1

Technical Support Document for the Modeling and Weight of Evidence 

(WOE) Portions

Of the State of Delaware’s Ozone State Implementation Plan (SIP) 

Entitled “Delaware State Implementation Plan

For Attainment of the 8-Hour Ozone National

Ambient Air Quality Standard

Reasonable Further Progress and

Attainment Demonstration, June 2007”

Purpose of the Technical Support Document

This Technical Support Document (TSD) describes the Environmental Protection 

Agency’s (EPA’s) evaluation of the modeling and WOE portions of Delaware’s State 

Implementation Plan (SIP) revision entitled “Delaware State Implementation Plan for 

Attainment of the 8-Hour Ozone National Ambient Air Quality Standard Reasonable 

Further Progress and Attainment Demonstration, June 2007” (Delaware SIP revision for 

the Philadelphia area). Delaware is part of the Philadelphia-Wilmington-Atlantic City

ozone nonattainment area (Philadelphia area). The Philadelphia area is comprised of 18 

counties in Pennsylvania, New Jersey, Delaware and Maryland. Classified as a 

“moderate” nonattainment area, the Philadelphia area is required to attain the NAAQS by 

June 2010, in effect, by the ozone season of 2009. 

The purpose of this TSD is to provide more detailed information than can be contained in 

the official notice published in the Federal Register. Readers who need more information 

than we provide in this TSD or want to review the modeling in more detail should read 

the above referenced Delaware SIP revision for the Philadelphia area.

Introduction to the Delaware State Implementation Plan for Ozone

The Delaware SIP revision for the Philadelphia area includes plans to reduce ozonecausing emissions for Delaware’s portion of the Philadelphia Area. The Plan also 

includes modeling that will predict whether the area will meet the ozone standard by the 

due date. This TSD reviews modeling and WOE portions of the Delaware SIP revision 

for the Philadelphia area. This TSD reviews Delaware’s documentation of modeling and 

WOE that attempts to show attainment of the eight-hour ozone standard by the 2009 

ozone season. All of the states in the northeastern United States cooperated via the 

Ozone Transport Commission’s (OTC) Modeling Committee to prepare the modeling 

that was performed by the New York State Department of Environmental Conservation

(NYSDEC) and supporting organizations.

Delaware believes that their SIP modeling when combined with additional supporting 

analyses (Weight of Evidence) (WOE) “Demonstrates that the entire PhiladelphiaWilmington-Atlantic City, PA-NJ-DE-MD moderate non-attainment area will attain the 

8-hour ozone NAAQS in 2009”.
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What Are The Components Of A Modeled Attainment Demonstration?

Modeling Process Overview

The OTC Modeling Committee used the Community Multi-scale Air Quality model

(CMAQ) version 4.4 as its photochemical grid model. The model uses simulations of 

chemical reactions, emission of ozone precursors and a sophisticated meteorological 

model to produce ozone concentrations over the eastern United States. The 

meteorological data used in the meteorological model was for the base year ozone season 

of 2002. The photochemical grid model was run with the base year meteorology and 

base year emissions to determine if the model performance was satisfactory. Once the 

model performance was determined to be adequate, 2009 ozone concentrations were 

modeled by running the model with projected emissions for 2009 and the original 2002 

meteorology. The meteorology was held constant so that the results of changing the 

emissions would not be influenced by changing meteorology. The EPA modeling 

guidance1recommends that states use the modeled ozone concentrations in a relative 

sense and not rely solely on the concentrations the model predicts for the attainment year 

(2009). The relative response factor (RRF) used in the modeled attainment test is 

computed by taking the ratio of the mean of the 8-hour daily maximum predictions in the 

future to the mean of the 8-hour daily maximum predictions with baseline emissions, 

over all relevant days. The base year ozone design values at each monitor are then 

multiplied by the monitor-specific RRF to produce an attainment year design value. If 

the result is less than or equal to 84 ppb, at all locations, the modeling system will have 

predicted attainment of the ozone air quality standard. 

The Regulatory Framework

Section 110 (a) (2) (k) of the Clean Air Act requires states to prepare air quality modeling 

to show how they will meet ambient air quality standards. EPA determined that areas 

classified as ‘moderate’ or above must use photochemical grid modeling or any other 

analytical method determined by the Administrator to be at least as effective to 

demonstrate attainment of the ozone health-based standard by the required attainment 

date (40 CFR 51.908, published at 70 FR 71612 on November 29, 2005). In 40 CFR 

51.903, published at 69 FR 23951 on April 30, 2004, EPA specified how areas would be 

classified with regard to the eight-hour ozone standard set by EPA in 1997. At 69 FR 

23858, on April 30, 2004, EPA followed these procedures and classified the 

Philadelphia-Wilmington-Trenton, PA-NJ-DE-MD and ozone nonattainment area as 

moderate, so they must attain the 1997 eight-hour ozone standard by June 2010. Since 

the attainment date is June 2010 for moderate areas, states must achieve emission 

reductions by the ozone season of 2009 in order for ozone concentrations to be reduced, 

and show attainment, during the last complete ozone season before the 2010 deadline. 

The modeling guidance lists nine steps for preparing modeling to demonstrate attainment 

of the ozone standard. 

1 Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality Goals 

for Ozone, PM2.5, and Regional Haze, EPA -454/B-07-002, April 2007
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1. Develop a conceptual description of the problem to be addressed.

2. Develop a modeling/analysis protocol.

3. Select an appropriate model to support the demonstration.

4. Select appropriate meteorological time periods to model.

5. Choose an appropriate area to model with appropriate horizontal/vertical resolution

and establish the initial and boundary conditions that are suitable for the application.

6. Generate meteorological inputs to the air quality model.

7. Generate emissions inputs to the air quality model.

8. Run the air quality model with base case emissions and evaluate the performance.

Perform diagnostic tests to improve the model, as necessary.

9. Perform future year modeling (including additional control strategies, if necessary) and

apply the attainment test.

How Did Delaware Address All Of The Components Of A Modeled Attainment 

Demonstration?

The Delaware SIP revision for the Philadelphia area addresses each of the elements of a 

modeled attainment demonstration as follows:

Conceptual description of the problem

A conceptual model describes how weather patterns affect the formation and transport of 

ozone, accounting for emissions and photochemistry. The Delaware SIP revision for the 

Philadelphia area describes the climatology that produces high ozone concentrations in 

the Philadelphia area. On warm sunny days, winds from the southwest and west blow 

emissions from sources of ozone-forming chemicals both at the surface and aloft toward 

the Philadelphia area. In addition, emissions from large combustion sources in the Ohio 

Valley and other areas are brought eastward by upper level winds to the east coast, 

augmenting the ozone formed locally. A complete conceptual description of the ozone 

problem in the Philadelphia area is contained Delaware explains this in Section 7.7 of 

their SIP revision for the Philadelphia area.

Modeling/analysis protocol

The Ozone Transport Commission’s (OTC’s) Modeling Committee developed a protocol 

for modeling the ozone problem in the northeastern United States. The modeling 

protocol is located in Appendix 7-1 of the Delaware SIP revision for the Philadelphia 

area. It adequately describes how the photochemical grid model will be used in the ozone 

attainment plans. 

The Model Used in the Attainment Demonstration

By agreement of the OTC, NYSDEC ran the Community Multi-scale Air Quality Model 

version 4.4 (CMAQ) for the states in the northeast ozone transport region, including 

Pennsylvania. CMAQ is an acceptable model, listed in the photochemical modeling 

guidance as a currently used photochemical grid model. EPA agrees CMAQ is 

appropriate for this modeling demonstration. The inputs to the model are described in 

Section 7 of the Delaware SIP revision for the Philadelphia area.
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Meteorological Time Periods Used in the Modeling

Section 7.3 of the Delaware SIP revision for the Philadelphia area notes that the OTC 

Modeling Committee agreed to model the entire ozone season of 2002. Using the entire 

ozone season of 2002 covers many different kinds of ozone episodes and exceeds EPA’s 

recommendations for episode selection. 2002 was a good year to model because it was

the base year for the attainment plans and as a whole is representative of meteorological 

conditions conducive to elevated concentrations of ozone.

Meteorological Data Used in the Air Quality Model

The OTC Modeling Committee decided to use a prognostic meteorological model that 

provides life-like meteorological inputs to the photochemical grid model. The 

Pennsylvania State University/National Center for Atmospheric Research Mesoscale

Meteorological Model (MM5) version 3.6 was chosen for the modeling analysis. The 

MM5 model provides a reasonable representation of weather conditions at the surface 

and aloft. A summary of the MM5 application is contained in Section 7.4 of the 

Pennsylvania SIP revision for the Philadelphia area. 

Domain of the Model, Horizontal/Vertical Resolution and the Initial and Boundary 

Conditions

The modeling domain extends from Maine to Florida and out in the Atlantic Ocean on 

the east and west to the Mississippi River. The size of the modeling domain was made 

large enough to include all emission sources that affect ozone formation in the 

northeastern United States. Even this boundary is defined by a larger photochemical 

modeling domain that covers much of North America. Over the northeastern United 

States, the model used 12 kilometer grid cells. The Philadelphia area is included in the 12 

kilometer grid cell area. The OTC Modeling Committee used a 12-kilometer grid size for 

the areas in and near its states to provide a fine enough grid resolution to adequately 

capture the ozone patterns experienced in the ozone transport region (OTR). The OTC 

Modeling Committee chose to use a 12 kilometer grid resolution based on its states’ 

experiences with the one-hour ozone attainment modeling that indicated that modeling at 

finer resolutions than 12 kilometers would not result in increased model accuracy. 

Outside the local areas the grid resolution used in the modeling is 36 kilometers. The 

selection of model domains and horizontal grid resolution was deemed acceptable to 

EPA. The grid resolution, modeling domain and boundary conditions are described in 

Appendix 7-2 of the Delaware SIP revision for the Philadelphia area.

Vertical resolution is the number of layers and the size of each layer in the model. The 

layers in the photochemical grid model were set up to be compatible with the model that 

produced weather conditions for the photochemical grid model. The vertical grid 

resolution used in the modeling is described in Appendix 7-2 of the Delaware SIP

revision for the Philadelphia area. The vertical resolution used in the modeling exercise 

followed EPA’s modeling guidance and therefore adequately represents the atmosphere 

where ozone forms and is transported. 
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Emissions Used in the Air Quality Model

The emissions data for 2002 were generated by individual states within the OTR and 

assembled and processed through the Mid-Atlantic Northeast Visibility Union (MANEVU), a Regional Planning Organization (RPO). These emissions were then processed by

NYSDEC using the SMOKE emissions processor to provide CMAQ compatible inputs. 

The 2002 emissions for the non-OTR areas within the modeling domain were obtained 

from the corresponding RPOs and were processed using SMOKE, in a manner similar to 

that of the OTR emissions.

The OTR states, through MANE-VU, contracted MACTEC Federal Programs (called 

Contractor) to develop 2009, 2012 and 2018 inventories based upon 2002 inventories that 

the states had previously developed for use in the base-year model work. The Contractor,

in consultation with the states, developed the necessary growth and control factors and 

applied to the 2002 inventory. Emissions for mobile sources and the electric energy

generating units (EGUs) was not part of the Contractor’s effort. . To generate on-road 

mobile emissions, the states provided the Virginia Department of Environmental Quality 

(VADEQ) and the Northeast States for coordinated Air Use Management (NESCAUM)

with appropriate MOBILE 6 input files along with the projected vehicle miles traveled 

(VMT), which was coupled with hourly “gridded temperature” information. As for the 

emissions from the EGU sector, the inter-RPO work group utilized the Integrated 

Planning Model (IPM) to develop the state and unit-level emissions. These inventories 

are identified as 2009 on the way (2009OTW), since they reflect all emission control 

measures that were promulgated or would become effective on or before 2009.

Details of emissions processing are provided in Appendices 7-7, 7-8 and 7-9 of the 

Delaware SIP revision for the Philadelphia area.

Base Case Run Model Performance Evaluation

NY DEC performed a model evaluation for the OTC to determine how well CMAQ 

reproduced the Philadelphia nonattainment area’s 2002 ozone season concentrations. 

Model evaluation followed performance statistics outlined in EPA modeling guidance.

The OTC’s evaluation included performance statistics for all monitors inside the 

Philadelphia nonattainment area. The model performance evaluation results are 

presented in included in Appendix 7-6 the Delaware SIP. The analysis indicates the 

modeling system does an adequate job of estimating the eight-hour surface ozone 

concentrations throughout the Philadelphia area.

2009 Control Case Modeling and the Modeled Attainment Test

The photochemical grid model used with projected emissions for 2009, including 

emission changes due to regulations the states are planning to implement and expected 

growth by the 2009 ozone season. 

Meteorological conditions from 2002, the same as the base year modeling, were used in 

the 2009 modeling. Using the base case meteorology allows the effect of changes in 
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states’ emissions to be determined without being influenced by yearly fluctuations in 

meteorology and is consistent with EPA guidance. 

As described earlier in this TSD, the attainment test used in the Philadelphia area

modeling demonstration involved the application of model-based RRFs to base year 

design values at each monitor to produce projected future year design values (2009). The 

projected 2009 design values represent design values that should result from emission 

controls Delaware and other states planned to have in place in 2009. The 2009 design 

values should be less than or equal to 84 ppb at all monitoring stations to meet the 

attainment test2. The SIP modeling predicts that in 2009, Philadelphia area will not pass 

the attainment test since design values are projected to be over the 84 ppb standard (see 

Table 1 below). 

Table 1 Modeled vs. Monitored Design Value

Summary of Photochemical Grid Modeling Results

In summary, the basic photochemical grid modeling presented in the Delaware SIP 

revision for the Philadelphia area meets EPA’s guidelines and when used with the 

methods recommended in EPA’s modeling guidance, is acceptable to EPA. When EPA’s 

attainment test is applied to the modeling results, the 2009 ozone design value is 

predicted to be 91 ppb in the Philadelphia-Wilmington-Atlantic City, PA-NJ-DE-MD 

ozone nonattainment area. Thus, based on EPA’s modeled attainment test, Philadelphia 

area has not demonstrated that it will reach attainment of the ozone standard in the 2009 

attainment year with the modeled emission reduction strategies committed to by the OTC 

states. To make a case for attainment, Delaware decided to use WOE to demonstrate 

attainment of the ozone standard in the Philadelphia area. 

2 The guidance also states that additional analyses are recommended to determine if attainment will be 

likely, even if the modeled attainment test is “passed”. The guidance recommends supplementary analyses 

in all cases, and specifically recommends a WOE analysis if modeled ozone is greater than or equal to 82 

ppb.

Monitor Site ID Number 2009 Modeled DV

Fairhill - CECIL CO, MD 240150003 81

Colliers Mills - OCEAN 

CO, NJ

340290006 91

Rider - MERCER CO, NJ 340210005 86

Ancora State Hospital -

CAMDEN CO, NJ

340071001 87

Camden - CAMDEN CO, 

NJ

340070003 88

Clarksboro -

GLOUCESTER CO, NJ

340155001 88

Bristol - BUCKS CO, PA 420170012 88

Northeast Airport -

PHILADELPHIA CO, PA

421010024 87
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Weight of Evidence Demonstration

EPA=s modeling guidance describes how to use a photochemical grid model and 

additional analytical methods to complete a WOE analysis to estimate if emissions 

control strategies will lead to attainment of the eight-hour NAAQS for ozone. A WOE 

analysis is a supporting analysis that helps to determine if the results of the 

photochemical modeling system are correctly (or not correctly) predicting future air 

quality. 

The WOE portion of the Delaware SIP revision for the Philadelphia area describes the 

analyses performed, databases used, key assumptions and outcomes of each analysis, and 

why the evidence, viewed as a whole, supports a conclusion that the Philadelphia 

nonattainment area will attain the NAAQS despite the model prediction that some 

monitors’ future design values exceed the current eight-hour ozone standard.

Table 2 outlines under what circumstances a WOE demonstration is needed. Modelpredicted design values are summarized above in Table 1. Of the twenty-two (22) ozone 

monitors in the Philadelphia nonattainment area only eight (8) exceed the threshold (82 

ppb) where the EPA modeling guidance recommends a WOE demonstration. Three of 

the monitors fall within the 82-87 ppb threshold outlined in Table 1 and four others fall 

within the last category listed in the WOE table included in the US EPA guidance. The 

Roxboro monitor is excluded from the WOE analysis since its current design value is 

significantly lower than the eight-hour standard (modeled 82 ppb, actual monitored 78 

ppb).

The WOE analysis for the remaining seven monitors includes the following:

· A comparison of model-predicted 2009 ozone design values and monitored design 

values for 2006, 

· An analysis of recent ozone trends in the Philadelphia nonattainment area,

· Alternative methods for calculating the 2009 ozone design value,

· An analysis of model-predicted regional transport,

· University of Maryland’s analysis of model sensitivity to emission changes.

Table 2. EPA Guidelines for Supplemental Analyses and Weight of Evidence 

Determinations

Results of Modeled Attainment Test Supplemental Analyses

Future Design Value < 82 ppb, all 

monitor sites

Basic supplemental analyses should be 

completed to confirm the outcome of the 

modeled attainment test

Future Design Value 82 - 87 ppb, at 

one or more sites/grid cells

A weight of evidence demonstration 

should be conducted to determine if 

aggregate supplemental analyses support 

the modeled attainment test

Future Design Value > 88 ppb, at one 

or more sites/grid cells

More qualitative results are unlikely to 

support a conclusion differing from the 

outcome of the modeled attainment test.
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Modeled Concentrations and Current Design Values

Table 3 below lists the OTC modeled 2009 design values and the 2006 and 2007 design 

values and 2008 preliminary design values. The modeled 2009 and projected 2006 

design values are surprisingly close to one another with most modeled concentrations 

slightly lower than the projected 2006 design values. The Delaware SIP revision for the 

Philadelphia area states that the 2006 design values suggest additional reductions over the 

next three ozone seasons might bring several of the monitors that are currently just over 

the eight-hour ozone standard into compliance. If this is true, air monitoring data from 

2007 and 2008 should support Delaware’s conclusion, but it does not.

Table 3. Modeled vs. Monitored Design Values

Delaware did not have access to 2007 ambient data or preliminary 2008 ambient data 

when it wrote its SIP revision for the Philadelphia area. However, even at that time, 

information was available that showed a comparison between a 2006 design value with 

modeled 2009 results was biased toward an optimistic result because the summer of 2004 

was a much cooler than normal (according to the National Climatic Data Center, the 

summer of 2004 was ranked as the 20th coolest of the last 110 years in the Northeast). 

Data from 2005 and 2006 showed that the number of 4th high values over the ozone 

standard was much higher in 2005 and 2006 than 2004’s in the Philadelphia 

nonattainment area. (See Figure below) We also know, based on the 2007 design values

and the preliminary 2008 design values, that the 2006 design values were biased low due 

to the extremely low values measured in 2004 due to cool weather conditions in 2004. 

Monitor Site ID Number 2009 Modeled DV 2006 Monitored 

DV

2007 Monitored DV Preliminary 2008 

Monitored DV

Fairhill - CECIL CO, MD240150003 81 90 93 90

Colliers Mills - OCEAN 

CO, NJ

340290006 91 93 92 87

Rider - MERCER CO, NJ340210005 86 87 91 87

Ancora State Hospital -

CAMDEN CO, NJ

340071001 87 89 88 86

Camden - CAMDEN CO, 

NJ

340070003 88 84 88 90

Clarksboro -

GLOUCESTER CO, NJ

340155001 88 86 91 87

Bristol - BUCKS CO, PA 420170012 88 86 92 92

Northeast Airport -

PHILADELPHIA CO, 

PA

421010024 87 90 91 89
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Based on the 2007 and preliminary 2008 ambient data, the modeled 2009 modeled design 

values now appear to be either reasonable or maybe even too low in some cases. 

When the 2007 and preliminary 2008 design values (92 ppb) and the preliminary 2008 4th

high ozone value (90 ppb) are examined, it is not reasonable to assume the Philadelphia 

area will be in attainment of the ozone standard after the 2009 ozone season. The 2008 

design values also suggest that the 2009 model-predicted design values are very 

reasonable and do not seem to be over-predicted by the model. 

Design Value Trends

The Delaware SIP revision for the Philadelphia area states that there have been 

significant declines in the Philadelphia nonattainment area’s eight-hour ozone design 

values over the last several decades. Significant declines did occur after enactment of the 

NOx SIP Call in the 2003/2004 period. The data in Table 4 show that ozone design 

values have decreased sharply after 2003. EPA’s study of the effect of the NOx SIP Call 

has documented the beneficial effects of the NOx SIP Call emission reductions that were 

in place by the 2004 ozone season across the eastern United States 

Since 2004, design values have not continued to decrease as sharply for several reasons. 

First, the 2004 ozone season was cooler than normal, so the decreases in ozone due to 

additional controls have been offset somewhat by higher concentrations as the areas have 

returned to more typical summertime conditions. Second, current reductions in ozone 

and precursors transported from the Midwest are not as large as the decreases that 
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occurred while the NOx SIP Call was implemented. Even if EPA’s Clean Air 

Implementation Rule (CAIR) is implemented, most of the ozone season reductions due to 

CAIR are not likely to be implemented until after 2009. Any additional reductions in 

ozone in the 2008 and 2009 ozone seasons will be limited to mostly local controls in each 

nonattainment area and a few reductions in major sources due to States’ initiatives in the 

Ozone Transport Region. 

While the observed design values for 2007 are only 1 to 3 ppb higher than the modelpredicted value for 2009, an ozone reduction of 9 ppb would be needed to reduce design 

values to attainment by 2009 and 6 ppb to reduce the 4th high value to less than or equal 

to 84 ppb (level needed to qualify for a one year extension of the attainment date). 

Unless emission reductions that will occur between now and 2009 are to equal to, or 

more effective than the reductions from 2002 to 2007 when ozone decreased by up to 10 

ppb, the Philadelphia nonattainment area will not reach attainment by the 2009 ozone 

season. Even if local controls in 2008 and 2009 could reduce emissions by the amount of 

local reductions observed during the NOx SIP Call implementation years, comparable 

reductions in ozone needed to attain the standard in 2009 will not happen since the 

present ozone trends will be not be assisted by the large reductions in transported ozone 

and precursors that occurred before 2005. The recent trend in 8-hour ozone 

concentrations at the Philadelphia area ozone monitors presented in Table 4 indicate that 

attainment of the 8-hour ozone standard by the end of the 2009 ozone season is unlikely.
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Table 4. Recent Ozone Design Value Trends

Alternative Baseline Design Value

Seven monitors within the Philadelphia area are projected to exceed the current eighthour ozone standard following the US EPA’s recommended method. The recommended 

baseline concentration used in the attainment demonstration is the average of the three 

eight-hour ozone design values that include the emission base year (2002). Thus the 

baseline concentration is the average of the 2002, 2003 and 2004 eight-hour ozone design 

values.

Delaware asserts that by using the US EPA recommended method for calculating a 

monitor’s baseline concentration, undue weight is placed on the 2002 ozone season, one 

of the worst ozone seasons since the late 1990s. As alternative to the US EPA’s baseline 

concentration calculation, Delaware took the straight average of the 4th highs over the 

same years (2000-2004). This approach weighs each year equally. EPA did not 

recommend this method as it is not as robust as EPA’s recommended method. The 

Delaware method gives equal weight to the 2004 ozone season that has the advantage of 

emission reductions resulting from the NOx SIP Call. Additionally, in 2004 the 

Philadelphia area had the fewest number of days with eight-hour ozone 85 ppb or above 

during the period 1980 to 2007. According to the National Climatic Data Center, the 

summer of 2004 was ranked as the 20th coolest of the last 110 years in the Northeast. In 

the Midwest, upwind from the northeast and a significant source of ozone precursors, 

2004 was ranked 5th coolest. 2004 was significantly cooler than normal, which means 

2004 was not typical according to the conceptual model described in the SIP revision for 

the Philadelphia area. With a cooler than usual summer and lower concentrations due to 

implementation of the NOx SIP Call, including 2004 in a pre-control strategy baseline 

Site 2003 2004 2005 2006 2007 Preliminary

2008

Camden

101 93 85 84 88 88

Ancora 

S.H. 

101 95 91 89 88 86

Clarksboro98 94 88 86 91 87

Rider 99 91 86 87 91 87

Colliers 

Mills

106 99 94 93 92 87

Bristol 100 93 86 86 92 92

NE 

Airport

97 95 90 90 91 89

Fairhill 98 92 89 90 93 90
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creates a low bias for any averaging method that includes it equally with other years. An 

examination of the recent design value trends presented earlier in Table 4 shows that the 

extraordinary decrease in ozone in 2004 does not continue in the following years, when 

increasing ozone occurred while emissions did not increase. For these reasons, EPA 

believes the recommended method for calculating the baseline design value provides 

more stability to the baseline design value and better represents the meteorology and 

emissions of the 2002 emissions baseline year.

Methods (see below) equally as reasonable as Delaware’s method that were not evaluated 

in the Delaware SIP revision for the Philadelphia area produce baseline design values the 

same as EPA’s recommended method or even higher for the peak ozone site of the 

Philadelphia area ozone nonattainment area at Colliers Mills. 

EPA guideline method baseline = 105.7 ppb 

Delaware’s alternative baseline = 104.2 ppb

2002 design value used by EPA and the State for designation purposes = 113 ppb

2003 design value, centered on 2002 = 106 ppb. 

The baseline value calculated using EPA’s recommended method is well within the range 

of values calculated by other methods, so using EPA’s recommended method is 

reasonable and does not produce an anomalous value. 

Table 5 lists the alternative baseline value and the projected 2009 concentration for the 

seven monitors that are projected to exceed the current eight-hour ozone standard in 

2009. This reduces the modeled 2009 values slightly but still leaves them close to the 

preliminary 2008 design values. Even with this adjustment the 2009 Colliers Mills 

monitored design value (90 ppb) still remains above the highest concentrations listed in 

EPA’s WOE cut-offs.

Table 5. Alternative Baseline Concentration Analysis

Site Name State Alternative 

Baseline

OTW/OTB V4

RRF

Alternate 2009

Camden NJ 94.0 0.8996 84

Ancora S.H. NJ 98.6 0.8733 86

Clarksboro NJ 96.4 0.9004 86

Rider College NJ 95.6 0.8908 85

Colliers Mills NJ 104.2 0.8703 90

Bristol PA 96.6 0.8976 86

NE Phila PA 94.6 0.9035 85
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Alternative RRF

Delaware calculated 2009 modeled ozone design values by multiplying the modeling 

baseline design values with a relative response factor (RRF) as recommended by EPA’s 

photochemical modeling guidance. At each monitoring site, the state calculates the RRF 

associated with the maximum eight-hour ozone concentration in the nine grid cells 

nearest with the monitoring site on high ozone days. Lower RRFs produce larger

decreases in ozone between the base and future years. 

The Delaware ozone Plan recalculates RRFs for several different ozone levels; 2002 

baseline model concentrations 3 85 ppb, 2002 baseline model concentrations 3 90 ppb 

and 2002 baseline model concentrations 3 95 ppb. The idea is to see if the air-quality

model predicts more reductions on days with higher ozone concentrations (more benefit 

on the worst days). Table 6 contains the different RRFs based on the 2002 baseline 

model concentrations.

Delaware then recalculated the projected modeled 2009 design values using the most 

beneficial alternative RRFs. This lowered the projected modeled design values at nearly

all seven monitors by 1 ppb, except Bristol and Colliers Mills which were unchanged.

Despite the use of alternative methods for calculating the RRF, the resulting future year 

concentrations for the Bristol and Colliers Mills monitors still exceed EPA’s 

recommended WOE cut-off concentrations (see above in Table 2).

Table 6. Alternative Projected 2009 Modeled Values Using Alternative RRFs

Site Alt RRF DV Base Alt Projected 2009

Camden 0.8915 98.0 87

Ancora S.H. 0.8723 99.7 86

Clarksboro 0.8875 98.0 86

Rider 0.8908 97.7 86

Colliers Mills 0.8703 105.7 91

Bristol 0.8892 99.0 88

NE Airport 0.8991 96.7 86

Combining Alternative Baseline Concentrations and Alternative RRFs

Table 7 contains the projected 2009 modeled design values from combining the 

alternative baseline concentrations and the alternative RRF calculations described in the 

previous two sections. The combination of these two alternative approaches lowers the 

projected 2009-modeled concentrations significantly, but still leaves the Colliers Mills 

monitor above the highest concentrations listed in the U.S. EPA’s WOE chart. Five of 

the other six monitors still remain above the levels needed for attainment of the ozone 

standard. 
14

Table 7. Combined Effects of Alternative Baseline Concentrations and Alternative 

RRFs

Site Alt RRF Alt DV Base Alt Projected 2009

Camden 0.8915 94.0 83

Ancora S.H. 0.8723 98.6 86

Clarksboro 0.8875 96.4 85

Rider 0.8908 95.6 85

Colliers Mills 0.8703 104.2 90

Bristol 0.8892 96.6 85

NE Airport 0.8991 94.6 85

Regional Transport Analysis

The NOx SIP Call reduced ozone precursor emissions over a large region of the eastern 

US. These reductions reduced regional transport from the large power plants along the 

Ohio River into the Philadelphia nonattainment area. Pennsylvania maintains an elevated 

monitoring site (Methodist Hill) on South Mountain in south-central Pennsylvania 

approximately 40 miles southwest of the City of Harrisburg. Methodist Hill sits at 

approximately 1900 ft above mean-sea level and is in a good position to sample ozone 

concentrations entering the eastern ozone transport region (OTR).

The Delaware SIP revision for the Philadelphia area contends that one way to gauge how 

well the OTC air quality model is simulating regional transport is to examine how well 

the modeled 2009 eight-hour ozone design value compares to the 2006 actual design 

value. Table 8 contains the modeled 2009 concentration and the actual 2006 ozone 

design value. The model appears to be over predicting Methodist Hill’s design value by

approximately 6 ppb. This seems to indicate the model may not be adequately

characterizing the effects of the NOx SIP Call on upwind sources (under predicting the 

benefit). Delaware contends that the 6 ppb difference represents an overestimation of 

background (regional) concentrations entering the eastern OTR and suggests the modeled 

2009 design values may be overestimated by as much as 6 ppb within the Philadelphia 

Nonattainment Area. This may be true for this particular monitor, but it is inappropriate 

to generalize this statement to the entire Philadelphia area based on one monitor. 

Additionally, when a comparison is made using the 2007 and preliminary 2008 design 

values for the Methodist Hill monitor, the difference is only 4 ppb, which is closer to the 

projected 2009 model-predicted design value and not nearly enough of a reduction to 

reduce modeled 2009 modeled ozone design values to levels needed to demonstrate 

attainment. 
15

Table 8. Modeled 2009 vs. Monitored Ozone Design Values at Methodist Hill

EPA 

Baseline

RRF 

OTB/OTW V4

Modeled 

2009

2006 DV 2007 DV Preliminary 

2008 DV

90.6 0.8488 76 70 72 72

Analysis of Air-Quality Model Sensitivity to Emission Changes

The Delaware SIP revision for the Philadelphia area cites a study of the 2003 Northeast 

Blackout (Marufu et al., 2004) that shows that the blackout caused a drop of at least 7 

ppb ozone, and likely considerably more, and a modeling study of the same event (Hu et 

al., 2006) that used CMAQ to predicted only a 2.2 ppb change. The modeled reduction 

in ozone due to the blackout on August 14, 2003 was compared to the difference in 

aircraft measurements of ozone made by the University of Maryland at Selinsgrove PA 

on the blackout day and on reference non-blackout days with similar meteorology. 

Modeled ozone reductions due to the large number of power plant shutdowns during the 

blackout were on the order of 2.2 ppb while reductions of up to 7 ppb were noted in the 

aircraft data when measurements made on the blackout day were compared to 

measurements made on non-blackout days (August 4, 2002 and August 3, 2005) that had 

meteorology thought to be similar to the meteorology on the blackout day. The 

University of Maryland concluded air-quality models such as CMAQ may be under 

predicting ozone reductions due to control programs such as the NOx SIP Call by up to a 

factor of two. To be conservative, the University of Maryland lowered their estimate of 

model under- prediction of ozone reductions from 100% to 50%. Application of the 50 

% under prediction results in adjusted modeled 2009 design values of less than 85 ppb for 

all WOE monitors inside the Philadelphia Nonattainment Area (see Table 9 below). 

Given the CMAQ 2009 design value predictions and the 2007 and preliminary 2008 

monitored 2008 design values shown earlier in Table 3, it appears that the model is doing 

a good job predicting the future year design values in the Philadelphia area. CMAQ 

predicted a ~10-13 percent ozone reduction for the 2002-2009 time period which is very

close to the measured ozone reductions for the same period. For this reason and for other 

reasons detailed below, we believe the 50 % adjustment to the 2009 design values

discussed earlier to be inappropriate. 

Table 9. Modeled Design Value Adjustments Due To Model Under-Prediction of 

Benefits

SITE BASELINE 

DV

MODELED 

2009 DV

∆ PPB 1.5 x ∆ PPB ADJUSTED 

2009 DV

Camden 98.0 88 -10.0 -15.0 83.0

Ancora S.H. 99.7 87 -12.7 -19.0 80.7

Clarksboro 98.0 88 -10.0 -15.0 83.0

Rider 97.7 87 -10.7 -16.0 81.7

Colliers Mills 105.7 91 -14.7 -22.0 83.7

Bristol 99.0 88 -11.0 -16.5 82.5

NE Airport 96.7 87 -9.7 -14.5 82.2
16

Analysis of the Blackout Study Adjustments

Measured changes in ozone at Selinsgrove PA during the August 14, 2003 blackout are 

used to argue that CMAQ does not respond adequately to upwind emissions reductions. 

In order to determine what ozone should have been if the blackout had not occurred, the 

University of Maryland (UM) paper compared ozone measurements on the blackout day

with ozone measurements on meteorologically similar days. As a result of this 

comparison, the UM paper concludes that ozone should have been reduced by twice as 

much as the amount predicted by photochemical grid modeling done by Georgia Tech 

(Hu et al., 2006) for the blackout day.

The vast majority of emissions reductions from the August 14, 2003 Northeast blackout 

were from NY and Ontario (not the Midwest). The ozone and ozone precursors were 

likely reduced in central PA, but the emissions reductions were generally not coming 

from the locations that transport ozone into the Baltimore or Philadelphia Areas. 

Even if the blackout had a large impact in central PA, there is no way to translate that 

impact on ozone to the entire Philadelphia area. EGU emissions likely account for a 

large fraction of the ozone in rural PA, but a much smaller fraction of ozone in the urban 

Northeast corridor. In the urban corridor, transported ozone may be important, but NOx 

from mobile sources is still the biggest source of ozone in the Philadelphia area (See 

Tables 3-4, 6-10, 6-11 and 6-12 in the Delaware SIP revision for the Philadelphia area). . 

This is not the case in rural PA. 

The only modeling of the blackout study was done by Georgia Tech (Hu et al., 2006).

The Georgia Tech paper argues that because the emissions reductions were coming from 

NY and Ontario (and not the Midwest), they would not expect a large ozone reduction in 

the urban areas. Georgia Tech's modeling only saw a 2.2 ppb ozone reduction in the 

Selinsgrove PA area. . Additionally, the Georgia Tech modeling used the SAPRC 

chemistry. No additional modeling was performed by the University of Maryland to 

support their conclusions from the blackout study. No effort was made to separate out 

the impacts of transported ozone and precursors from local sources. 

As mentioned above August 4, 2002 and August 3, 2005 were the reference days with

ostensibly similar meteorology to the August 15, 2003 blackout meteorology that the 

University of Maryland used in their analysis. EPA understands that comparing different 

days with similar weather conditions is only an approximation as no two weather patterns 

are exactly alike. It is almost impossible to find a period with identical meteorological 

conditions, much less the same chemical state of the air parcels entering the regions as 

occurred on August 15, 2003. Furthermore, the weather on the days used to compare 

with the blackout day was significantly different because the wind trajectories on the 

blackout day were mostly from Western New York and Northern Ontario, as opposed to 

northern Ohio and Western Pennsylvania on the comparison days. The reduction in 

ozone on the blackout day would be less than if the blackout occurred on the comparison 

days since the blackout day’s emissions came from an area with lower emissions than the 

comparison days. For these reasons, and other discussed in the Georgia Tech paper, the 

modeling may not be wrong when it predicts only a 2.2 ppb decrease in ozone on the 
17

blackout day in the area upwind of the northeast corridor’s ozone nonattainment areas. 

Also, the studies compared the model with observations in central Pennsylvania, not the 

northeast corridor. Central Pennsylvania is not representative of ozone impacts affecting 

the urban areas of the northeast corridor, where mobile source NOx emissions dominate

(see Tables 3-3, 3-4 , 6-11 and 6-12 in the Delaware SIP revision for the Philadelphia 

area). Mobile source emissions were not significantly affected during the blackout. 

Monitored ozone on the reference days was used to reach the conclusion that the model is 

under-predicting the benefits of upwind NOx reductions.

Even though the recent measurements don’t support the claim of an underestimate of 

ozone reduction from the model, there may still be a valid issue with regard to 

underestimation of upwind ozone reductions. If the model underestimates the impact of 

upwind NOx reductions, then there may be a compensating error which overestimates the 

ozone reductions from other sources. Therefore it is possible that the modeling system 

overestimated the benefits of low level NOx reductions. This makes it inappropriate to 

make any across the board adjustments to the DV. Any reductions in DV from transport 

may need to be compensated by increases to the DV from other (local low level) sources

Given all of the facts, there is no technical basis for the adjustment factor that is used to 

adjust the design values. Increasing the modeled ozone reduction by 50% is not 

conservative and is not supported by the current modeling contained in the Delaware SIP 

revision for the Philadelphia area and ambient measurements.

The Effect of 2007-2009 and 2008-2009 Emission Reductions

Any measures that reduce emissions that cause ozone pollution, even if not committed to 

by Delaware will help the Philadelphia area meet the Clean Air Act’s requirement in 

section 172 (c) that all nonattainment areas reach attainment as soon as practicable. Even 

if Delaware’s attainment plan does not conclusively show attainment by 2009 and this 

portion of their SIP revision for the Philadelphia area is not approved, attainment must 

still occur as soon as possible.

Table 10 below contains NOx and VOC emission reductions that are expected to occur 

between 2007-2009 and 2008-2009 in the Philadelphia area. A detailed discussion of 

how EPA estimated these emission reductions is contained in Attachment 1 of this TSD. 

Using the sensitivity of the CMAQ modeling contained in the Delaware SIP revision for 

the Philadelphia area to reductions in emissions, EPA estimated the potential reduction in 

ozone concentration that might be expected from emission reductions that will occur 

between 2007- 2009 and 2008-2009 in the Philadelphia area. The modeled ozone 

reduction factor (0.021 ppb/ton) is estimated by dividing the average model predicted ppb 

reduction in design values for all the Philadelphia area monitors by the total 2002-2009 

Philadelphia area emission reductions that were included in the modeling. If both VOC 

and NOx contribute proportionally to ozone reductions (not necessarily true, but used 

here as a simplifying assumption), then EPA’s modeled ozone reduction factor is 

multiplied by the total expected 2007-2009 emission reduction (VOC +NOx) expected to 

occur in the Philadelphia area. The result is a 2.25 ppb reduction in ozone for the 2007-

2009 period. When this concentration is subtracted from the 2007 design value (93 ppb) 
18

and the 2007 4th high ozone value (102 ppb) for the Philadelphia area, the result is 91 ppb 

and 99 ppb respectively, which does not show attainment. When a similar process is 

applied to the emission reductions expected to occur between 2008 and 2009 using the 

preliminary 2008 design value (92 ppb) and the preliminary 2008 4th high ozone value 

(90 ppb), the result is projected 2009 design value of 90 ppb and a projected 2009 4th

high ozone level of 88 ppb which again does not show attainment. Additionally, the 

potential 2.25 ppb and 1.21 ppb reductions in ozone are probably over estimates of the 

actual ozone reductions that could result from 2007-2009 and 2008-2009 emission 

reductions. Emission controls between 2007 and 2009 may have the potential to reduce 

ozone concentrations in 2009, however; the actual ozone reduction will most likely be 

less than the ozone reduction predicted from the model based ozone reduction factor. 

The 2007-2009 emission reductions will not be assisted by the large reductions in 

transported ozone and precursors that occurred before 2005 due to the NOx SIP Call that 

are accounted for in the modeling. Any additional reductions in ozone in the 2008 and 

2009 ozone seasons will be limited to mostly local controls in the nonattainment area and 

a few reductions in major sources due to States’ initiatives in the Ozone Transport 

Region. 
19

Table 10. Estimated Ozone Benefit from 2007-2009 and 2008-2009 Emission 

Reductions

Area Name Major NOx 

Emission 

Reduction In 

SIP Revision

NOx 

Emission 

Reduction 

(TPD) 

(2007-

2009)

NOx Emission 

Reduction 

(TPD) 

(2008-2009)

Major VOC 

Reductions in 

SIP Revision

VOC Emission 

Reduction 

(TPD) 

(2007-2009)

VOC Emission 

Reduction 

(TPD) 

(2008-2009)

Ozone Benefit 

(PPB) ( .021 

ppb/tpd) x (tpd 

reduction) 

(2007-2009)

Ozone Benefit 

(PPB) (.021 

ppb/tpd) x (tpd 

reduction) 

(2008-2009)

Philadelphia Fleet Turnover 43.3 20.31 Fleet 

Turnover 

30.88 14.29

DE Multi-P 13.01 to 

19.01 

9.394 to 11.23 

Certain 

Categories of 

ICI Boilers

2 (NJ 

estimate)

2 (NJ 

estimate)

The below 

are all 2008 to 

2009

Refinery 

Enforcement 

Initiative

0.3 (NJ 

estimate)

0.3 (NJ 

estimate)

Refinery 

Enforcement 

Initiative

0.9 (NJ 

estimate)

0.9 (NJ 

estimate)

NOx RACT 

rule 2006 

2.1 (NJ 

estimate)

OTC -

Consumer 

Products Phase 

2

0.02 (MD 

estimate)

0.02 (MD 

estimate)

OTC - PFC 

Phase 1

0.06 (MD 

estimate)

0.06 (MD 

estimate)

OTC - PFC 

Phase 2

0.03 (MD 

estimate)

0.03 (MD 

estimate)

OTC –

Industrial 

Adhesives

0.10 (MD 

estimate)

0.10 (MD 

estimate)

EPA MACT 

Standards

1.1 (NJ 

estimate)

1.1 (NJ 

estimate)

Consumer 

Products 2009 

Amendments

0.4 (NJ 

estimate)

0.4 (NJ 

estimate)

Adhesives and 

Sealants

2.2 (NJ 

estimate)

2.2 (NJ 

estimate)

Asphalt Paving 

(Cutback and 

Emulsified 

Asphalt)

1.5 (NJ 

estimate)

1.5 (NJ 

estimate)

Portable Fuel 

Containers 

(2005 + 2009)

0.8 (NJ 

estimate)

0.8 (NJ 

estimate)

Regulation 

1124 Section 

46, Crude Oil 

Lightering 

Operations

2.58 (DE 

estimate)

2.58 (DE 

estimate)

Totals .... 66.71 33.84 Totals .... 40.57 23.98 2.25 1.21

In summary, for ozone reductions to continue as they have since 2002, then emission 

reductions would have to continue as they have in the past. Even if reductions in local 

emissions continue to be about the same as past years, reductions in transported 

emissions will not be at the same level seen during the implementation of the NOx SIP 

Call in the 2003-2004 period. Therefore, we should not assume that a continuation of 

local emission reductions per the rate achieved during the earlier part of this decade will 
20

mean the same reduction in ozone now, as transported ozone will not be reduced at the 

same rate as in previous years. States have not provided quantitative information about 

how much emissions in the Ozone Transport Region outside the Philadelphia 

nonattainment area will be reduced compared to earlier years. To make an estimate of 

reductions, EPA used information in Delaware SIP revision for the Philadelphia area and 

those of surrounding states to calculate the amount of emission reductions in states’ plans 

for the Philadelphia nonattainment area. 

Ozone design values in the past five years have decreased enough to get the Philadelphia 

ozone nonattainment area more than halfway to attaining the standard by 2009. 

However, that large drop in ozone concentration was aided by the NOx SIP Call. The 

expected emission reductions over the next two years are less than the past five year 

period. There will be no comparable reduction in long-range transported pollutants 

scheduled to occur in the next two years, even if the Clean Air Interstate Rule (CAIR)

program was implemented on schedule.

EPA believes that the arguments presented above indicate that it is not likely that planned 

emission reductions between 2007 and 2009 are sufficient for the Philadelphia area to 

attain the ozone standard. 

Summary of EPA’s Evaluation Pennsylvania’sDelaware’s WOE Analysis

Delaware’s photochemical grid modeling results predict a 2009 projected design value 

well above the air quality health standard for the Philadelphia ozone nonattainment area. 

Even after accounting for the potential ozone benefits from additional measures not 

included in the modeling, attainment of the ozone standard by 2009 for Philadelphia area 

is not likely. The Delaware SIP revision for the Philadelphia area makes a possible, but 

ultimately, not convincing argument to show that ozone will be less than or equal to 84

ppb in 2009. With the modeled result at least 7 ppb away from attainment, EPA’s 

modeling guidance reasonably points out (see Table 2 above) that Delaware needs to 

supply a very substantial amount of evidence that the model is significantly overestimating future ozone concentrations. 

Modeling studies give conflicting results as to whether Delaware will get more reductions 

in ozone than predicted by the model. Current air quality data seem to indicate that the 

model is not significantly over predicting ozone concentrations in 2009. Recent air 

quality data also confirm the large gap between present air quality and attainment by 

2009. 

Air quality data through 2007 and preliminary data for 2008 are so far above the level 

needed for attainment that it is doubtful if those states in the Philadelphia area will able 

to implement enough emission controls to reach the standard by 2009. The present air 

quality also does not support the hypothesis that the models are incorrect. Present air 

quality concentrations should be closer to the standard since the Philadelphia area is only

one year from when it should be attaining the standard. Instead, present air quality is 
21

very similar to the 2009 model-predicted air quality. Emission reductions anticipated to

occur between now and 2009 are also not enough to close the ambient air quality gap. 

The information and calculations provided in the Delaware SIP revision for the 

Philadelphia area emphasizes methods or data that support their claims that is reasonable 

to assume the Philadelphia nonattainment area could attain the ozone standard by the 

deadline. EPA’s review of the WOE analyses must evaluate a spectrum of likely

alternative calculations, not only those that tend to enhance arguments that the area could

attain the ozone standard. As noted before, the method recommended by EPA’s guidance 

and other reasonable variations on EPA’s methods predict the area will not attain the 

ozone standard by 2009. Delaware has not provided convincing evidence that the 

proposed adjustments to the photochemical grid model’s attainment year forecast will 

give a more accurate answer than the calculations based on EPA’s recommendations in 

its modeling guidance. Additionally, recent air quality trends when combined with an 

analysis of ozone benefits from emissions controls expected to come online between 

now and the start of the 2009 ozone season do not support the case for attainment. 

Impact of the Vacatur of EPA’s Clean Air Interstate Rule (CAIR)

On June 11, 2008 the United States Court of Appeals for the District of Columbia Circuit 

issued an opinion vacating the Clean Air Interstate Rule (CAIR) in its entirety. This 

opinion did not become final before December 23, 2008, when the Court reconsidered its 

June 11 decision and issued an opinion remanding CAIR to EPA without vacating. North 

Carolina v. EPA, Docket No. 05-1244 (D.C. Cir. Dec. 23, 2008). The Court stated in its 

December 23 opinion that EPA was still under an obligation to remedy what the Court 

characterized as “fundamental flaws” with CAIR, though the Court declined to impose a 

deadline for EPA to finalize that remedy.

Whether or not CAIR and its associated emissions reductions remain in effect does not 

alter EPA’s analysis of the approvability of the attainment demonstration. The modeling 

and WOE analysis in the proposed SIP revision took into account the anticipated CAIR 

reductions. EPA has determined that the Area is not likely to attain the standard even 

with these reductions. If additional proceedings were to result in vacatur of CAIR, our 

conclusion that the Area us unlikely to attain the NAAQS would not change; however, 

loss of the CAIR reductions would further increase the odds of the Area reaching 

attainment by the applicable June 2010 deadline. 

Summary of EPA’s Technical Findings

The result of the photochemical grid modeling analysis using EPA’s recommended 

methods predicts that the Philadelphia area ozone nonattainment area will not attain the 

standard in the attainment year of 2009. In response to this, Delaware has offered a 

number of WOE arguments that when taken together present an outside chance of truly
22

attaining the ozone standard. However, EPA’s review of the attainment demonstration 

portion of the Delaware SIP revision for the Philadelphia area determined the following:

· The Delaware SIP revision for the Philadelphia area modeling uses an 

appropriate photochemical grid model and follows EPA’s guidance methods

and does not predict attainment in 2009.

· The Delaware SIP revision for the Philadelphia area relies on emission 

reductions resulting from a commitment to adopt and implement a number of 

regulations prior to the start of the 2009 ozone season. Some of these were 

included in the photochemical grid modeling. These regulations would provide 

for additional reductions from boilers, refineries, power generation, consumer 

products and portable fuel containers. Delaware’s own analysis shows that 

Delaware must adopt all these measures, and additional measures as well to 

show attainment by 2009. Delaware must adopt these measures as soon as 

possible to insure that these emission reductions occur by the start of the 2009 

ozone season to have any chance of attaining the standard in 2009.

· Regardless of the issues raised by Delaware regarding the performance of 

EPA’s recommended air quality models, the 2007 air quality data and 

preliminary 2008 air quality data continue to exceed the ozone standard by a

significant margin. Even a linear comparison of the percentage of additional 

emission reductions planned by the state with the needed improvement in air 

quality between 2007 and 2009 indicates it is unlikely that air quality will 

improve enough for the Philadelphia area to meet the ozone standard or qualify

the area for a one year extension of the attainment date in 2009.

· When comparing the measured ozone concentrations in 2007 to the ozone 

concentrations predicted for 2009 by using EPA’s recommended application of 

the photochemical grid modeling, the photochemical grid model does not 

exhibit the magnitude of inaccuracies suggested in the Delaware SIP revision 

for the Philadelphia area. 

· In order to insure attainment, Delaware refers to additional measures which 

were not included in the original photochemical modeling analysis. Most of 

these measures are not contained in the SIP revision for the Philadelphia area

and are not committed to by Delaware as part of its attainment demonstration. 

While EPA encourages Delaware to continue to promote these worthwhile and 

important emission reduction programs, the amount of tangible air quality

benefit is difficult to estimate with any degree of certainty. Based on EPA’s 

estimates of the ozone benefits that the proposed additional measures could 

provide, it is not likely that the additional ozone benefit will be sufficient for the 

Philadelphia area to demonstrate attainment of the ozone standard or qualify for 

a one year extension of the attainment date in 2009. However, implementing 

these measures will move the State’s nonattainment areas closer to attainment. 
23

· The Philadelphia area attainment demonstration relied on adjustments to the 

baseline design values and on adjustments to the model predicted relative 

response factors form the basis of the photochemical modeling analysis. These 

adjustments to the base year starting value and the amount of reduction in ozone

from 2002 to 2009 differ from EPA’s modeling guidance and, more 

importantly, are not sufficiently justified and are artificially weighted toward a 

conclusion that Philadelphia area will attain the ozone standard in 2009.

· The Philadelphia area attainment demonstration relied on research which 

evaluated the impact of a widespread power blackout to develop an alternative 

approach to estimating anticipated air quality improvements from upwind 

power plants. While EPA believes that this approach provides some insight 

into the transport of ozone precursors, a critical review of all the research 

available to EPA and Delaware leads EPA to disagree with the premise that the 

air quality modeling results should be adjusted using Delaware’s alternative 

approaches.

· EPA encourages Delaware to implement all of its planned measures as soon as 

possible to meet the Clean Air Act’s requirement that all nonattainment areas 

reach attainment as soon as practicable. 

Recommendation of Disapproval of Attainment Demonstration

EPA has carefully evaluated the information provided by Delaware and other information 

it deems relevant to help understand what the air quality is likely to be by the 2009 ozone 

season. EPA concludes that Delaware’s SIP revision for the Philadelphia does not satisfy

the Clean Air Act’s requirement that State Implementation Plans provide for attainment 

of the National Ambient Air Quality Standard by the applicable attainment date.

For the Delaware SIP revision for the Philadelphia area to show attainment of the air 

quality standard, several factors would have to coincide. The photochemical grid model 

would have to be under-predicting the change in ozone by at least 40 percent. All of the 

measures Delaware committed to implement as part of its SIP revision for the 

Philadelphia area as well as measures not committed to as WOE would have to be 

implemented by the beginning of the 2009 ozone season. To expect all of these factors to 

occur is not realistic. In addition, based on recent air quality data, Delaware and its 

surrounding states would have to reduce emissions by levels equaling perhaps 50 percent 

of the total emission reductions contained in the SIP revision for the Philadelphia area. 

These emission reductions would have to occur between 2007 and the beginning of the 

2009 ozone season. Based on EPA’s evaluation of Delaware’s rate of progress plans for 

the Philadelphia area, it is unlikely that more than 20 percent of the Philadelphia area’s 

emission reductions will occur in the next few years, making it implausible that ozone in 

the Philadelphia area will be reduced from 2007 design value of 93 ppb or the 

preliminary 2008 design value of 92 ppb to 84 ppb by 2009.
24

Based on EPA’s review of the technical information provided in the Pennsylvania SIP 

revision for the Philadelphia area, EPA concludes that the SIP revision for the 

Philadelphia area does not show attainment of the ozone standard by 2009 and EPA 

should disapprove the attainment demonstration portion of the SIP.
25

Attachments:

Attachment 1