trading ratio. 

EPA expects that the implications of different ratios for 24-hour and annual PM2.5 conformity 

budgets will be addressed more systematically in future plans, since the outcome could be different than 

for the current Plan. 

C. IPT for Aggregate Commitments 

The 2015 PM2.5 Plan relies on interpollutant trading in the regulatory context of Aggregate 

Commitments, the emissions reductions committed to in the previous plan. Demonstrating that these 

commitments were met is a precondition for EPA granting a request to extend the attainment deadline 

under CAA section 188(e). For direct PM2.5, NOx, and SOx, the Plan includes such demonstration in 

 

27 80 FR 1816 at 1841 (January 13, 2015). 

28 The 2012 PM2.5 Plan also relied on a 1:8 DPM for NOx ratio and a 1:1 DPM for SOx ratio in meeting the 

contingency measure requirements. See 2012 PM2.5 Plan, Chapter 9, pp. 9-8 and 9-14. However, EPA did 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 required. 80 FR 1816 at 1837 (“January 13, 2015). 
25 

Chapter 6 for SJV District commitments29 and in the CARB Staff Report for the combined SJV District 

and CARB commitments.30

In a manner very similar to the RFP measure accounting discussed above, when there is a 

shortfall in one pollutant, equivalent reductions of a different pollutant were used to make up for it; 

equivalence was determined via an interpollutant trading ratio derived from modeled sensitivity to 

emission reductions. An apparent shortfall in the committed reductions for direct PM2.5 emissions was 

addressed by using NOx and SOx reductions that were in excess relative to the commitments for those 

pollutants. The NOx for DPM trading ratio used was 9:1, and the SOx 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 pollutant 

amounts are independent of the trading direction. Had pollutant geographic distribution and 

conservativeness been considered in the calculations, the ratios for the two directions might not be 

numerical reciprocals as assumed in the Plan. 

For the NOx:DPM ratio, Plan Chapter 6 (p.6-3) cites Appendix H (“Weight of Evidence”) of the 

2008 PM2.5 Plan, but this appears to be in error as that document does not provide any trading ratios. 

The CARB Staff Report refers to a DPM for NOx trading ratio of 1:9 in the context of Aggregate 

Commitments, referring to modeling performed in the 2008 PM2.5 Plan, and EPA’s approval of use of 

the ratio for contingency.31 As was explained above for conformity, the ultimate source for the 9:1 NOx

for DPM trading ratio used was a supplemental document included in the 2011 update to the 2008 PM2.5

plan; it was derived from sensitivity to area-wide reductions. 

 

29 2015 PM2.5 Plan, Chapter 6, pp. 6-3 and p.6-4. 

30 2015 PM2.5 Plan, CARB Staff Report, pp. 21 and 22. 

31 2015 PM2.5 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 PM2.5 Plan. This must be the same 2012 PM2.5 Plan material32 cited above under 

Method 2a in section III on “IPT Ratio Estimation Methods in the SJV 2015 PM2.5 Plan,” which stated a 

value of 5, apparently from averaging the ratios calculated for multiple sites in the SJV, derived from 

sensitivity to area-wide reductions. It is not explicitly stated where the 2015 PM2.5 Plan’s value of 4 

came from. However, the site-specific ratios for the Bakersfield-California and Bakersfield-Planz sites 

are 4.3 and 3.6, based on the relative sensitivity of ambient PM2.5 concentrations to a ton of emission 

reduction of DPM or SOx;

33 these average to 3.9, or about 4. Note that the SOx for DPM ratio appears 

only in the Plan portion (Chapter 6, sec. 6.1.1) describing 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 for 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 reductions 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 desirable to consider the geographic distribution of the 

emission reductions. Still, the regulatory requirement is an accounting of a certain tpd of emission 

reductions, without regard for their distribution. 

The commitments are not tied to particular NAAQS averaging times, so it is not clear whether 

ratios based on 24-hour or annual PM2.5 modeling would be more appropriate.. Nor is it clear whether it 

is appropriate in this regulatory context to use ratios that are averages over those for individual sites, or 

ratios for the design value site. The former arguably represents the area as a whole, which would be 

reasonable for an area-wide emissions accounting like this; but for RFP there is a preference for 

 

32 2012 PM2.5 Plan, Appendix G, pp. G-64 and G-65. 

33 2015 PM2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Table 2. 
27 

evaluation of ratios at the design value site. For the Bakersfield-Planz site, which is the 2013 design 

value site for the 24-hour NAAQS, the site-specific NOx for DPM trading ratio is 3.6, again based on the 

relative sensitivities of ambient PM2.5 concentrations to emission reductions of DPM.34 Since this is 

lower than the ratio used in the 2015 PM2.5 Plan, the Plan arguably requires conservatively large NOx

reductions to replace the DPM. 

In the absence of EPA guidance on the appropriate basis for a ratio, and the conservativeness of 

the approach that was used, EPA finds that the area-wide approach and the resulting interpollutant used 

are acceptable for the aggregate commitments in this Plan. 

V. Conclusions for Interpollutant Trading in the SJV 2015 PM2.5 Plan 

Neither the 2008 PM2.5 Plan or the 2012 PM2.5 Plan, nor the 2015 PM2.5 Plan that relies upon the 

prior plans, addressed the various methodology considerations 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 ratio, 

conservativeness for trading in the other direction, and robustness for different tpd trading amounts. For 

example, in the 2008 PM2.5 Plan there was no discussion of why the average of the three computed 

annual Bakersfield trading ratios should be used, as 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 nearby 

sites within a single city explored. It is not unreasonable to use the average, but how reasonable it is may 

depend on the regulatory context in which the ratio is used. 

 

34 2015 PM2.5 Plan, CARB Staff Report, Appendix A, p. A-27, Table 2. 
28 

Plan’s citations for the interpollutant trading ratios used sometimes involve a chain through 

multiple other documents, and it mixes together ratios from different previous plans for various 

locations for different PM2.5 averaging times, without regard to the direction of the trade or the 

regulatory context for which the ratio was developed or was previously approved by EPA. Nevertheless, 

this was not a case of “cherry picking” ratios convenient for the particular places they were applied.

Rather, on the whole the 2015 PM2.5 Plan used ratios developed for the 2008 PM2.5 Plan when they were 

available (NOx for DPM and DPM for NOx), and when they were not, relied on 2012 PM2.5 Plan ratios 

(SOx for DPM and NOx for NH3). This mixture is partly a function of the time and resource constraints 

faced by the State and District for this Plan, which had to be assembled in a short time, and which was 

not able to include new modeling to address all the regulatory contexts in a uniform manner. 

The Plan’s method involving uniform area-wide reductions is a plausible one for assessing the 

effect of area-wide control programs or budgets, since they are also relatively uniform as a first 

approximation — though that is a rebuttable presumption — and because the particular distribution is 

less important for requirements focusing on accounting of emissions totals. Yet the details of the method 

can be significant for the specific emissions numbers 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 methodology considerations, and some of the 

issues raised may prove to be of minor importance once they are explored. In the absence of previous 

guidance, the State has some leeway in using a generic approach to IPT ratios for the various regulatory 

context. But EPA expects that, for each regulatory context, some of the methodology considerations will 

be addressed in modeling and documentation for upcoming plans for the 2006 and 2012 PM2.5 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 direction of the trade.