1U.S. EPA, (1996), Guidance on Use of Modeled Results to Demonstrate Attainment of the

Ozone NAAQS, EPA-454/B-95-007, (June 1996). Web site: http://www.epa.gov/ttn/scram/ (file

name: “O3TEST”).

March 28, 2002

SUBJECT: Mid-Course Review Guidance for the 1-Hour Ozone 

Nonattainment Areas that Rely on Weight-of-Evidence 

for Attainment Demonstration

FROM: Lydia N. Wegman, Director /S/ 

Air Quality Strategies & Standards Division, OAQPS

J. David Mobley, Acting Director /S/

Emissions, Monitoring and Analysis Division, OAQPS 

TO: Air Division Directors, Regions I - X 

SUMMARY

This memorandum–

1. Transmits the technical guidance for performing Mid-Course Reviews (MCR), Recommended

Approach for Performing Mid-course Review of SIP’s to Meet the 1-hour NAAQS for Ozone. 

(See Attachment.) 

 The guidance was developed primarily for certain eastern States with serious or severe

nonattainment areas that have committed in their 1-hour ozone SIPs to complete and submit a MCR or

related analysis (such as an early attainment assessment). This guidance may also prove useful to other

areas that may submit new or revised attainment demonstrations relying on weight-of-evidence. In

addition, this guidance may have other uses that EPA may reference from time to time.

2. Provides policy guidance for using the MCR technical guidance.

The EPA’s modeling and attainment demonstration guidance1 provides that States using long

term projections (which is the case for severe and extreme nonattainment areas, as well as serious areas

requesting attainment date extensions) commit in their SIPs to perform a mid-course review. Also,

EPA’s proposed rulemaking on the 1-hour ozone SIPs for ten nonattainment areas (December 16,
2

2See, e.g., Federal Register December 16, 1999 (64 FR 70318 at 70323).

3

Ibid.

4U.S. EPA (1996) op. cit.

1999) set forth a framework for reviewing and processing those 1-hour ozone SIPs; one element of

that framework was a commitment for a MCR.2 That proposed rulemaking also noted, “The EPA’s

1996 modeling guidance also recognizes a need to perform a mid-course review as a means for

addressing uncertainty in the modeling results. Because of the uncertainty in long term projections, EPA

believes a viable attainment demonstration that relies on weight of evidence needs to contain provisions

for periodic review of monitoring, emissions, and modeling data to assess the extent to which

refinements to emission control measures are needed.”3

This memorandum covers several topics:

– The overall MCR process and timing, including the potential consequences of findings that

progress toward attainment is or is not being made.

– Guidance for situations where failure to make progress is due to transport.

– Special schedule for other (e.g., moderate or serious) ozone nonattainment areas with

attainment dates of 2004 or earlier.

The Regional Offices should provide this guidance to the appropriate State air pollution control

agencies.

BACKGROUND

A mid-course review (MCR) provides for an opportunity to assess if a nonattainment area is or

is not making sufficient progress toward attainment of the one-hour ozone standard. The review will

utilize the most recent monitoring and other data to assess whether the control measures relied on in a

SIPs attainment demonstration have resulted in adequate improvement of the ozone air quality. The

EPA believes that a commitment to perform a MCR is a critical element in any attainment

demonstration that employs a long term projection period and relies on a weight of evidence test.4 In

proposing to approve the attainment demonstration SIPs for ten serious and severe nonattainment areas

for the 1-hour ozone NAAQS on December 16, 1999, EPA indicated that in order for EPA to

approve the SIPs, the States would need to commit to perform a MCR, since they relied on a weight of

evidence test with long term projections. EPA also requested the States to work with EPA in a public

consultative process to develop a methodology for performing the MCR and develop the criteria by

which adequate progress 

would be judged. The States have participated in such a consultative process with EPA, which 
3

resulted in the development of the attached MCR technical guidance. A methodology other than the

one developed from the public consultative process would need to be approved in advance by EPA. 

The attached technical guidance contains three basic steps: (1) perform an administrative test

(e.g., demonstrate whether the appropriate emission limits were adopted and implemented); (2) analyze

available air quality, meteorology, emissions and modeling data and document findings; and (3)

document conclusions regarding whether progress toward attainment is being made using a weight of

evidence determination (which may or may not include new modeling analyses).

In the December 16, 1999, notices of proposed rulemaking, EPA did not request that States

commit in advance to adopt new control measures as a result of the MCR process. Based on the

MCR, if EPA determines additional control measures are needed for attainment, EPA would determine

whether additional emission reductions are necessary from the State or States in which the

nonattainment area is located or upwind States, or both. The EPA would then require the appropriate

State or States to adopt and submit the new measures within a specified period. The rulemaking

proposals noted that EPA anticipated that these findings would be made as calls for SIP revisions under

section 110(k)(5) and, therefore, the period for submission of the measures would be no longer than 18

months after the EPA finding.

GUIDANCE

A. Overall Process and Timing

The basic MCR process has several steps:

1. At the mid-course period, the State performs an assessment of progress toward attainment using

available data. The attached technical document provides guidance on various analyses that can be

used in a MCR. Table 1 below provides a summary of the analyses that EPA highly recommends be

included in this assessment, together with additional analyses that the State may want to consider. The

State also performs an “administrative” test, which includes several determinations, such as whether the

necessary emission limits have been adopted and implemented. 

2. Using this information, the State determines whether sufficient progress is being made toward

attainment. The State may rely on a weight of evidence demonstration in which a number of factors

may be considered, such as fluctuations due to meteorology or impacts of transported ozone and

precursors. These results are presented to EPA for review.

3. The EPA evaluates the MCR and determines whether sufficient progress is being made.

4. If EPA determines that sufficient progress is not being made, EPA may call for a SIP revision that

includes either or both of the following:
4

5Based on the meteorology in the year the MCR is performed and the previous year (e.g.,

2003 and 2004) and an anticipated report from EPA on the status of regional NOx emission reductions.

a. A near-term correction, for which the State could apply one or more of its contingency

measures (contained in the SIP for failure to make reasonable further progress) or additional

measures. One way to identify additional emission reductions needed is found in the attached

technical guidance document.

b. A longer term correction, which may require new modeling, following the June 1996, EPA

“Guidance on Use of Modeled Results to Demonstrate Attainment of the Ozone NAAQS,”

and which may result in the need for adoption of new additional emission control measures.

TABLE 1

HIGHLY RECOMMENDED AND OPTIONAL ANALYSES FOR MID-COURSE REVIEW

Highly Recommended Analyses Additional Optional Analyses

1. Review/Assessment of monitored ozone trends from

1994 to the year the MCR is performed (e.g., 2004). 

This should address year-to-year variations with

emphasis on the most recent 5 years.

Tech. Guidance citation: 2.1

Trends assessment over a longer period

(that corresponds to the time period of

the SIP) that accounts for year-to-year

variation.

2. Adjustment of monitored ozone trends for year-toyear meteorological variations using curve fitting

technique.

Tech. Guidance citation: 2.1

Adjustment for Meteorology using

CART or other Technique

Tech. Guidance citation: 2.1

3. An assessment of observed ozone trends in relation to

previous projections of emission reductions from 1996 to

the year the MCR is performed (e.g., 2004), including an

interpretation of observed changes in ozone

concentrations resulting from the NOx SIP call.5 It is not

intended to suggest that an entire emission inventory

needs to be developed for the year for which the MCR is

performed. But, some assessment of the relative change

in ozone concentrations compared to the relative change

in emissions is needed. EPA expects that any emissions

assessment should be based on MOBILE6.

Tech. Guidance citation: 2.1
5

Highly Recommended Analyses Additional Optional Analyses

6The target needs to be derived only if results of analyses 1, 2 and 3 above does not project

attainment.

7Estimates of inventory change from the year the MCR is performed (e.g., 2004) to the

attainment date are derived from interpolation of ROP SIP 2002, 2005, and (where appropriate) 2007

inventories and new control measure emission benefits and implementation schedules. A new inventory

would not be needed for the year the MCR is performed (e.g., 2004). This approach assumes that

Regional NOx reductions are substantially implemented by 5/2004. The emission change from the year

the MCR is performed (e.g., 2004) to the attainment date can be multiplied by previously-derived

ozone/emissions sensitivity factors, and the result added to 124 ppb to obtain the target.

4. An identification of an ozone “target”6 for the year the

MCR is performed (e.g., 2004) using techniques cited in

the technical guidance.7

Tech. Guidance citation: 3.2

Identification of an ozone target for the

year the MCR is performed (e.g.,

2004) using Air Quality Modeling. 

Tech. Guidance citation: 2.3

5. A discussion of observed differences in upwind and

downwind monitors to attempt to ascertain if any lack of

progress toward attainment is caused by local or

transported emissions and ozone concentrations

Tech. Guidance citation: 2.2

6. A statement of the state’s current overall VOC / NOx

emission reduction strategy/approach. Where data are

available, PAMs ambient hydrocarbon data and NOy

measurements should be considered in this assessment.

Tech. Guidance citation: 2.1
6

Highly Recommended Analyses Additional Optional Analyses

7. Administrative reviews:

a. Review of progress on previous State and local

control measures implemented or underway. At a

minimum, the review must address whether all the

rules needed to effect emission reductions have been

adopted. The review must assess the degree of

implementation of local measures using available

information.

b. A conclusion as to whether attainment is expected by

the attainment date (on or off-track). This would

account for an anticipated EPA assessment of

national and regional measures.

c. If attainment is not anticipated by the attainment date,

the review should contain an identification of reasons

why attainment is not anticipated.

d. If attainment is not anticipated by the attainment date,

and the MCR analysis indicates that local short-term

corrective measures could be taken to help put the

area back on track toward attainment, then the

submission could identify such measures.

8. Consultation with EPA regarding any analyses

contemplated that is not in Columns 1 or 2 above.

Table 2 below outlines the process and recommended timing of the MCR process. The dates in this

table are based on performance of the MCR in 2004, but comparable timeframes should be used

where the MCR is performed in a different year. Deviations in timing or approach should be discussed

in advance with the appropriate Regional Office.

TABLE 2

MID-COURSE REVIEW TIMEFRAME

DATE* ACTIVITY

As soon as

possible

Using the technical guidance for performing the MCR, State identifies air quality 

expected to be achieved at “mid-course” period (at the end of the year the MCR

is performed (e.g., 2004)
7

DATE* ACTIVITY

between

12/31/03 and

6/30/04

Recommended effort: States should identify analyses and data bases which will be

used to support a mid-course review and discuss these with the appropriate U.S.

EPA Regional Office. 

10/1/04 EPA to provide any available assessments of national and regional control

measures.

12/31/04 State submits its MCR to EPA that determines whether sufficient progress toward

attainment is being made. Even though the MCR is not a SIP revision, EPA

recommends that the State provide an opportunity for public comment on the

MCR before submission to EPA.
8

DATE* ACTIVITY

3/31/05 to

12/31/05

If the State determines that sufficient progress is being made, the State would

continue implementing the SIP. If the State determines that progress is not being

made, the State would proceed as follows depending on the degree of progress:

a. In all cases, the State should identify the already adopted control measures for

which emission reductions phase-in after the year in which the MCR is performed

(e.g., post-2004), and potential new control measures that could form the bases

for the emission reduction inventory input for future air quality modeling if needed.

b. EPA recommends that the State implement some or all of the short-term

corrections identified in its MCR to expedite getting the area back on course

toward attainment.

c. If the problem is more severe and not readily addressed by short-term

corrections, EPA recommends that the State voluntarily and expeditiously begin

the process to revise its SIP, including new modeling as appropriate.

EPA will also review the MCR. If EPA determines that sufficient improvement in

ozone air quality is not occurring within the State, and that the State is not

implementing sufficient corrective measures to address its contribution to that lack

of progress, EPA would either find that the State has failed to implement a part of

its SIP and/or issue a call for SIP revision for the State. Also, if EPA determines

that sufficient progress is not being made by any upwind State that had previously

been found to contribute significantly to the State’s ozone problem, EPA would

concurrently either find that an upwind State has failed to implement a part of its

SIP and/or issue a call for a SIP revision from such an upwind State. (Likely

timeframes for proposed and final rule on call for SIP revision: 4/30/05 and

8/31/05.) See section B below regarding transport.

The EPA would specify the time period for a State to act; that time would be no

longer than 18 months from the determination of lack of progress or call for SIP

revision (e.g., 2/28/07). Actions the State may need to take are--

–any corrective action identified in the MCR

–submitting a new attainment demonstration with new modeling based on a new

base year emission inventory and appropriate meteorological episodes.

3/31/05 to

2/28/07

State voluntarily completes the corrective actions or submits a SIP revision in

response to an EPA SIP call.
9

DATE* ACTIVITY

* In the notices of proposed rulemaking of December 16, 1999, EPA originally asked the States

with serious and severe ozone nonattainment areas to commit to submit their mid-course review by

the end of 2003; the selection of this time by EPA and the States was based in large part on the

expectation that NOx emission reduction controls under EPA’s NOx SIP call would be implemented

by that time. However, in August 2000, the U.S. Court of Appeals for the D.C. Circuit ruled that

EPA could not require compliance with the NOx SIP call reductions before 5/31/04. Therefore,

many States affected by the NOx SIP call emission reductions revised their commitment to submit

the MCR on the schedule suggested above. 

It is recognized that a revised attainment demonstration submitted in response to the MCR may not

provide emission reductions in sufficient time to result in attainment for areas with an attainment date

earlier than November 2007 (however, see section D below). If implemented in the ozone season of

2007, these reductions should be sufficient for an area with a 2007 attainment date to qualify for the

first of two possible one-year attainment date extensions. 

In cases where the analysis will not result in corrective actions by the ozone season of the

attainment year, the analysis should more correctly be termed an “accelerated attainment assessment”

rather than a true mid-course review, since it will serve the purpose of beginning corrective analyses

and actions early when it appears that the SIP will not result in attainment by the attainment date.

B. Where Failure to Make Progress Is Due to Transport

Section 2.2 of the MCR technical guidance provides a tool for assessing the relative importance of

transported emissions compared to local emissions. The analysis may show that an area’s failure to

make adequate progress toward attainment may be largely due to transported ozone and precursors. 

The State should identify this situation in its MCR and recommend or propose ways of addressing

control, including filing a petition with EPA under section 126 to require control of upwind sources

contributing to the continuing nonattainment. To the extent that EPA determines that the lack of

adequate progress is due to transport from across State lines, EPA will use its authority under the Clean

Air Act to address the contributing emission sources. To the extent that the lack of progress is due to

local emissions, EPA will address that situation through an appropriate remedy (e.g., a call for a SIP

revision if additional control is needed or a finding of failure to implement and/or federal enforcement if

existing controls requirements are not being enforced).

Staff from some of the northeast Ozone Transport Commission (OTC) States have recommended

that the issue of transport be addressed in a more comprehensive manner and have provided EPA with

some ideas for the format of such an activity. OAQPS intends to follow up with the affected States on

their ideas.
10

C. Special Schedule for Serious Areas with Proposed Attainment Dates of 2004 or Earlier

The December 1999 notices of proposed rulemaking (NPRs) on the 1-hour O3 SIPs for these

areas (Atlanta and Western Massachusetts) acknowledged that in order to approve the attainment

demonstration SIP for the serious areas requesting an attainment date extension to a year prior to 2005,

a review that occurs at a midpoint prior to the attainment date would be impractical in terms of timing. 

Therefore, for these areas, EPA requested the State’s commitment to an MCR be a commitment to

perform an early attainment assessment to be submitted by the end of the attainment year. Such an

early attainment assessment should follow the MCR technical guidance noted above. This early

attainment assessment will help guide the State and EPA in determining what further action might be

required if the area does not attain by its attainment date.

D. New Attainment Demonstration and Modeling as Part of MCR

Some States may wish to perform new modeling and develop a new attainment demonstration as

part of the their MCR. This approach is actually preferable to a mid-course assessment without new

modeling, since it will result in a more timely assessment of the magnitude of the problem that exists at a

point in time prior to the attainment date. More importantly, it will address any problems (i.e., identify

and plan for additional emission reductions that might be needed to ensure attainment) much sooner

than any SIP revision that EPA would require subsequent to submission of a mid-course review.

New attainment demonstration and modeling should make use of the most up-to-date guidance

(e.g., EPA 1996 modeling guidance), analytical techniques, and models such as MOBILE6, BEIS3

and/or NONROAD. In many cases this will require an assessment of the impact changing from one

analytical technique or model to another will have on the original SIP analysis results. 

E. Pilot Demonstration of MCR Guidance

Several State agency representatives have recommended that EPA perform a pilot application of

the MCR technical guidance. At this time, EPA plans to perform such an effort.

F. Documentation

A State should include the following documentation for its mid-course review:

- the administrative review;

- descriptions of the data bases used to support each analysis;

- identification of models and analytical techniques used to perform the analysis;

- the outcome of each analysis and whether or not it is consistent with a conclusion that the SIP is

on track;

- a narrative describing the rationale used to conclude that the weight of evidence does or does not

suggest that the SIP is on track toward timely attainment.

G. Relationship to SIP Revisions Needed as a Result of MOBILE6 Model
11

8Memorandum of January 18, 2002, from John S. Seitz and Margo Tsirigotis Oge to EPA

Regional Air Division Directors re: “Policy Guidance on the Use of MOBILE6 for SIP Development

and Transportation Conformity.”

The EPA recognizes that for some areas, States are required to submit revisions to their SIP to

account for the new MOBILE6 mobile source emissions model on a schedule earlier than that required

for the mid-course review. SIP revisions that revise interim MOBILE5 Tier 2 estimates with

MOBILE6 are not intended to duplicate any technical analyses completed for mid-course reviews in

those areas. The MOBILE6 SIP and budget revisions are primarily intended to revise the motor

vehicle emissions inventories with the new model. Although the overall SIP must continue to

demonstrate attainment or maintenance with these revised MOBILE6 inventories as described in

question 5 of the guidance memorandum, “Policy Guidance on the use of MOBILE6 for SIP

Development and Transportation Conformity,”8 for areas completing mid-course reviews, EPA

believes that new attainment modeling or additional control measures to ensure attainment may be

delayed until the mid-course reviews. EPA will work with these States on a case-by-case basis to

decide what additional documentation is necessary to show that the MOBILE6 SIP revision

demonstrates attainment.

If the State cannot demonstrate that the SIP shows attainment with the revised MOBILE6

inventories as described in question 5 of EPA’s guidance on use of MOBILE6, the State can submit an

enforceable commitment to do one of the following in its mid-course review: 1) submit additional

measures needed to fill any emission reduction shortfall in the SIP at the time of the submission of the

mid-course review; or 2) document that there is no emission reduction shortfall as demonstrated

through the mid-course review. Such a commitment would be submitted as part of the MOBILE6 SIP

revision, and this commitment is necessary for EPA to find the revised MOBILE6-based motor vehicle

emission budgets adequate for conformity purposes.

If the time between the submission of the MOBILE6 revision and the submission of the mid-course

review is not sufficient for adoption of the additional measures needed as a result of the MOBILE6

revision, the State may submit the additional measures pursuant to these commitments as a separate

SIP revision as expeditiously as practicable after the submission of the mid-course review, but no later

than 12/31/04. Also, some areas may have committed to perform the MOBILE6 revision at the same

time as the MCR; these areas would be given a sufficient amount of time to adopt any measures needed

as a result of their MOBILE6 revision, but no later than 12/31/04.

CONCLUSION

The EPA believes that a mid-course review is an important element of the SIP planningimplementation-assessment-revision process. It is particularly important for cases where the attainment

demonstration relies on a weight of evidence demonstration.

The Regional Offices should provide this guidance to the appropriate State air pollution control

agencies.
12

cc: Air Program Managers, RO 1-10, Regional Meteorologists, RO 1-10, Tom Helms, Joe

Tikvart, John Silvasi, Sharon Reinders, Denise Gerth, Ellen Baldridge, Jan Tierney, Sara

Schneeberg, Kevin McLean, Meg Patulski

EPA:OAR:OAQPS:AQSSD:OPSG:JSILVASI\LLassiter:N. Campus C545E\1-5526\C-539-02 

File Name: I:\SEC\SILVASI\POLICYMEM33D.WPD March 21, 2002
Recommended Approach

For Performing Mid-course Review of SIP’s

To Meet The 1-hour NAAQS For Ozone

U.S. Environmental Protection Agency

Office of Air and Radiation

Office of Air Quality Planning and Standards

Emissions, Monitoring and Analysis Division

Research Triangle Park, NC 27711

January 2002
1

INTRODUCTION

In this paper, we identify several methods for reviewing whether a State is on track

toward attaining the 1-hour national ambient air quality standard (NAAQS) for ozone within

prescribed time limits. These methods may be used during a mid-course review (MCR). In

the event that a required attainment date occurs too soon to permit a mid-course review,

these methods may also be used to perform an early attainment assessment submitted by the

end of the attainment year. Given the fact that attainment determinations are based on three

years of data, the early attainment assessment is especially important for areas in which,

during the two years prior to the attainment year, the areas have registered violations or

continue to observe exceedances. An early assessment will help determine why the air

quality has not improved as expected and provide an early opportunity to develop additional

emission reductions. 

1.0 How Do I Determine If A SIP Is On Track Toward Attainment?

First we recommend an administrative review to determine whether the scheduled number

of measures called for in the SIP have been implemented. Then we recommend data analyses

to determine if it is likely that the State implementation plan (SIP) is on track toward

attainment. We recommend that analyses be performed to address several issues which need

to be understood to make a reliable judgment about whether a SIP is on track. Information

resulting from these analyses may be used in a weight of evidence determination to see if the

preponderance of evidence suggests attainment will occur within prescribed time limits. 

Analysis to support the MCR should make use of the most up-to-date guidance (e.g., EPA

1996 modeling guidance), analytical techniques, and models such as MOBILE6, BEIS3 and/or

NONROAD. In many cases this will require an assessment of the impact changing from one

analytical technique or model to another will have on the original SIP analysis results.

1.1 Perform Administrative Review 

A State should document whether it has implemented the measures planned prior to the

date of the mid-course review. This determination will need to be confirmed by the

appropriate U.S. EPA Regional Office(s). A State also should show whether

administrative/legal prerequisites are in place to implement previously agreed to measures

prior to the required attainment date.

If a State’s implementation plan relies on regional control measures, for a MCR to be

productive, a substantial portion of these need to have been implemented prior to the most

recent ozone season in the nonattainment area for which the mid-course review is being

performed. For example, if NOx SIP call measures are implemented by Spring, 2004, and this

constitutes an important part of the strategy for meeting the NAAQS in a particular

nonattainment area, the mid-course review should include data from the Summer 2004. 
2

Further to facilitate a productive review, a substantial portion of a State’s local control

measures needs to have been put in place prior to the most recent ozone season preceding the

mid-course review. Unless substantial progress has been made implementing local and

(where necessary) regional measures, it is unlikely that the signal resulting from

implementation of a SIP will be sufficiently strong to permit a meaningful mid-course analysis.

1.2 Analyze Available Air Quality, Meteorological, Emissions And Modeling Data

These analyses should address several key issues to help judge whether a SIP is on track

toward attainment. 

(1) Taking estimated changes in precursor emissions and meteorological factors affecting

air quality into account, do measured air quality and measured changes in air quality

observed between the time of the mid-course review and earlier suggest improvement at a

sufficient rate so that attainment is likely by the required date? Will the remaining

measures provide a rate of air quality improvement that is consistent with attainment by

the attainment date?

(2) To what extent does transport of ozone and/or precursors from areas upwind of a

nonattainment area account for air quality in the nonattainment area subject to the midcourse review? 

(3) Is progress toward meeting the ozone NAAQS (reflected by changes in air quality

normalized for meteorological differences) consistent with relative progress toward the

ozone NAAQS predicted earlier with air quality models?

Prior to performing an analysis to address one or more of the issues, indicate why the

particular analysis is relevant for making a judgment on whether the SIP is on track. In

addition, prior to performing each analysis, identify an outcome which is consistent with a

hypothesis that the SIP is on track toward attainment.

It takes time to perform analyses needed to address these issues. Therefore, we

recommend that 6 months to a year prior to the time a mid-course review is due, States should

identify analyses and data bases which will be used to support a mid-course review and

discuss these with the appropriate U.S. EPA Regional Office. Analyses to address each of

these issues are described more fully in Section 2.0.

1.3 Perform A Weight Of Evidence Determination 

A weight of evidence determination consists of a review of the results of the analyses

mentioned in Section 1.2. Analysis results for each of the three identified issues should be

considered. Make a determination on how the preponderance of information from each set of

results are consistent with the hypothesis that the SIP is on track toward attainment. If, for

some reason, it is not feasible to address one of these issues, document the reasons why not. 
3

Include the following documentation for the weight of evidence determination used in the

mid-course review:

- descriptions of the data used to support each analysis;

- identification of air quality models, statistical models or analytical techniques used;

- outcome of each analysis and whether it is consistent with the hypothesis that the SIP

is on track;

- assessment of the credibility of each type of analysis used in the mid-course review;

- a narrative describing the rationale used to conclude that the weight of evidence

supports or does not support a conclusion that the SIP is on track. 

The mid-course review should make use of the most up-to-date analytical techniques and

models such as MOBILE6, BEIS3 and/or NONROAD. In many cases this will require an

assessment of the impact changing from one analytical technique or model to another will

have on the original SIP analysis results.

2.0 What Types Of Analysis Should I Consider To Address Key Issues In The Mid-course

Review?

In this section, we identify several means of analysis for addressing the issues identified

in section 1.2. States should perform the set of analyses that is most appropriate based on

available data bases for the area under review. 

(1) What do air quality trends (normalized for meteorological differences) at midcourse review suggest about the likelihood of meeting the NAAQS on time?

(2) What does aerometric analysis of the role of transport vs. local emissions suggest

about the likelihood of meeting the NAAQS?

(3) Are aerometric data observed at mid-course review time consistent with modeled

projections used earlier to conclude that the most recent SIP revision is sufficient to

meet the NAAQS on time?

2.1 Use Air Quality Trends To Assess Whether A SIP Is On Track Toward Attainment

These analyses require decisions to be made about four factors to make well informed

judgments about whether timely attainment is likely: (1) choosing one or more trend

parameters, (2) adjusting (i.e., normalizing) observed ozone trends in the selected trend
4

parameters for meteorological differences, (3) deciding whether ozone trends are likely to be

most responsive to changes in VOC or NOx emissions or changes in both, and (4) selecting

the area for which trends in emissions will be estimated.

Choosing trend parameters. There are several things to consider when choosing trend

parameters in a mid-course review. Closeness of the parameter to the definition (i.e., form) of

the NAAQS, susceptibility to extreme or unusual meteorological conditions, stability of the

parameter, dependence on the number of sites with measured values. Table 2.1 presents

some potential trend parameters along with comments about the suitability of each. Table 2.1

is not comprehensive. Other trend parameters may be used, so long as a rationale for doing

so is presented. Since trend parameters could behave differently from one another, it is

advisable to select several parameters to get a better overall sense of how air quality is

changing. 

Table 2.1. Some Candidate Ozone Trend Parameters

Measure Comments

# of exceedances (monitor with most

exceedances)

Identity of monitor may change; closest

measure of the NAAQS, however number of

exceedances may often be small and

reflects use of a step function, making this

an unstable trend parameter.

Ozone design value (monitor with highest 4th

high hourly daily maximum over a 3-year

period)

Close to definition of NAAQS & more

stable than # of exceedances, however the

design value could be dominated by a single

year and could be sensitive to

meteorological differences.

Total number of exceedances (all monitors

in nonattainment area or nearby)

More stable than similar measure for a

single monitor, but still likely subject to

large annual fluctuations.

Highest 2nd high daily maximum ozone

concentration observed each year

Avoids problem of one bad year dominating

for 3-years, but still subject to large annual

fluctuations.

Highest running average 2nd high daily

maximum ozone concentration, averaged

over 3 consecutive years

Somewhat more stable, but diverges more

from the form of the NAAQS.

Highest 95th percentile daily maximum

ozone concentration 

More stable, but differs from the form of the

NAAQS.
5

Total number of hours with observed

concentrations greater than 124 ppb

Differs from form of NAAQS, may be

subject to some of the same problems as

parameters dealing with number of

exceedances, but likely less so.

In addition to ozone trend parameters, States with data from Photochemical Air

Monitoring Stations (PAMS) and/or reliable NOx measurements can consider trends in

precursor concentrations to see whether they appear to track emission control programs

which have been implemented. Use of PAMS data to estimate trends is described by Main

and Roberts (2000). Because precursor data may be subject to larger fluctuations than ozone

concentrations, we suggest using trend parameters like median or mean concentrations

observed during the summer (or ozone season) months. Since some of these data are not

likely to be measured continuously, mean values measured at a specific time of day (e.g., 6-9

am) may need to be used. The standard deviation in the chosen precursor trend parameter

may also be useful to compute to get a sense of how variable precursor concentrations are

during the course of each ozone season. Comparing this intra-annual variability with trends in

mean or median observations may provide a sense of the strength of the trend evidence

resulting from the precursor measurements. 

Normalizing observed trends in ozone for meteorological differences. Generally, the trend

parameters we identify for ozone reflect the fact that the NAAQS focuses on extreme values. 

Year to year fluctuations in meteorology may have a more pronounced effect on extremes

than on concentrations in the middle of a range of observations. Thus, in order to be able to

interpret observed changes in selected trend parameters, it is necessary to adjust the trend so

that it reflects differences in meteorology. Any credible procedure for adjusting air quality

trends for meteorological differences is acceptable. Several procedures for doing this are

reported in the literature.

Cox, et al. (1993) describe a method which has been used to adjust daily maximum ozone

concentrations for severity in meteorological conditions during any particular year. The

methodology develops a regression relationship between daily maximum ozone (dependent

variable) and several meteorological variables. This relationship defines each day’s ozone

forming potential. By going back over a long period of record (e.g., 40 years), severity of

meteorological conditions accompanying any given incident of high ozone can be characterized

in terms of climatological norms. This information can be used as a basis for adjusting the raw

observed air quality values. Use of the Cox, et al. methodology is illustrated in U.S. EPA

trend reports, as well as in U.S. EPA (1996).

Another method for adjusting observed trends is use of the Classification of Regression

Tree (CART) analysis procedure, as described in Dueul, et al. (1996). The widely-used

CART procedure identifies combinations of meteorological parameters which frequently

appear to coincide with high observed ozone concentrations. Noting differences in the
6

frequency of favorable meteorological combinations provides a means for considering

meteorological differences as a mitigating factor in observed ozone trends.

More recently, Milanchus, et al. (1998) and Hogrefe, et al. (2000) have described a

procedure which is able to suppress short term variability in observed ozone attributable to

diurnal and synoptic differences (i.e., general differences over a wide area or seasonal

variations) so that underlying base level trends can be discerned.

Perhaps the simplest procedure for normalizing an observed trend for meteorological

differences is to fit a curve through observed values of a trend parameter over the 10 years

preceding the mid-course review. In the following discussion, this is illustrated for the

maximum observed ozone design 9across all monitors in the area) value in each year. The

fitted curve is used in concert with observed maximum design values in each year to calculate

residual values for each year (i.e., the difference between the design value indicated by the

fitted curve and the actual observation). These residuals may be used to account for

variability in meteorology as described in the following paragraphs.

To illustrate, Table 2.2 shows hypothetical data for a 10-year period.

Table 2.2. Example Of Best Fit Design Values And Residuals

Year Residual, ppb

(Absolute Value)

Best Fit Design

Value, ppb

r/(best fit)

1995 12 185 .06

1996 6 185 .03

1997 10 179 .06

1998 9 166 .05

1999 8 166 .05

2000 7 155 .05

2001 9 153 .06

2002 0 148 0

2003 7 137 .05

2004 7 134 .05

MEAN = 0.046
7

This says that, on average, an area’s observed maximum design value for a given year varies

by about 4.6% from a best fit design value based on a review of 10 years’ data. So as not to

connote greater precision than warranted, we recommend rounding this variability upward to

the next whole integer. Thus, in this example, so long as the most recent observed design

value is not more than 5% above a pre-determined air quality target for the year of the midcourse review (2004 in this example), this factor would support a conclusion that the SIP is on

track. 

Deciding whether ozone is sensitive to reductions in VOC, NOx or both. This step is

needed in order to judge whether to pay most attention to changes in VOC emissions, NOx

emissions or changes in emissions for both precursors to make a judgment about whether an

observed (adjusted) trend in ozone is likely to continue at a sufficient pace to meet the

NAAQS within prescribed times. Since ozone’s sensitivity to changes in VOC, NOx, or both

emissions may vary from one location to another, and from one date and time to another, the

approach selected must consider a composite of results from several days and locations

throughout the area. 

There are at least two approaches which may be used to judge whether an estimated

change in emissions of VOC, NOx or changes in both should be tracked when interpreting

normalized trends in ambient ozone concentrations. A State may use another approach if it

presents sufficient justification for doing so to the appropriate U.S. EPA Regional Office. If it

is not clear whether an ozone problem is limited by available VOC or NOx emissions, a State

should estimate changes in emissions for both precursors.

Use results of model sensitivity tests

Air quality model applications may be used to indicate whether future reductions in ozone

are most likely to result from reductions in VOC emissions or NOx emissions. If the results

are ambiguous (e.g., differ for different modeled days), then the State should assume that it is

necessary to track projected reductions in emissions of both precursors. For purposes of a

mid-course review, simulations of uniform reductions in anthropogenic VOC emissions and

uniform reductions in anthropogenic NOx emissions may be performed to assess which

precursor to focus on when assessing likelihood of future attainment. The preferred approach

for doing this is to use a photochemical grid model. However, if it appears that an area’s

nonattainment problem is dominated by local emissions and/or there is another means for

considering effects of ozone/precursors transported regionally into the nonattainment area,

the OZIPP/EKMA approach may be used for judging whether local changes in VOC or local

changes in NOx should be tracked to understand observed normalized trends in ozone. 

Methods for judging potential significance of transport are discussed in Section 2.2.

It may happen that conclusions reached concerning which of the two precursors to track

depend on the size of the modeling domain used in the sensitivity tests. For example, if a

regional modeling domain is used, a State might conclude that predicted ozone is most
8

sensitive to changes in NOx emissions. In contrast, if an urban scale domain is used (e.g., < ~

300 km on a side), results might suggest future changes in VOC emissions are more

important. In fact, a finding like this is important if the analysis described in Section 2.2

suggests transport is an important part of an area’s nonattainment problem. It implies that

local changes in VOC emissions along with regional changes in NOx emissions should be

tracked in order to interpret the meaning of observed normalized trends in ambient ozone.

Use measured ambient precursor data 

Ratio of indicator species. The indicator species approach is described at length in such

references as Sillman (1995), Sillman (1998), Lu, et al., (1998) and Blanchard, et al., (1999). 

The approach relies on past modeling simulations (performed by others) which have noted

that predicted reduction in daily maximum ozone is sensitive to reductions in VOC if the

predicted ratio of certain indicator species (e.g., O3/NOy) is low, but is sensitive to reductions

in NOx if the ratio is high. If measurements of indicators exist, a State can review these data

on days where monitored ozone exceeds 0.12 ppm, to note whether the ratio is in the VOCsensitive or NOx-sensitive range. There is generally also a range of values for the ratio in

which it is unclear whether maximum observed ozone is limited by availability of VOC or

NOx. If the observations fall in this latter range, it would be necessary to estimate future

changes in both NOx and VOC emissions in order to interpret whether the ozone trend data

suggest timely attainment is likely.

Trends in ambient precursor concentrations. If a downward trend in ambient ozone

concentrations is accompanied by changes in one precursor (e.g., VOC) but not the other, this

may be considered in judging which precursor is likely affecting observed changes in ozone.

Deciding on the area for which to estimate past and future changes in emissions. In order

to decide whether a nonattainment area is on track toward attainment, a State needs to (1)

review available normalized ozone trend data, (2) review changes in emissions accompanying

these observed trends, (3) note the reduction in ozone still needed to meet the NAAQS, (4)

estimate the corresponding additional reductions in emissions needed to realize the needed

additional reduction in ozone, and (5) compare additional emission reductions provided for in

the SIP revision with the necessary additional reductions estimated as necessary in (4). 

Earlier, we discussed how a State may determine whether it needs to focus on VOC

emissions, NOx emissions or both. However, it is also necessary to define the area for which

past and future changes in emissions should be estimated. The nonattainment area is one

such area for which States should estimate changes in emissions. If a State believes that

regional transport is an important factor affecting observed exceedances of 0.12 ppm,

changes in VOC and/or NOx emissions also need to be calculated for an additional, larger

geographic area. To determine how large this area should be, we recommend that States note

every day with one or more observed exceedances within the nonattainment area between the

time of the most current SIP revision (e.g., 1999) and the time of the mid-course review (e.g.,
9

2004). For each day, states should compute a back trajectory for 36 hours, beginning at the

time and site with the highest observed exceedance. For this purpose, we recommend using

the HY-SPLIT model developed by NOAA (NOAA, 1999). However, other peer reviewed

trajectory models may also be used. This analysis will yield a bundle of 36-hour backtrajectories. States should use the smallest geographic area for estimating emission changes

which is consistent with including 90% of the total hours represented by the trajectories (i.e.,

90% of 36 hours times number of days with exceedances). This example assumes that most

of the impact at a monitoring site occurs as a result of emissions occurring within 36 hours’

travel time, and provides a means to avoid having to consider a very large area on the basis

of a small number of trajectories. Periods differing from 36 hours may be considered if

justified on a case-by-case basis.

2.2 Use Air Quality Data To Assess Relative Importance Of Regional Transport And

Local Emissions

States may use this analysis to determine whether to be concerned with past and

projected changes in emissions, (1) only within the nonattainment area , or (2) within a larger

geographical area (including the nonattainment area) as well. The analysis consists of several

steps:

(1) determine the resultant wind direction for each day with a monitored exceedance;

(2) identify monitors which are upwind and downwind on each day with an observed

exceedance;

(3) select air quality measures to consider at upwind and downwind sites;

(4) compare and note differences in upwind and downwind ozone as well as trends in

upwind and downwind ozone;

(5) use the information developed in (4) to decide whether it is sufficient to consider

only emission changes occurring within the nonattainment area or whether a larger

geographical area should also be considered.

Determining resultant wind direction. This may be done in one of two ways: (a) using

surface wind velocity measurements, or (b) through use of a meteorological model. If surface

wind velocity measurements are used, the resultant wind direction should be estimated solely

on the basis of daytime measurements (e.g., 7am-8pm) made on each day with a recorded

exceedance. If the resultant wind direction is estimated using a trajectory or other

meteorological model, a 24-hour period (ending at the time of the observed exceedance) may

be considered, so long as an effort is made to account for overnight wind shear.

Identifying upwind and downwind monitors. This step is used to identify which monitored

data are likely to be most impacted by emissions in the nonattainment area. Using previously

estimated resultant wind directions, a group of monitors is identified as downwind and another

group is identified as upwind. To illustrate, for a nonattainment area characterized by large

concentrations of emissions in the center of the designated nonattainment area, downwind
10

sites on a day might be characterized as those included within a relatively large arc (e.g., 120o)

whose vertex is at the center of the emissions and whose axis is the resultant wind direction. 

All other sites in this example would be upwind sites. 

Select air quality measures to consider at upwind and downwind sites. Since the identity

of downwind and upwind sites is a function of the resultant wind direction, individual

monitoring locations may be upwind on some days and downwind on others. Thus, to assess

relative importance of transport, it is more appropriate to look at concentrations and trends

for the group of upwind vs. the group of downwind sites rather than focus on all observations

at individual sites. We recommend looking at measures such as the highest, second highest

and spatially averaged daily maximum observed ozone concentrations at downwind vs. upwind

sites on individual days, as well as values for these variables which have been averaged over

an ozone season. In addition, we recommend looking at trends in these parameters observed

over the period between the SIP revision and the mid-course review and, perhaps, over a

longer period stretching back 10 years from the mid-course review. 

Compare absolute ozone concentrations and trends at upwind and downwind sites. The

purpose of this step is to contrast ozone concentrations and trends in ozone concentrations for

each of the air quality measures selected in the previous step. This comparison is used to

judge whether transport is an important factor affecting a nonattainment area’s attainment

status. Large contrasts between upwind and downwind sites suggest that local emissions are

an important factor affecting the area’s attainment status. A mixed set of results is probably

consistent with both local emissions and transport playing a significant role. If results for

upwind and downwind monitors are nearly identical, this suggests that an area’s

nonattainment status is dominated by regional transport.

Determine which area(s) should be used to characterize estimated changes in emissions. 

If there are consistent, major contrasts between downwind and upwind sites, States may focus

their emission estimates on those emissions which occur within the nonattainment area. If

there is a contrast using some measures, but a less apparent or small difference using other

measures, States should focus emission estimates on the nonattainment area and on a larger

geographical area determined as described in the last paragraph in Section 2.1. If there are

only small or indiscernible differences between measures at upwind and downwind sites,

States should focus their emission estimates on a larger geographical area, determined as

described in Section 2.1.

2.3 Use Air Quality Model Estimates

In Section 2.1, we noted that air quality models may be used in a relatively non-resource

intensive fashion to determine whether the mid-course review should focus on changes in

VOC emissions, changes in NOx emissions or changes in both. In Section 2.3, we describe

other uses of model-generated results which we believe would increase the credibility of a

mid-course review.
9

 If per chance States have conducted regional scale modeling for 2002, we recommend

consideration of this information, especially it’s viable use in assessing the role of transport.

11

Using model projections as a target against which to compare air quality observations. 

Model results, obtained prior to the mid-course review, may be used to check whether

changes in air quality observed at the time of the mid-course review are consistent with likely

attainment. We suggest the following procedure. Model most recent available emissions

present during a base period (i.e., 1996 emissions 9), projected emissions for the year of the

mid-course review (e.g., 2004 assuming the mid-course review is to be completed by the end

of 2004) and emissions projected for two years prior to the required attainment date (e.g.,

2005 for a 2007 attainment date). Select a modeled air quality metric to use as predicted

values corresponding to each of these three emission scenarios. For the 1-hour ozone

NAAQS, this metric would ordinarily be the highest 1-hour daily maximum ozone

concentration predicted within the non-attainment area(s) or at locations clearly impacted by

the area(s) being reviewed. Other metrics can be considered on a case by case basis. 

Next, note the relative progress made by 2004 in reducing the predicted metric from its

value corresponding to 1996 emissions to its value corresponding to 2005 emissions. This

relative modeled progress is the value against which observed progress in reducing the area’s

monitored design value (or other selected measure) is compared. The procedure for using

modeled results as a means for assessing whether a monitored design value indicates a SIP

revision is on track toward attainment is illustrated in the following example.

EXAMPLE

Given: An area’s average maximum monitored design value for 1994-96, 1995-97 and 1996-

98 is 155 ppb, and the observed design value in 2004 is 136 ppb. Using a curve fitting

technique like that described in Section 2.1, variability in the maximum monitored design

value attributable to fluctuations in meteorology has been determined to be 5%. Modeling

results available prior to the mid-course review show predicted peak 1-hour daily maximum

ozone predictions of 150, 139 and 131 ppb corresponding to 1996, 2004 and 2005 emissions.

Find: Is the SIP revision on track toward attainment of the 1-hr NAAQS for ozone by 2007?

Solution:

(1) Calculate the relative modeled progress predicted for 2004.

Rel.modeled progress = ((150 - 139)/(150 - 131)) x 100 = 58%

(2) Apply this relative progress to the average maximum design value for the three 3-year

periods including the year of the current inventory. Thus, if the base inventory reflects
10We recognize that the design value in 2004 may not accurately reflect current air quality at

that time. The design value at a monitoring site is ordinarily the 4th highest daily maximum concentration

observed over a continuous 3-year period. However, if a major change in emissions occurs during this

3-year period (e.g., in Spring, 2004) the 4th highest value may well be observed before these changes

(e.g., in 2002 or 2003). If none of the observed top 4 daily maximum ozone concentrations at a

monitor occurs in 2004, we also recommend that States compare the observed 2nd high daily

maximum concentration observed in 2004 with the mid-course review target, and present supporting

information that 2004 is not an unusually low year due to benign meteorological conditions.

12

emissions from 1996, the average maximum design value is determined using the maxima

observed in 1994-96, 1995-97 and 1996-98. For purposes of illustration, let us assume this

average maximum design value is 155 ppb. The target for the year of the mid-course review

projected inventory (i.e., 2004 in this example) is derived using the expression below. 

Mid-course review target = 155 ppb - (155 ppb - 124 ppb) ( 0.58) = 137 ppb

So long as this value (137 ppb) is not 5% or more below the observed air quality design value

in 2004, the SIP is on track toward attainment.10 Earlier, we noted that the observed current

maximum design value is 136 ppb. We note that 95% of this value is 129 ppb. Clearly, the

predetermined target of 137 ppb is not 5% or more below the observed design value (136

ppb). Thus, in this example, the SIP is on track toward attainment.

Estimating relative importance of regional transport vs. local emissions. States should

review results of past model applications which may be relevant for addressing this issue.

Applications in which regional control measures were added to previously simulated locally

applied control measures may be germane. Alternatively, applications in which locally applied

control measures are superimposed over previously simulated regional measures may be

reviewed. Comparing effects of regional/superimposed on local measures vs.

local/superimposed on regional measures may provide further insight into the relative

importance of regional transport vs. local emissions as contributors to a nonattainment

problem. 

3.0 Using Weight Of Evidence To Estimate If A SIP Is On Track

In this Section, we describe how to combine information from the previous analyses to

determine if the weight of available air quality monitoring, meteorological, emissions and

modeling evidence supports a hypothesis that a SIP is on track toward attainment.

3.1 Compute Normalized (For Meteorology) Trend For Ozone Within And Nearby A

Nonattainment Area
13

Select appropriate ozone trend parameters. If available, use robust measures of ambient

precursor trends to help confirm emissions reductions. Normalize ozone trends for

meteorological differences using a curve fitting approach like the one described in Section 2.1

or other approaches like the Cox, et al. (1993), CART or filtering approach described by

Milanchus, et al. (1998).

3.2 Compare The Observed Trend With A Target Consistent With Attainment By The

Prescribed Date

This comparison may be performed in several ways. 

Comparison with photochemical grid model estimates

For example the first way may be to compare the monitored observations with modeled

estimates previously obtained for a base period, mid-course and attainment date. This

method is illustrated in Section 2.3. If a fitted curve and residuals are calculated, actual

observations may be compared with model estimates, as described. If another method is used

in which residuals are not estimated, the normalized, rather than actual observations should

be compared with the target. Use of model results for this purpose is advantageous, because

it does not require a State to make a number of explicit assumptions about which emissions

trends to consider or to make assumptions about the role of transport. The disadvantages are

resources needed to do the modeling and the likelihood that the model results will be based on

a relatively limited sample (i.e., few days).

Using proportional extrapolations

If new model results are unavailable to establish a target for the mid-course year which is

consistent with attainment, a proportional approach may be needed to assess whether it is

likely that a SIP is on track. This approach uses ozone/emission sensitivity factors from prior

modeling or observed correspondence between normalized ozone trends (which ever is most

appropriate) at mid-course review time with net estimated reduction in emissions between the

SIP’s base period (e.g., 1999) and mid-course review time (e.g., 2004). Either approach may

be used to estimate a unit sensitivity factor of ozone to emission changes.

Unit Sensitivity = (ÄO3 / Ä(emissions)) (1)

The preceding information may be used in concert with (1) information about the net reduction

in emissions anticipated between time of the mid-course review and the attainment date, and

(2) the remaining difference between the design value at mid-course review time and 124 ppb

to estimate if attainment is likely by the required time. If the method suggests attainment is

likely by the required time, the SIP is assumed to be on track toward attainment. This is

shown by the expression below.
14

(O3)attainment = (O3)@mid-course review + (Unit Sensitivity) (Ä(future emissions)) < 124 ppb

where 

(O3)attainment is the estimated ozone design value for the attainment year.

(O3)@mid-course review is the normalized design value at mid-course review.

Ä(future emissions) is the change in emissions anticipated between mid-course review time

and the required attainment date—note that this is generally a negative number. 

Note, if the estimated ozone design value for the attainment year is greater than 124 ppb,

the “Unit Sensitivity” parameter may be used to estimate the level of emission reductions

needed to achieve 124 ppb. However, since this is a linear extrapolation, the farther the

estimated ozone design value is from 124 ppb the less reliable these results are. 

For the proportional extrapolation methodology to be applied, it is necessary to make a

series of assumptions about which emissions to consider in the preceding expressions. We

make recommendations about this in the following subsection.

3.2.1 Which Emissions Trends Should I Use In The Proportional Extrapolation

Technique?

First, use the analysis of transport described in Section 2.2 and results of available 

modeling, described in Section 2.3 to assess whether transport is a major reason for

nonattainment of the NAAQS in the designated nonattainment area being reviewed. If it is

not, consider only emission trends within the nonattainment area. If transport and local

emissions both appear to play important roles, then consider emission trends in the

nonattainment area and emissions trends in a larger geographic area, determined using a

method like the one described near the end of Section 2.1. If transport seems to clearly

dominate local emissions, consider only the trends in the larger geographical area.

Next, determine which emissions trends to consider. This requires two questions to be

addressed: (1) do I consider VOC, NOx or emissions of both? and (2) what trend parameter

should I use to characterize the change in emissions? To address the first question, use

available urban scale modeling or ambient precursor data (Section 2.1) to identify the

precursor of principal concern in the local (i.e., nonattainment) area. Use results from past

regional modeling analyses to identify the precursor of principal concern in the larger,

regional area. If it is not clear from the available information which precursor is the

controlling one in the local area or the regional area, consider trends for both precursors in

the area(s) for which this is unclear.
15

To address the second question, we recommend using seasonal mean emissions estimates. 

This, in effect normalizes year-to-year emissions for meteorological differences. Further,

uncertainties in estimating emissions factors and activity levels for specific days are likely to

be sufficiently large so as not to warrant this level of detail.
16

3.2.2 Some Concerns And Caveats

We have several concerns about the proportional extrapolation approach. Thus, for 

various reasons, we recommend that States use results from photochemical grid models to

establish targets for comparison whenever feasible. First, the relationship between ozone

formation and precursor emissions can be a non-linear process under a number of

environmental conditions. Linear approximations work best if the current ozone (i.e., at midcourse review time) is close to 124 ppb. If this difference is more than, say, 5 ppb, States

should consider developing an EKMA isopleth diagram to ensure that there is no intervening

ridge line between the current and desired air quality if a given precursor is reduced. 

Presence of a ridge line indicates that reductions of one precursor may result in little or no

change in ozone while reductions in the other precursor may result in large changes in ozone. 

Another analysis which might be performed to increase the weight of evidence produced by an

extrapolation methodology is to review correspondence between observed air quality trends

and estimated emissions trends to see whether something other than a linear curve provides a

better description of this correspondence. An extrapolation methodology which is analogous

to the one described herein (but reflecting a non-linear correspondence) could be applied. 

Second, the extrapolation requires one to make previous assumptions about what kind of

emissions are important determinants of observed ozone, as well as where these emissions

are located. It is unlikely that these assumptions will be as reliable as those made with

models, as the models are better able to consider meteorological factors which influence the

importance of emission configurations on ozone concentrations. 

4.0 Summary

States should first do an administrative review to determine whether the scheduled

number of measures called for in the SIP have been implemented and that

legal/administrative prerequisites are in place to implement the remaining measures identified

in the SIP revision. If these targets are not met, there is cause for concern that the SIP is not

on track toward attainment.

States should also perform a series of analyses to ascertain trends in ozone and

precursors, emission trends, relative importance of transport vs. local emissions and to review

modeling efforts to identify ozone-limiting precursors and the role of transport. 

A weight of evidence determination may be performed to assess the likelihood that a SIP

is on track toward attainment. Two example analytical methods are provided and are

recommended for use as part of the weight of evidence determination to see if observed air

quality at the time of the mid-course review meets a target considered to be consistent with

meeting the NAAQS in a timely manner. The first and preferred approach is to use modeled

results to establish a target at the time of a mid-course review which is consistent with

ultimate attainment of the NAAQS. The second approach is to use proportional

extrapolations based on past observed correspondence between air quality and emissions and
17

anticipated future changes in emissions. Other factors that may be considered in the weight

of evidence analysis include consideration of the assumptions and methods underlying the two

analytical approaches or other analytical approach used and use of refinements which appear

more appropriate based on data and analyses performed for a specific area.

For a mid-course review to be acceptable, a State should include the following

documentation:

- results of the administrative review;

- descriptions of the data used to support each analysis;

- identification of models and analytical techniques used to perform the analysis 

- outcome of each analysis and whether it is consistent with a conclusion that the SIP is

on track;

- a narrative describing the rationale used to conclude that the weight of evidence

supports or does not support a conclusion that the SIP is on track. 

5.0 Cited References

Blanchard, C.L., F.W. Lurmann, P.M. Roth, H.E. Jeffries and M. Korc, (1999), “The Use of

Ambient Data to Corroborate Analyses of Ozone Control Strategies”, Atmospheric

Environment 33, pp.369-381.

Cox, W.M. and S.H. Chu, (1993), “Meteorologically Adjusted Ozone Trends in Urban Areas:

A Probabilistic Approach”, Atmospheric Environment 27B, (4), pp.425-434.

Deuel, H.P. and S.G. Douglas, (1996), Regional Ozone Patterns in the Eastern U.S. 1985-

1995: Statistical Pattern Analysis and Classification of Regression Tree (CART)

Analysis in Support of the Ozone Transport Assessment Group (OTAG) Modeling Effort,

Prepared for Southeast Modeling Center by Systems Applications< Inc., SYSAPP-96/50,

San Rafael, CA., (October 1996).

Hogrefe, C., S.T. Rao, I.G. Zurbenko and P.S. Porter, (2000), “Interpreting the Information in

Ozone Observations and Model Predictions Relevant to Regulatory Policies in the

Eastern United States”, Accepted for publication in Bulletin of the American

Meteorological Society.

Lu, C.H. and J.S. Chang, (1998), “On the Indicator-Based Approach to Assess Ozone

Sensitivities and Emissions Features”, J. Geophysical Research 103, pp.3453-3462.
Main, H.H. and P.T.Roberts, (2000), PAMS Data Analysis Workbook: Illustrating the Use of

PAMS Data to Support Ozone Control Programs, Prepared for U.S. EPA, Office of Air

Quality Planning and Standards, Research Triangle Park, NC 27711. To be posted on

http:\\www.epa.gov/oar/oaqps/pams/

Milanchus, M.L., S.T. Rao and I.G. Zurbenko, (1998), “Evaluating the Effectiveness of

Ozone Management Efforts in the Presence of Meteorological Variability”, J.Air & Waste

Management Association 48, pp.201-215.

NOAA, (1999), HY-SPLIT model, available at www.arl.noaa.gov/ss/models/hysplit.html

Sillman, S., (1995), “The Use of NOy, H2O2 and HNO3 as Indicators or O3-NOx-ROG

Sensitivity in Urban Locations”, J. Geophysical Research 100, pp.14175-14188.

Sillman, S., (1998), Evaluating the Relation Between Ozone, NOx and Hydrocarbons: The

Method of Photochemical Indicators, EPA/600R-98/022. 

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