1

Technical Support Document

For the Modeling and Weight of Evidence (WOE) Portions of the Document Entitled 

“Cecil County, Maryland

8-Hour Ozone

State Implementation Plan and Base Year Inventory

SIP Revision:

07-05

June 15, 2007

 

 

TSD Prepared June, 2012

Todd A. Ellsworth

Office of Air Monitoring and Analysis, 3AP40

U.S. Environmental Protection Agency, Region 3

1650 Arch Street

Philadelphia, Pennsylvania 19103

/S/

____________________________________________

Reviewed by Walter Wilkie, Associate Director,

Office of Air Monitoring and Analysis (3AP40)

June 22, 2012

_________________

Date Signed
2

Technical Support Document

For the Modeling and Weight of Evidence (WOE) Portions of 

the Document Entitled “Cecil County, Maryland

8-Hour Ozone

State Implementation Plan and Base Year Inventory

SIP Revision:

07-05

June 15, 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 Maryland’s State 

Implementation Plan (SIP) entitled “Maryland State Implementation Plan for Attainment 

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

Progress and Attainment Demonstration, June 2007”, hereafter referred to as the 

Maryland SIP. Maryland 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 Maryland SIP. 

Introduction to the Maryland State Implementation Plan for Ozone

The Maryland SIP includes plans to reduce ozone-causing emissions for Maryland’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 Maryland SIP. This TSD reviews Maryland’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.

Maryland 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”.
3

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
4

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 Maryland Address All Of The Components Of A Modeled Attainment 

Demonstration?

The Maryland SIP 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 Maryland SIP 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 ozoneforming 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 in Section 12.1 of the Maryland SIP.

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 H-1 of the Maryland SIP. 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 

Maryland. 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 12.2 of the Maryland SIP.
5

Meteorological Time Periods Used in the Modeling

Section 12.2 of the Maryland SIP 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 12.2 of the 

Maryland SIP. 

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 

Section 12.2 of the Maryland SIP.

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 Section 12.2 of the Maryland SIP. 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. 

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 (MANE-
6

VU), 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 H-10 of the Maryland SIP.

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 H-12 the Maryland 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 

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 
7

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 Maryland 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 Design Value

Monitor ID Site Name County State

DVC

(ppm)

RRF 

OTB/OTW V4

2009 

Model 

DV 

(ppm)

2009 

Monitored 

(ppm) 

100010002 Killens Pond Kent DE 88 0.8934 78 75

100031007 Lums Pond New Castle DE 91 0.8462 77 75

100031010 Brandywine New Castle DE 93 0.8781 81 78

100031013 Bellefonte New Castle DE 89 0.8759 77 74

100051002 Seaford Sussex DE 90 0.8462 76 76

100051003 Lewes Sussex DE 86 0.8956 77 76

240150003 Fair Hill Cecil MD 98 0.8336 81 84

340010005 Nacote Creek Atlantic NJ 88 0.8762 77 72

340070003 Camden Camden NJ 98 0.8996 88 87

340071001 Ancora State Hospital Camden NJ 100 0.8733 87 81

340110007 Millville Cumberland NJ 94 0.8486 79 78

340150002 Clarksboro Gloucester NJ 98 0.9004 88 83

340210005 Rider College Mercer NJ 97 0.8908 86 81

340290006 Colliers Mills Ocean NJ 107 0.8703 91 80

420170012 Bristol Bucks PA 99 0.8976 88 88

420290100 New Garden Airport Chester PA 94 0.8387 78 77

420450002 Chester Delaware PA 91 0.8705 79 77

420910013 Norristown Montgomery PA 92 0.8861 81 79

421010004 AMS Lab Philadelphia PA 72 0.9081 65 64

421010014 Roxboro Philadelphia PA 91 0.9070 82 81

421010024 NE Airport Philadelphia PA 97 0.9035 87 84

421010136 Elmwood Philadelphia PA 84 0.9070 76 82

 

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.
8

Summary of Photochemical Grid Modeling Results

In summary, the basic photochemical grid modeling presented in the Maryland SIP 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 PhiladelphiaWilmington-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, 

Maryland decided to use WOE to demonstrate attainment of the ozone standard in the 

Philadelphia area. 

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 Maryland SIP 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 and 2009

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

 Alternative methods for calculating the 2009 ozone design value.
9

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.

 

Modeled Concentrations vs. Monitored Design Values

Table 3 below lists the OTC modeled 2009 design values and the 2006-2011 monitored 

design values. Maryland did not have access monitored ambient ozone data from 2007-

2011ambient data when it wrote its SIP. Because the modeled 2009 and projected 2006 

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

only slightly lower than the monitored 2006 design values, Maryland SIP suggests that

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 attainment by the 

attainment date. Classified as a “moderate” nonattainment area, the Philadelphia area is 

required to attain the NAAQS by June 2010, in effect, by end of 2009 ozone season. As 

shown in Table 3 by the end of the 2009 ozone season only two ozone monitors (Bristol 

& Camden) were still monitoring nonattainment. However the 4

th high monitored 8-hour 

values in 2009 were less than or equal to 84 ppb, the level needed to qualify for a one 

year extension of the attainment date. As a result, the EPA granted the Philadelphia area 

a one year extension of the attainment date to June 2011 or effectively by the end of the 

2010 ozone season.

Additionally, the 2009 data presented Table 3 and earlier in Table 1 show that the model 

is over predicting the monitored design values by anywhere from 1-13 ppb in 19 out of 

the 22 cases. 
10

Table 3. Modeled vs. Monitored Design Values

 

Design Value Trends

The Maryland SIP 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 

occurred while the NOx SIP Call was implemented. However additional emission 

reductions have continued to occur due mostly local controls in each nonattainment area 

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

Region. 

As Table 4 shows, the monitored ozone design values for each of the Philadelphia area 

monitors continued to decline and now show attainment beginning in 2010 and continue 

to show attainment in 2011. 

Monitor Site ID Number 2009 

Modeled 

DV

2006 

Monitored 

DV

2007 

Monitored 

DV 

2008 

Monitored 

DV

2009 

Monitored 

DV

2010 

Monitored 

DV

2011 

Monitored 

DV

Fairhill - CECIL CO, 

MD

240150003 81 90 93 90 84 80 81

Colliers Mills -

OCEAN CO, NJ

340290006 91 93 92 87 80 81 81

Rider College -

MERCER CO, NJ

340210005 86 87 91 87 81 78 78

Ancora State 

Hospital - CAMDEN 

CO, NJ

340071001 87 89 88 86 81 80 80

Camden - CAMDEN 

CO, NJ

340070003 88 84 88 87 87 83

Clarksboro -

GLOUCESTER CO, 

NJ

340155001 88 86 91 87 83 81 82

Bristol - BUCKS 

CO, PA

420170012 88 86 92 92 88 83 80

Northeast Airport -

PHILADELPHIA 

CO, PA

421010024 87 90 91 89 84 82 83
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Table 4. Recent Ozone Design Value Trends for Philadelphia Area Monitors

Monitor ID Site Name 2003 2004 2005 2006 2007

 

2008

 

2009

 

2010

 

2011

100010002 Killens Pond 0.089 0.084 0.080 0.080 0.081 0.081 0.075 0.074 0.071

100031007 Lums Pond 0.093 0.084 0.080 0.078 0.082 0.080 0.075 0.075 0.075

100031010 Brandywine 0.093 0.089 0.082 0.082 0.083 0.083 0.078 0.076 0.075

100031013 Bellefonte 0.090 0.085 0.082 0.081 0.081 0.078 0.074 0.075 0.077

100051002 Seaford 0.091 0.085 0.082 0.080 0.082 0.081 0.076 0.077 0.076

100051003 Lewes 0.088 0.085 0.084 0.082 0.082 0.079 0.076 0.077 0.075

240150003 Fair Hill 0.100 0.094 0.088 0.090 0.093 0.090 0.084 0.080 0.081

340010005 Nacote Creek 0.091 0.085 0.092 0.079 0.077 0.074 0.072

340070003 Camden 0.101 0.093 0.085 0.084 0.088 0.087 0.087 0.083

340071001

Ancora State 

Hospital 0.101 0.095 0.091 0.088 0.089

0.086 0.081 0.080 0.080

340110007 Millville 0.095 0.89 0.084 0.083 0.083 0.081 0.078 0.076 0.071

340150002 Clarksboro 0.099 0.095 0.088 0.086 0.087 0.087 0.083 0.081 0.082

340210005 Rider College 0.100 0.092 0.086 0.087 0.091 0.087 0.081 0.078 0.078

340290006 Colliers Mills 0.109 0.102 0.094 0.093 0.092 0.087 0.080 0.081 0.081

420170012 Bristol 0.100 0.093 0.086 0.086 0.092 0.092 0.088 0.083 0.080

420290100

New Garden 

Airport 0.100 0.094 0.087 0.086 0.085

0.082 0.077 0.076 0.074

420450002 Chester 0.093 0.089 0.082 0.083 0.085 0.083 0.077 0.074 0.073

420910013 Norristown 0.094 0.089 0.086 0.085 0.086 0.084 0.079 0.078 0.077

421010004 AMS Lab 0.075 0.068 0.063 0.062 0.068 0.067 0.064 0.066 0.068

421010014 Roxboro 0.093 0.086 0.081 0.078 0.080 0.078 0.081

421010024 NE Airport 0.097 0.095 0.090 0.090 0.091 0.089 0.084 0.082 0.083

421010136 Elmwood 0.083 0.079 0.070 0.074 0.077 0.081 0.082

 

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.

Many States in the Philadelphia area believe 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, a straight average of the 4th highs over 
12

the same years (2000-2004) was used to calculate the baseline concentration. This 

approach weighs each year equally. 

EPA guideline method baseline = 105.7 ppb 

Maryland’s alternative baseline = 104.2 ppb

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 the model is still over 

predicting the actual monitored 2009 design values in six out of the seven cases. 

Table 5. Alternative Baseline Concentration Analysis

Site Name State Alternative 

Baseline

OTW/OTB V4

RRF

Alternate 2009 2009 Monitored 

Design Value

Camden NJ 94.0 0.8996 84 87

Ancora S.H. NJ 98.6 0.8733 86 81

Clarksboro NJ 96.4 0.9004 86 83

Rider College NJ 95.6 0.8908 85 81

Colliers Mills NJ 104.2 0.8703 90 80

Bristol PA 96.6 0.8976 86 88

NE Phila PA 94.6 0.9035 85 84

Alternative RRF

Maryland 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. 

RRFs were calculated for several different ozone levels; 2002 baseline model 

concentrations  85 ppb, 2002 baseline model concentrations  90 ppb and 2002 baseline 

model concentrations  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.

The projected modeled 2009 design values were recalculated using the most beneficial 

alternative RRFs. This reduces the modeled 2009 values slightly but the model is still 

over predicting the actual monitored 2009 design values in six out of the seven cases. 
13

Table 6. Alternative Projected 2009 Modeled Values Using Alternative RRFs

Site Alt RRF DV Base Alt Projected 2009 2009 Monitored 

Design Value

Camden 0.8915 98.0 87 87

Ancora S.H. 0.8723 99.7 86 81

Clarksboro 0.8875 98.0 86 83

Rider 0.8908 97.7 86 81

Colliers Mills 0.8703 105.7 91 80

Bristol 0.8892 99.0 88 88

NE Airport 0.8991 96.7 86 84

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. This reduces the modeled 2009 values slightly but the model is 

still over predicting the actual monitored 2009 design values in five out of the seven 

cases. 

Table 7. Combined Effects of Alternative Baseline Concentrations and Alternative 

RRFs

Site Alt RRF Alt DV Base Alt Projected 

2009

2009 Monitored 

Design Value

Camden 0.8915 94.0 83 87

Ancora S.H. 0.8723 98.6 86 81

Clarksboro 0.8875 96.4 85 83

Rider 0.8908 95.6 85 81

Colliers Mills 0.8703 104.2 90 80

Bristol 0.8892 96.6 85 88

NE Airport 0.8991 94.6 85 84

In summary, with the benefit of having the 2009 monitored design values, it’s obvious 

that the model is over predicting the actual concentrations, even when model adjustments

are made to account for model over prediction. 

 

Summary of EPA’s Evaluation Maryland’s WOE Analysis

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

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

However, after taking into account WOE arguments regarding model over prediction of 

the 2009 monitored design values and recent ozone design value trends, which show 

attainment of the standard by 2010, the Maryland SIP has demonstrated attainment of the 

ozone standard by the extended attainment date of June 2011. 
14

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, a number of WOE arguments

have been made that when taken together with recent air quality trends show attainment 

of the ozone standard by the extended date of June 2011. EPA’s review of the attainment 

demonstration portion of the Maryland SIP determined the following:

 The Maryland SIP modeling uses an appropriate photochemical grid model and 

follows EPA’s guidance methods and does not predict attainment in 2009.

 The Maryland SIP 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. Maryland’s own analysis shows that Maryland must adopt all these 

measures, and additional measures as well to show attainment by 2009. 

 In order to insure attainment, Maryland refers to additional measures which 

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

these measures are not contained in the SIP and are not committed to by

Maryland as part of its attainment demonstration. While EPA encourages 

Maryland 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. However, implementing these measures 

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

 An evaluation of recent air quality data indicates that the Philadelphia area has 

attained the 1997 8-hour ozone standard by the applicable attainment date of 

June 2011. 

Recommendation of Approval of the Attainment Demonstration

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

it deems relevant to help understand what the air quality was likely to be by the extended 

attainment date of June 2011 or effectively by the end of the 2010 ozone season. EPA 

concludes that Maryland’s SIP satisfies 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.

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

concludes that the SIP demonstrates attainment of the ozone standard by the June 2011 

attainment date and EPA should approve the attainment demonstration portion of the SIP.