Technical Support Document (TSD)

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

“Delaware State Implementation Plan

For Attainment of the PM2.5 Annual National Ambient Air Quality Standard,” 

January 3, 2008

TSD Prepared May, 2010

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 15, 2010

_________________

Date Signed
2

Technical Support Document for the Modeling Portion Of the State of 

Delaware’s Ozone State Implementation Plan (SIP) Entitled “Delaware

State Implementation Plan

For Attainment of the PM2.5 Annual National Ambient Air Quality 

Standard, January 3, 2008”

Purpose of the Technical Support Document

This Technical Support Document (TSD) describes the Environmental Protection 

Agency’s (EPA’s) evaluation of the modeling and weight of evidence (WOE) portion of 

Delaware’s State Implementation Plan (SIP) revision entitled “Delaware State 

Implementation Plan For Attainment of the PM2.5 Annual National Ambient Air Quality

Standard, January 3, 2008.” This SIP revision will hereafter be referred to as the 

Delaware PM2.5 SIP.

In April 2005, EPA designated 126 areas of the country as “non-attainment” under the 

annual fine particle (PM2.5) National Ambient Air Quality Standard (NAAQS). Among 

those non-attainment areas is the Philadelphia-Wilmington, PA-NJ-DE PM2.5 NonAttainment Area (NAA). This NAA includes: New Castle County Delaware, Burlington 

and Camden Counties in New Jersey and Bucks, Chester, Delaware, Montgomery and 

Philadelphia Counties in Pennsylvania. According to the federal Clean Air Act (CAA) 

and the EPA Implementation Rule1, this entire NAA must attain compliance with the 

annual PM2.5 NAAQS by 2010.

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. 

The Regulatory Framework

On July 18, 1997 (62 FR 36852), EPA established PM2.5 NAAQS, including an annual 

standard of 15.0 μg/m3based on a 3-year average of annual mean PM2.5 concentrations, 

and a 24-hour (or daily) standard of 65 μg/m3based on a 3-year average of the 98th 

percentile of 24-hour concentrations. EPA established the standards based on significant 

evidence and numerous health studies demonstrating that serious health effects are 

associated with exposures to PM2.5.

Following promulgation of a new or revised NAAQS, EPA is required by the CAA to 

designate areas throughout the United States as attaining or not attaining the NAAQS; 

this designation process is described in section 107(d)(1) of the CAA. In 1999, EPA and 

 

1 Environmental Protection Agency, Clean Air Fine Particle Implementation Rule, 40 CFR Part 51, 

March 29, 2007.

 
3

State Air-quality Agencies initiated the monitoring process for the 1997 PM2.5 NAAQS, 

and, by January 2001, established a complete set of air-quality monitors. On January 5, 

2005, EPA published initial air-quality designations for the 1997 PM2.5 NAAQS (70 FR 

944), based on air-quality monitoring data for calendar years 2001-2003.

On April 14, 2005, EPA published a final supplemental rule amending the Agency’s 

initial designations (70 FR 19844). EPA did not consider modifications made in this rule 

to be “re-designations” because the changes were made before April 5, 2005, the 

effective date of the initial designations. As a result of the final supplemental rule, PM2.5

nonattainment designations are in effect for 39 areas, comprising 208 counties within 20 

States (and the District of Columbia) nationwide, with a combined population of about 88 

million. The Delaware portion of the Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA 

which is the subject of this proposed rulemaking, is included in the list of areas not 

attaining the 1997 PM2.5 NAAQS. The only Delaware County included in the 

Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA is New Castle County.

On October 17, 2006, EPA strengthened the 24-hour PM2.5 NAAQS to 35 μg/m3, and 

retained the level of the annual PM2.5 standard at 15.0 μg/m3(see 71 FR 61144). On

November 13, 2009, EPA designated areas as either attainment/unclassified or 

nonattainment with respect to the revised 24-hour NAAQS (see 74 FR 58688). In the

November 2009 designation action, EPA established a deadline of December 14, 2012, 

for States to submit attainment plans for areas designated as nonattainment for the revised 

24-hour PM2.5 NAAQS. Therefore the Delaware PM2.5 SIP does not address the current 

24-hour PM2.5 NAAQS.

Of relevance to the proposed rulemaking herein, 74 FR 58688 clarified designations for

the 1997 PM2.5 NAAQS by relabeling the existing designation tables to identify

designations for the annual NAAQS, and by providing a separate table identifying 

designations for the 1997 24-hour NAAQS (i.e., 65 μg/m3). In that table, the

Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA is designated as attaining the 1997 

24-hour PM2.5 NAAQS.

 

Introduction to the Delaware State Implementation Plan for the Annual Average 

PM 2.5 Standard

On July 18, 1997, EPA established a health based PM2.5 NAAQS at 15.0 micrograms per 

cubic meter (μg/m3), annual average. New Castle County, Delaware was designated by 

the EPA as being in non-attainment for this annual PM2.5 NAAQS in April 20052. Kent 

and Sussex Counties were designated as attainment; however, sources within these 

counties emit PM2.5 and PM2.5 precursors which contribute to PM2.5 non-attainment in 

New Castle County. 

The PM2.5 SIP includes plans to reduce PM2.5 emissions for Delaware’s portion of the 

Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA. The SIP also includes modeling that 

 

2 70 FR 944, January 5, 2005
4

will predict whether the area will meet the PM2.5 standard by the attainment date. This 

TSD reviews Delaware’s documentation of modeling that shows attainment of the annual 

average PM2.5 standard by the 2010 attainment date. 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 PM2.5 SIP demonstrates that all of the CAA and Rule 

requirements associated with the annual PM2.5 NAAQS have been met; and that 

Delaware and the entire Philadelphia-Wilmington-DE-PA-NJ NAA will attain 

compliance with the annual PM2.5 NAAQS as expeditiously as practicable, and not later 

than 2010. 

What Are The Components Of A Modeled Attainment Demonstration?

Modeling Process Overview 

Delaware is a member of the Mid-Atlantic/Northeast Visibility Union (MANE-VU) 

Regional Planning Organization (RPO). The MANE-VU RPO was tasked with the 

assignment of preparing a PM-2.5 modeling platform that all member states could use to 

demonstrate compliance with the 1997 PM2.5 NAAQS.

The New York State Department of Environmental Conservation (NYSDEC) was the 

lead agency for coordinating and running the modeling platform used by the MAVE-VU 

RPO. Modeling centers responsible for running the platform included the NYSDEC, the 

University of Maryland at College Park (UMD), the Northeast States for Coordinated Air 

Use Management (NESCAUM), the New Jersey Department of Environmental 

Protection (NJ DEP), and the Virginia Department of Environmental Quality (VA DEQ). 

Each modeling center was responsible for installing the modeling platform, conducting 

diagnostic tests and completing a benchmark run to ensure accurate results. 

 

The MANE-VU RPO 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 PM2.5 and PM2.5 precursors and a sophisticated meteorological 

model to produce PM2.5 concentrations over the eastern United States. The 

meteorological data used in the meteorological model was for the base year PM2.5 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.
5

The EPA modeling guidance3recommends that States use the modeled PM 

concentrations in a relative sense and not rely solely on the concentrations the model 

predicts for the attainment year (2009). A modeled attainment test is an exercise in 

which an air quality model is used to simulate current and future air quality. If future 

estimates of PM2.5 concentrations are less than the NAAQS, then this element of the 

attainment test is satisfied. EPA’s recommended test is one in which model estimates are 

used in a “relative” rather than “absolute” sense. Ratios of the model’s future (2009) 

predicted PM2.5 concentrations to base year (2002) predictions are calculated for all PM2.5 

monitor locations. These ratios are called relative response factors (RRF). Future PM2.5 

concentrations are estimated at existing monitoring sites by multiplying a modeled 

relative response factor at locations “near” each monitor by the observation-based, 

monitor-specific, “baseline” design value. The resulting predicted “future 

concentrations” are compared to NAAQS. The PM2.5 attainment test is complicated and 

reflects the fact that PM2.5 is a mixture species. Therefore, the attainment test for PM2.5 is 

called the Speciated Modeled Attainment Test (SMAT). In the test, ambient PM2.5 is 

divided into major components. These are:

- mass associated with sulfates

- mass associated with nitrates

-mass associated with ammonium

- mass associated with organic carbon 

- mass associated with elemental carbon

- mass associated with particle bound water

- mass associated with “other” primary inorganic particulate matter

- and passively collected mass

A separate RRF is calculated for each of the PM2.5 components (except passive mass).

Each of these site-specific ratios are called component-specific RRFs. Future PM2.5 

design values are estimated at existing monitoring sites by multiplying modeled relative 

response factors “near” each monitor times the observed “component specific design 

value.” This latter quantity is estimated using measured site-specific design values for 

PM2.5 in concert with available measured composition data. Future site-specific PM2.5 

design values at a site are estimated by adding the future year values of the seven PM2.5 

components. If all future site-specific PM2.5 design values are less than or equal to 15.0 

micrograms per cubic meter (μ/m3) at all locations, the test is passed and the modeling 

system will have predicted attainment of the PM2.5 annual air quality standard. 

 

Steps Required in a Modeled Attainment Demonstration

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

of the PM2.5 standard. 

1. Develop a conceptual description of the problem to be addressed.

 

3 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
6

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 PM2.5 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 

PM2.5, accounting for emissions and photochemistry.

The conceptual model for the Delaware PM2.5 SIP is described in the Attachment 1 

entitled: The Nature of the Fine Particle and Regional Haze Air Quality Problems in the 

Mid-Atlantic Northeast Visibility Union(MANE-VU) Region: A Conceptual Description 

(NESCAUM), November 2006. This document was prepared by Northeast States for 

Coordinated Air Use Management (NESCAUM) for use by the OTC member States, and 

it provides the conceptual description of the fine particle issues in the OTC states, 

consistent with the EPA’s guidance.

The NESCAUM document contains a conceptual description that explains how elevated 

regional PM2.5 peak concentrations in the summer differ significantly from the largely 

urban peak concentrations observed during winter. On average, summertime 

concentrations of sulfate in the northeastern United States are more than twice that of the 

next most important fine particle constituent, organic carbon (OC), and more than four 

times the combined concentration of nitrate and black carbon (BC) constituents. 

Episodes of high summertime sulfate concentrations are consistent with stagnant 

meteorological flow conditions upwind of the Philadelphia-Wilmington, PA-NJ-DE 

PM2.5 NAA and the accumulation of airborne sulfate (via atmospheric oxidation of SO2) 

followed by long-range transport of sulfur emissions from industrialized areas within and 

outside the area. National assessments have indicated that in the winter, sulfate levels in 

urban areas are higher than background sulfate levels across the eastern U.S., indicating 

that the local urban contribution to wintertime sulfate levels is significant relative to the 
7

regional sulfate contribution from long-range transport. A network analysis for the 

winter of 2002 suggests that the local enhancement of sulfate in urban areas of the 

MANE-VU region ranges from 25 to 40% and that the long-range transport component 

of PM2.5 sulfate is still the dominant contributor in most eastern cities.

In the winter, urban OC and sulfate each account for about a third of the overall PM2.5 

mass concentration observed in Philadelphia and New York City areas. Nitrate also 

makes a significant contribution to urban PM2.5 levels observed in the northeastern United 

States during the winter months. Wintertime concentrations of OC and nitrate in urban 

areas can be twice the average regional concentrations of these pollutants, indicating the 

importance of local source contributions. This is likely because winter conditions are 

more conducive to the formation of local inversion layers which prevent vertical mixing. 

Under these conditions, emissions from tailpipe, industrial and other local sources 

become concentrated near the Earth’s surface, adding to background pollution levels 

associated with regionally transported emissions. 

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 

Delaware. 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 PM2.5 SIP. 

Meteorological Time Periods Used in the Modeling

The Delaware PM2.5 SIP notes that the OTC Modeling Committee agreed to model all 

days in 2002. Modeling an entire year covers many different kinds of PM episodes and 

exceeds EPA’s recommendations for episode selection. Due to the fact that the 

attainment demonstration is being conducted using a resource intensive photochemical 

grid model, EPA accepts the use of a single, recent “representative” year to be used for an 

annual model simulation. Two factors were used in selecting 2002 as the 

“representative” year - the observed annual mean concentrations of PM2.5 are close to the 

3-year observed design value at all, or most monitoring sites, and the pattern of quarterly 

mean values is similar to the pattern of quarterly mean concentrations averaged over 3 

years.

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 
8

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

and aloft. 

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 PM2.5 concentration 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-Wilmington, PA-NJ-DE 

PM 2.5 NAA 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 PM patterns experienced in the 

ozone transport region (OTR). 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.

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 resolution 

used in the modeling exercise followed EPA’s modeling guidance and therefore 

adequately represents the atmosphere where PM2.5 is emitted, 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 (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 NESCAUM with appropriate MOBILE 6 input files along with the 

projected vehicle miles traveled (VMT), which was coupled with hourly “gridded 
9

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.

Emission inventory details are provided in Sections 3, 4 and 6.3.5 of the Delaware PM2.5 

SIP.

Base Case Run Model Performance Evaluation

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

reproduced the Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA 2002 PM2.5

concentrations. 

CMAQ was employed to simulate PM2.5 for the calendar year 2002. A review of PM2.5 

and its individual species was conducted for the study domain.

The CMAQ model performance for surface PM2.5 is good with acceptable bias and error. 

Several observations can be made with respect to model performance, including the 

following:

1. Approximately 80-90% of organic mass (OM) is in the primary fraction. 

Observed OM has distinct maximum during summer when secondary

formation is highest; CMAQ exhibits substantial under-prediction in 

secondary organic aerosols (SOA). The predicted primary OM is highest 

during the winter.

2. CMAQ captures seasonal variation in SO4 well.

3. CMAQ appears to overestimate primary PM2.5 components (EC, soil, primary

OM), especially during colder months.

4. CMAQ appears to underestimate secondary OM during the summer.

These issues are not of great regulatory concern since attainment tests are based on the 

application of relative response factors. In summary, the regional and local model 

performance is acceptable for PM2.5. While there are some differences between the 

spatial data between sub-regions, there is nothing to suggest a tendency for the model to 

respond in a systematically different manner between regions. Examination of the 

statistical metrics by sub-region confirms the absence of significant performance 

problems arising in one area but not in another, building confidence that the CMAQ 

modeling system is operating consistently across the full OTC domain. This confidence 

in the modeling results allows for the modeling system to be used to support the State 

Implementation Plan to meet the 24-hour and annual PM2.5 NAAQS.
10

2009 Control Case Modeling and the Modeled Attainment Test

As previously mentioned, the PA-NJ-DE PM2.5 nonattainment area has an attainment date 

of April 5, 2010. The PM2.5 NAAQS include an annual standard of 15 μg/m3based on 

the 3-year average of annual mean PM2.5 concentrations.

The purpose of a modeling assessment is to determine if control strategies currently being 

implemented (“on the books”) will lead to attainment of the annual average NAAQS for 

PM2.5 by 2009. The modeling is applied in a relative sense, similar to the 8-hour ozone 

attainment test. However, the PM2.5 attainment test is more complicated and reflects the 

fact that PM2.5 is a mixture. In the test, ambient PM2.5 is divided into major components, 

with a separate relative response factor (RRF) and future design value (DVF) calculated 

for each of the PM2.5 components. Since the attainment test is calculated on a per species 

basis, the attainment test for PM2.5 is referred to as the Speciated Modeled Attainment 

Test (SMAT)4. The following sections outline the process to determine that 2009 

projections of PM2.5 will meet the NAAQS from regional modeling, as suggested in 

EPA’s modeling guidance.

Annual SMAT Results 

Table 1 presents the results of the annual SMAT results for the PA-NJ-DE nonattainment 

area. The SMAT results demonstrate that the projected average annual arithmetic mean 

PM2.5 concentration calculated at each FRM monitor attains the annual PM2.5 NAAQS. 

Specifically, all future design value (DVF) calculations are less than15 μg/m3. Table 1

presents the results of the annual SMAT results for a suite of regional modeling runs 

conducted by OTC each representing OTB/OTW – “On the Books, On the Way” control 

measures. All runs demonstrate compliance with the annual NAAQS.

Table 1: Annual SMAT Results for PA-NJ-DE Nonattainment Area

2009 On-The-Books-On-The-Way Control Measures

AIRS ID Site Name County State

2000-2004 DVB 2009

Q1 Q2 Q3 Q4 DVF

10-003-1003 Bellefonte New Castle DE 14.91 15.54 15.45 13.02 12.3

10-003-1007 Lums Pond New Castle DE 13.30 14.49 15.69 11.10 11.4

10-003-1012 Newark New Castle DE 15.56 15.00 15.75 13.57 12.5

10-003-2004 MLK New Castle DE 16.79 15.60 16.66 14.26 13.3

34-007-0003 Copewood E Davis Camden NJ 13.5 14.3 16.6 12.7 12.1

34-007-1007 Pennsauken TWP Camden NJ 14.0 14.0 15.7 13.6 12.2

34-015-0001 Gibbstown Municipal

Bldg. Gloucester NJ 13.9 13.4 16.0 11.4

11.7

42-017-0012 Bristol Bucks PA 14.14 13.68 14.70 13.82 12.4

 

4 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. 
11

AIRS ID Site Name County State

2000-2004 DVB 2009

Q1 Q2 Q3 Q4 DVF

42-029-0100 Belmont Avenue Chester PA 14.44 14.07 16.72 14.12 12.8

42-045-0002 Chester Delaware PA 15.07 15.96 16.35 12.63 13.3

42-091-0013 Norristown Montgomery PA 13.61 13.77 14.90 12.71 12.0

42-101-0004 Philadelphia AMS Philadelphia PA 14.13 13.86 16.20 12.83 12.5

42-101-0020 Philadelphia Belmont Ave Philadelphia PA 14.0 14.4 15.4 13.1 12.0

42-101-0024 Philadelphia North East Philadelphia PA 12.43 12.37 15.56 11.45 11.0

42-101-0047 Philadelphia Broad Street Philadelphia PA 15.54 14.88 16.89 13.25 12.8

42-101-0136 Philadelphia Elmwood Philadelphia PA 13.75 13.04 15.75 11.93 11.5

Summary of Photochemical Grid Modeling Results

In summary, the basic photochemical grid modeling, presented in the Delaware PM2.5 

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 annual-average PM2.5 design value is predicted to be 

13.3μg/m3in the Philadelphia-Wilmington, PA-NJ-DE NAA. Thus, based on EPA’s 

modeled attainment test, the Philadelphia-Wilmington, PA-NJ-DE PM2.5 NAA will reach 

attainment of the annual average PM2.5 standard in 2009 before the attainment date of 

April 5, 2010. 

Unmonitored Area Analysis

The modeled attainment test does not address future air quality at locations where there is 

not a PM2.5 monitor nearby. To guard against the possibility that air quality levels could 

exceed the standard in areas with limited monitoring, EPA suggests that additional 

review is necessary, particularly in nonattainment areas where the PM2.5 monitoring 

network just meets or minimally exceeds the size of the network required to report data to 

Air Quality System (AQS). This review is intended to ensure that a control strategy leads 

to reductions in PM2.5 and its constituent pollutants at other locations that could have 

baseline (and future) design values exceeding the NAAQS were a monitor deployed 

there. The test is called an “unmonitored area analysis”. The purpose of the analysis is 

to use a combination of model output and ambient data to identify areas that might 

exceed the NAAQS if monitors were located there. 

It is important to note that Delaware currently operates a network of six PM2.5 monitors. 

Four of these monitors are in New Castle County, which is part of the PA-NJ-DE 

nonattainment area. Some of these monitors were established as State and Local Air 

Monitoring Stations (SLAMS). These SLAMS monitors were selected based on specific 

monitoring objectives (background concentration, area of highest concentration, high 

population, source impact, transport, and rural impact) as required by EPA and siting

scales (micro, middle, neighborhood, urban, and regional) established by EPA. 
12

EPA believes that the density of the monitoring network in New Castle County relieves 

the necessity of applying this additional analysis.

Local Area Analysis

EPA modeling guidance recommends that a local area analysis be performed for those 

areas where local primary PM2.5 sources are thought to be contributing to a monitor and 

causing non-attainment of the NAAQS. At this time, no monitors within the PA-NJ-DE 

nonattainment area are projected to exceed the NAAQS so it does not appear to be a 

necessary requirement in this circumstance to conduct the local area analysis. 

Furthermore, existing monitoring data suggests a uniform regional pattern with respect to 

PM2.5 concentrations rather than any “hot spot” monitors. 

Supplemental Analyses and Weight of Evidence (WOE) Determination 

EPA=s modeling guidance 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. 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. 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. 

All models, including the CMAQ, model have inherent uncertainties. Over or under 

prediction may result from uncertainties associated with emission inventories, 

meteorological data, and representation of PM2.5 chemistry in the model. Therefore, EPA 

modeling guidance provides for other evidence to address these model uncertainties so 

that proper assessment of the probability to attain the applicable standards can be made.

EPA modeling guidance states that those modeling analyses that show that attainment 

with the NAAQS will be reached in the future with some margin of safety (i.e., estimated 

concentrations below 14.5 μg/m3for annual PM2.5 and 62 μg/m3for 24-hour PM2.5) need 

more limited supporting material. 

Due to the fact that the modeling results presented in Table 1 fall well below the 

aforementioned “weight of evidence” thresholds established by EPA, a limited 

supplemental analysis was deemed necessary to support the 2009 attainment 

demonstration.

 
13

Trends in PM2.5 Design Values 

Figure 1 below shows the annual PM2.5 design value trends for the period 2000-2009 for 

all monitors in the PA-NJ-DE PM2.5 NAA including the monitors in Kent and Sussex 

Counties. It is clear from this figure that all the monitors show a downward trend in the 

annual PM2.5 design value. It is noteworthy that from 2008 and after the design values at 

all monitors are in compliance with the annual PM2.5 standard. Please note that the 2009 

PM2.5 design values represented in Figure 1 are preliminary values. 

Figure 1

PM 2.5 Annual Avg. Design Value Trend at Each Monitor in 

the PA-NJ-DE PM2.5 NAA

10

11

12

13

14

15

16

17

20022003200420052006200720082009

Years

C

o

n

c

e

ntratio

n (u

g/m

3)

100031003

100031007

100031012

100032004

340070003

340071007

340155001

420170012

420290100

420450002

420910013

421010004

421010020

421010024

421010047

421010136

421010057

340150004

421010055

h

Summary of EPA’s Evaluation Delaware’s WOE Analysis

Design values at all monitors have trended downward since the year 2000. This 

information coupled with current annual average PM2.5 design all below the annual 

average PM2.5 standard since 2007 support the demonstration of attainment by the 2010 

attainment date.

 
14

Summary of EPA’s Technical Findings

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

methods predicts that the PA-NJ-DE PM 2.5 NAA will attain the annual average PM2.5

standard by the attainment date of April 5, 2010.

Recommendation of Approval of Attainment Demonstration

EPA has carefully evaluated the information provided by Delaware PM2.5 SIP.

EPA concludes that Delaware PM2.5 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.

Attachment:

Attachment 1: The Nature of the Fine Particle and Regional Haze Air Quality Problems 

in the Mid-Atlantic Northeast Visibility Union (MANE-VU) Region: A Conceptual 

Description (NESCAUM), November 2006.