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Technical Support Document (TSD) for 

AERMOD-Based Assessments of Long-Range 

Transport Impacts for Primary Pollutants
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EPA- 454/B-15-003

July 2015

Technical Support Document (TSD) for AERMOD-Based Assessments of Long-Range 

Transport Impacts for Primary Pollutants

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Air Quality Analysis Division

Air Quality Modeling Group

Research Triangle Park, North Carolina
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Preface 

This document provides the results and interpretation of AERMOD simulations for typical PSD sources. 

The focus of the analysis is to determine the impact of a range of source types on pollutant 

concentrations in the near and far field.
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Contents 

Preface ..........................................................................................................................................................4

Contents........................................................................................................................................................5

Tables............................................................................................................................................................6

Figures...........................................................................................................................................................7

1. Introduction ..............................................................................................................................................8

2. Background ...............................................................................................................................................8

3.0 Approach to evaluating near-source and long-range impacts .............................................................10

3.1 Source Types and Characteristics .....................................................................................................10

3.2 Modeling Assessment of Various Facility Types...............................................................................11

4. Summary of results.................................................................................................................................14

4.1 NO2 and SO2 screening ....................................................................................................................14

4.2 Refined analyses for NO2 and SO2 ...................................................................................................17

4.3 Refined analyses for PM10 and PM25..............................................................................................20

5. Conclusions.............................................................................................................................................21

6. Additional information............................................................................................................................21

References ..................................................................................................................................................22

Appendix A..................................................................................................................................................23

Plots from NO2 and SO2 screening analysis...........................................................................................23

Appendix B..................................................................................................................................................34

Plots from NO2 and SO2 refined analysis...............................................................................................34

Appendix C ..................................................................................................................................................47

Plots from PM refined analysis...............................................................................................................47
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Tables 

Table 1 - Significant Impact Levels (SILs) for Criteria Pollutant by Class 1, 2, and 3 Areas...........................9

Table 2 - PSD Increment for Criteria Pollutants by Class 1, 2, and 3 Areas. ...............................................10

Table 3 - Summary of Modeling Scenarios by Source Type from AERMOD Implementation Workgroup 

(AIWG).........................................................................................................................................................11

Table 4 - Results for Phase 1 screening modeling for NO2 and SO2 by Facility type5 ................................15

Table 5 - Results for Phase 1 screening modeling for NO2 and SO2 by Facility type, continued...............16

Table 6 - Summary of NO2 and SO2 emissions for the coal EGU facility....................................................17

Table 7 - Results from the Phase 2 refined analysis for NO2......................................................................18

Table 8 - Results from the Phase 2 refined analysis for SO2 ......................................................................19

Table 9 - Summary of 2011 NEI emission data used in refined modeling analysis.....................................20
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Figures 

Figure 1 – Location of receptors for screening and refined runs. ..............................................................13

Figure 2 - Results from the screening analysis for the asphalt plant..........................................................24

Figure 3 - Results from the screening analysis for the biomass plant ........................................................25

Figure 4 - Results from the screening analysis for the cement kiln............................................................26

Figure 5 - Results from the screening analysis for the coal EGU ................................................................27

Figure 6 - Results from the screening analysis for the ethanol plant.........................................................28

Figure 7 - Results from the screening analysis for the flare .......................................................................29

Figure 8 - Results from the screening analysis for the fuel oil turbine.......................................................30

Figure 9 - Results from the screening analysis for the landfill gas turbine.................................................31

Figure 10 - Results from the screening analysis for the natural gas compressor station...........................32

Figure 11 - Results from the screening analysis for the pulp and paper plant...........................................33

Figure 12 - Refined NO2 and SO2 results, ASX, max and design values, 1-hr NAAQS ................................35

Figure 13 - Refined NO2 and SO2 results, DHT, max and design values, 1-hr NAAQS ...............................36

Figure 14 - Refined NO2 and SO2 results, OAK, max and design values, 1-hr NAAQS ...............................37

Figure 15 - Refined NO2 and SO2 results, SMQ, max and design values, 1-hr NAAQS ..............................38

Figure 16 - Refined SO2 results, ASX, 3 & 24-hr increment........................................................................39

Figure 17 - Refined SO2 results, DHT, 3 & 24-hr increment .......................................................................40

Figure 18 - Refined SO2 results, OAK, 3 & 24-hr increment.......................................................................41

Figure 19 - Refined SO2 results, SMQ, 3 & 24-hr increment......................................................................42

Figure 20 - Refined NO2 & SO2 results, ASX, annual NAAQS .....................................................................43

Figure 21 - Refined NO2 & SO2 results, DHT, annual NAAQS.....................................................................44

Figure 22 - Refined NO2 & SO2 results, OAK, annual NAAQS.....................................................................45

Figure 23 - Refined NO2 & SO2 results, SMQ, annual NAAQS....................................................................46

Figure 24 - Refined PM25 results, ASX, 24-hr annual increment................................................................48

Figure 25 - Refined PM25 results, DHT, 24-hr annual increment...............................................................49

Figure 26 - Refined PM25 results, OAK, 24-hr annual increment...............................................................50

Figure 27- Refined PM25 results, SMQ, 24-hr annual increment...............................................................51

Figure 28 - Refined PM25 results, ASX, 24-hr NAAQS ................................................................................52

Figure 29 - Refined PM25 results, DHT, 24-hr NAAQS................................................................................53

Figure 30 - Refined PM25 results, OAK, 24-hr NAAQS................................................................................54

Figure 31 - Refined PM25 results, SMQ, 24-hr NAAQS...............................................................................55
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1. Introduction 

For long-range transport (LRT) applications of distances of more than 50 km from a source, the proposed 

revisions to EPA’s Guideline on Air Quality Models (published as Appendix W to 40 CFR Part 51) include 

recommendations of a screening approach for addressing PSD increment and removal of CALPUFF as an 

EPA preferred model for such applications. While there is no proposed replacement refined model =for 

LRT applications under Appendix W, the information provided in this report indicates that the need for 

LTR assessments for NAAQS and PSD increment violations for inert pollutants is rare thereby mitigating 

the necessity for a preferred model for regulatory LRT assessments. This document provides technical 

details of an EPA modeling assessment that supports this proposed change, including summarizing the 

model scenarios and approach used to determine the impact of a range of source types on pollutant 

concentrations in the near and far field. 

2. Background 

The permitting process for the Preventions of Significant Deterioration Program (PSD) requires that a 

new or modifying source demonstrate that the additional emissions will not cause or contribute to a 

violation of the NAAQS or PSD increment. The traditional approach for demonstrating compliance is a 

multi-step process. As described in Section 9 of the proposed version of Appendix W, this two-stage 

process entails:

1. Conduct a single-source impact analysis where only the new or modifying source is considered 

in the analysis. The new or modifying source will model its own emissions to determine the 

appropriate impacts for each applicable pollutant and each averaging time. The predicted 

impacts from the single source are compared to the Significant Impact Level (SIL) for each 

averaging time and pollutant. 

a. If all single source impacts are less than the SIL, then the new or modifying source is 

generally not expected to cause or contribute to a violation of a NAAQS or PSD 

increment and the compliance demonstration is considered complete. 

b. If the single source impacts are greater than the SIL anywhere, then there is a chance 

that the source will cause or contribute to a violation of a NAAQS or PSD increment and 

then a cumulative impact analysis should be undertaken. 

2. The cumulative impact analysis takes into account all sources affecting the air quality in an area 

including the project source impact and consideration of background, which includes 

contributions from natural, nearby, and unknown sources. The cumulative modeling 

demonstrations for NAAQS and PSD increment are different:

a. For a NAAQS compliance demonstration, the emissions from the new or modifying

source must be added to the background concentrations and the resulting ”design 

concentration(s)” are compared to the specific NAAQS to determine compliance. When

modeled violations are evident, the contribution from the new or modifying source to 

the modeled violations can be compared to the SIL to determine if the source is causing 

or contributing to the modeled violation(s). 

b. For a PSD increment compliance demonstration, the total increase in concentrations 

from all permitted sources is compared to the ‘baseline’ concentration. The total 

increase in concentrations is not allowed to be greater than the applicable PSD 

increment. 
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As indicated above, the SIL is used as demonstration tool to determine the culpability of a new or 

modifying source to any NAAQS or PSD increment violations. Table 1 shows the SIL levels for PM2.5, 

PM10, SO2, NO2, and CO, while Table 2 shows the PSD increment levels of fine particulate matter 

(PM2.5), coarse particulate matter (PM10), sulfur dioxide (SO2), and nitrogen dioxide (NO2). In 1996, a 

rulemaking was proposed with Class I specific SILs for NO2 (annual: 0.1 ug/m3), SO2 (annual: 0.1 ug/m3, 

24-hr: 0.2 ug/m3, 3-hr 1.0 ug/m3) and PM10 (annual: 0.2 ug/m3, 24-hr: 0.3 ug/m3) (U. S. EPA, 1996). 

While this rule never went final, such that these Class I SILs have never been promulgated, it is our 

understanding that they have been used in practice by some states and regions for a screening analysis 

to eliminate the need for increment analysis are longer transport distances. 

Pollutant Class 1 Class 2 Class 3 Source

Fine Particulate Matter (PM2.5) 

Annual mean 0.061 0.3 0.3 40 CFR 51.165 (b)(2)

24-hr maximum 0.07 1.2 1.2

Particulate Matter (PM10)

Annual arithmetic mean * 1 * 40 CFR 52.21 (k)(2)

24-hr maximum * 5 *

Carbon Monoxide (CO):

8-hr maximum * 500 * 40 CFR 51.165 (b)(2)

1-hr maximum * 2000 *

Sulfur Dioxide (SO2)

Annual mean * 1 * 40 CFR 51.165 (b)(2)

24-hr maximum * 5 *

3-hr maximum * 25 *

1-hr maximum * 3 ppb (~7.8 ug/m3) * (U. S. EPA, 2010b)

Nitrogen Dioxide (NO2):

Annual mean * 1 * 40 CFR 51.165 (b)(2)

1-hr maximum * 4 ppb (~7.5 ug/m3) * (U. S. EPA, 2010a)

Table 1 - Significant Impact Levels (SILs) for Criteria Pollutant by Class 1, 2, and 3 Areas.

For most PSD compliance demonstrations, the near-source impacts (e.g., those occurring within 50 km

of the new or modifying source) are the controlling factor in successfully meeting Clean Air Act 

requirements. For the inert criteria pollutants, these near-source impacts are assessed with the EPA’s 

preferred dispersion model, AERMOD (Cimorelli, et al, 2005). Due to variations in meteorology that is 

expected to occur beyond 50 km and the time required for a plume to travel this distance, steady-state 

plume models like AERMOD are expected to be overly conservative in the far-field. Thus, when long 

range transport (LRT) is expected to be important (i.e., impacts beyond the nominal distance of 50 km), 

 1 The annual and 24-hr Class I SILs were remanded and vacated by request from the EPA in January 22, 2013. These 

values are currently under review and the EPA is developing a proposed rule to update the PM25 SILs. See the 

PM25 modeling guidance for more discussion on this issue (U. S. EPA, 2014).
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an alternative model is necessary for assessing impacts for those distances with the current Appendix W 

recommending the use of the CALPUFF modeling system (U. S. EPA, 2003). Section 6.2.3 of the current 

version of Appendix W, published in 2005, discusses the regulatory needs for LRT impact assessments. 

The focus in section 6.2.3 is the need to protect Class I areas and in particular, Class I PSD increments are 

identified as the most stringent regulatory benchmarks in the PSD program. While refined LRT modeling 

could also be needed for NAAQS, it is uncommon that a facility can demonstrate compliance for a 

NAAQS and not also comply with applicable PSD increment(s). 

Pollutant Class 1 Class 2 Class 3

Fine Particulate Matter (PM2.5) 

Annual mean 1 4 8

24-hr maximum 2 9 18

Particulate Matter (PM10)

Annual arithmetic mean 4 17 34

24-hr maximum 8 30 60

Sulfur Dioxide (SO2)

Annual mean 2 20 40

24-hr maximum 5 91 182

3-hr maximum 25 512 700

Nitrogen Dioxide (NO2):

Annual mean 2.5 25 50

1-hr maximum NA NA NA

Table 2 - PSD Increment for Criteria Pollutants by Class 1, 2, and 3 Areas.

3.0 Approach to evaluating near-source and long-range impacts 

In order to assess the nature of long range transport aspects of a PSD compliance demonstration, a 

variety of facility types were modeled across several inert criteria pollutants for a range of meteorology 

conditions to improve our understanding of the source impacts in the near-field (i.e., within 50 km) and 

far-field (beyond 50 km). 

3.1 Source Types and Characteristics

Table 3 provides a summary of the source types that were included in a modeling study conducted by 

EPA state agencies under the AERMOD Implementation Workgroup (AIWG). In 2011, EPA re-instituted

the AIWG with a focus on the new 1-hour NO2 and SO2 NAAQS. The purpose of the workgroup was to 

provide insights into challenges being brought forward by stakeholders regarding modeling as part of 

compliance demonstrations for the new standards (Snyder & Thurman, 2012). The workgroup focused 

on modeling of “real world” examples utilizing existing and newly formed guidance for the NO2 (U. S. 

EPA, 2010a) and SO2 NAAQS (U. S. EPA, 2010b). The AIWG workgroup was composed of EPA staff from 

the Regional offices and the Office of Air Quality Planning and Standards (OAQPS) as well as modelers 

from state, territorial, and local air quality agencies. The workgroup compiled a list of source types or

facilities that were of interest to various state and local agencies. 
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For each modeled facility, emissions and source characteristics were based on actual facilities from past 

permitting experiences but were modified to be generic facilities. AIWG participants conducted several 

modeling scenarios across multiple regions of the country that reflected changes stack height, addition 

of controls, and modifications of facility boundaries reflecting changes in ambient air. Also for NO2 

sources, the modeling scenarios involved comparing the use of available Tier 3 methods under Appendix 

W: Plume Volume Molar Ratio Method (PVMRM) and Ozone Limiting Method (OLM). For complete 

details of the AIWG modeling study and results, the full report is available 

at:http://www.epa.gov/ttn/scram/10thmodconf/review_material/AIWG_Summary.pdf and 

http://www.epa.gov/ttn/scram/10thmodconf/review_material/AIWG_Summary_v2.pdf. 

Table 3 - Summary of Modeling Scenarios by Source Type from AERMOD Implementation Workgroup 

(AIWG)

Facility Base emission (tpy)

NO2/SO2

Stack heights (m)2

Asphalt plant 188/13.2 6, 3,0. 3

Biomass 244/174 55, 9, 9

Cement kiln 7140/3129 160, 160, 903, 901

Coal EGU 1863/4959 150, 100, 6, 5, 5

Ethanol plant 1180/890 11, 11, 2, 43, 25

Flare 104/6083 25

Fuel oil turbine 1184/417 25, 25, 25, 253, 253, 253, 251, 61, 61, 61

Landfill gas turbine 80/45 13,13,13,10

NG compressor 90/0.01 11, 11, 11, 11, 53

Pulp & paper plant 9657/3403 30, 30 ,29, 85, 85, 72, 72, 76, 8, 67, 67

3.2 Modeling Assessment of Various Facility Types

The goal of this modeling assessment is to determine what types of facilities may have significant 

impacts, as defined as modeled concentrations above the SIL, at distances greater than 50 km from the 

source. There are fundamentally 4 parameters that affect this result: 1) the source configuration (e.g., 

stack height), 2) the emission rate, 3) the meteorology in the geographic area, and 4) the terrain in the 

geographic area. The source configurations and meteorology are closely tied when determining 

maximum impacts from a facility. These two aspects of a modeling compliance demonstration under 

PSD are more constrained than the possible range of emissions. Any facility, whatever source 

configuration and meteorology, can have significant impacts at any distance if the emissions are high 

 2 Primary stacks for NO2, for SO2, for both 3 NO2 only, no SO2 emissions at this stack height. 
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enough. Thus, it is essential to have emissions that reflect reality with respect to the facility type, which 

makes the AIWG modeling scenarios ideal for this evaluation.

Each original AIWG scenario was evaluated with a limited number of meteorological scenarios, which 

generally originated in the vicinity of the physical location of the facility that the scenario was based 

upon. However, to expand the usefulness of these scenarios for the purposes of this assessment and to 

more efficiently evaluate maximum potential impacts, a three-phased analysis of was used here:

1. In the first phase, screening meteorology, generated by the MAKEMET tool included in 

AERSCREEN (U. S. EPA, 2011), was used to evaluate a source’s maximum 1-hour concentration 

impacts that could occur across these “worst-case” meteorological datasets. The screening 

meteorology includes conditions ranging from low wind & high stability cases, which would give 

highest concentrations for near-surface releases, to high wind and highly unstable conditions, 

which would give the highest concentrations for elevated release, with tall stacks. In addition to 

using screening meteorology, this initial phase used the SCREEN option in AERMOD, which 

determines plume centerline concentrations, regardless of the wind direction and 

source/receptor spatial relationships. When using this option with multiple sources in a single 

AERMOD run, the estimated concentrations are biased to be higher because each receptor will 

see the plume centerline from each source, which generally could not occur regardless of the 

wind direction. For these model simulations, receptors were placed at distances ranging from 

100 m to 60 km. Multiple receptor heights were used in order to evaluate the potential 

presence of terrain downwind, with receptor heights including 0, 25, 65, 100, 150, and 200 m

(Figure 1). Due to the plume centerline option being used, only a single receptor at each 

distance and height was required. Thus, the results from this initial phase represent an 

extremely conservative estimate of plume impacts. If the results from a facility showed impacts 

less than the SIL under this initial phase, then it is not expected for it to have impacts greater 

than the SIL for a refined single-source analysis. 

2. The second phase of more refined modeling was pursued for those facilities whose predicted 

impacts from the first phase were above the SIL at 50 km. For this phase, AERMOD’s CO SCREEN 

option was not used and several sets of actual meteorology were used instead of screening 

meteorology. One set of the actual meteorology selected was known to cause higher 

concentrations in surface releases, while one was known to cause higher concentrations with 

elevated releases. Additional meteorological data sets were used to provide more complete and 

robust findings. Since the plume centerline concentrations were not calculated in this phase, a 

polar receptor grid was used with 1-degree radial spacing through 360 degrees. Receptor 

distances and heights matched those from phase 1. For this second phase, we calculated 1-hour, 

3-hour, 24-hour and annual concentrations to compare to the SILs for various averaging periods. 

3. The third phase was pursued if results from the second phase still indicated impacts above the 

SIL at 50 km, wherein the facilities were evaluated against the relevant NAAQS for each 

averaging period. If the facility’s impact was above the NAAQS in the near field, then the 

emissions were scaled down such that the source’s impact satisfied the NAAQS in the near-field 

and its impacts at 50 km and beyond were evaluated and compared to the SIL for the 

appropriate averaging periods. This phase was based off the modeling in phase 2 by linearly 

scaling concentrations rather than scaling emissions and rerunning model simulations. 
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4.

Figure 1 – Location of receptors for screening and refined runs.4

 4 Panel A shows the x and y locations, with refined receptors colored in black and screening receptors in red. Panel 

B shows the x and z locations for all receptors along any x/y radial.
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The AIWG scenarios included NO2 and SO2 emissions only, with NO2 using full conversion. However, 

PSD increments exist for CO, PM10, and PM2.5. Since CO air quality levels and emissions are currently so 

low, this pollutant is rarely an issue in PSD permitting and therefore were not evaluated in this 

assessment. However, PM10 and PM2.5 are necessary to include in this assessment and evaluated along 

with NO2 and SO2. Since PM emissions were not included in the original AIWG scenarios, an analysis of 

the EPA’s 2011 National Emissions Inventory (NEI, 

http://www.epa.gov/ttn/chief/net/2011inventory.html) was conducted to determine scaling factors for 

PM emissions, based on the NO2 and SO2 emissions for a particular facility type. Additional details on 

this analysis are provided in the next section.

4. Summary of results 

4.1 NO2 and SO2 screening

The first phase of screening analysis was conducted for NO2 and SO2 only. The results are summarized 

in Table 4 and Table 5 and in multiple figures are provided in Appendix A. For these screening runs, the 

maximum concentration for both NO2 and SO2 at each receptor was recorded. In addition, the 8th high 

1-hour concentration for NO2 and the 4th high 1-hour concentration for SO2 were also recorded at each 

receptor. The 8th and 4th high 1-hour concentrations are the concentrations typically used for 

determining compliance in a refined cumulative modeling analysis. While results are conservative given 

the plume centerline concentrations calculated in the screening analysis, they give some sense of the 

distribution of the highest concentrations and provide valuable perspective on the maximum 

concentrations. 

Several of the facility impacts were below their respective 1-hour SILs at the 50 km distance. The flare 

was below the SIL for NO2 and the asphalt plant and biomass burning plant were below the SIL for SO2. 

The landfill gas turbine and natural gas (NG) compressor were below the SIL at 50 km for both NO2 and 

SO2. Several other facilities were very close to the SIL at 50 km or had some receptors above and some 

below the SIL, depending on the receptor height. Only the NG compressor was compliant with any 

NAAQS, which has negligible SO2 emissions. While several facilities were close to the NAAQS, most were 

orders of magnitude higher than the NAAQS for both the highest concentration and the associated ranks 

concentration.

The focus of the evaluation here is to determine if source impacts are above the SILs at 50 km. However, 

as noted in Table 4 and Table 5, most of the facilities are fairly close to the applicable SILs at 50 km but 

well above the NAAQS in the near field. While the analysis of the SILs at 50 km is largely within the 

context of the PSD increment, an analysis of PSD increment would be completed in conjunction with an 

analysis of NAAQS. Thus, it is reasonable to assume that NAAQS compliance in the near field would need 

to be met before considering the source impacts at 50 km and comparing to the applicable SIL, 

particularly if the concentrations in the near field are orders of magnitude above the NAAQS (i.e.., the 

NAAQS is the controlling standard). When the emissions from these facilities are scaled back to meet the 

NAAQS in the near field, the impacts from most of the facilities at 50 km fall well below the applicable 

SILs. For example, for the fuel oil turbine, the equivalent of the design value from the screening runs for

NO2 (the 8th maximum 1-hr average) was 1115 ppb (not shown), while the standard is 100 ppb and the 

maximum impact at 50 km was 12.5 ppb. The emissions would need to be reduced by a factor of 11 to 

pass a NAAQS demonstration. This reduction would also apply to the impact at 50 km, resulting in an
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Table 4 - Results for Phase 1 screening modeling for NO2 and SO2 by Facility type5

Facility Receptor 

Elevation

NO2 NAAQS (impacts in ppb) SO2 NAAQS (impacts in ppb)

Maximum 1-

hour impact

Impact at 50 km

(SIL 4 ppb)

Maximum 1-

hour impact

Impact at 50 km

(SIL 3 ppb)

Asphalt 

plant

0 291.0 5.7 96.1 0.7

25 856.8 6.2 115.4 0.7

65 884.7 4.4 50.0 0.2

100 408.3 2.3 49.7 0.2

150 181.1 1.6 49.3 0.1

200 129.8 1.4 49.0 0.1

Biomass

0 17.5 1.0 11.2 0.7

25 17.7 1.0 11.3 0.7

65 548.5 1.7 351.4 1.1

100 899.4 5.5 576.2 3.5

150 534.6 2.8 342.5 1.8

200 243.7 1.5 156.1 1.0

Cement 

kiln

0 148.7 12.6 60.8 5.2

25 149.1 12.7 61.0 5.2

65 150.4 12.7 61.5 5.2

100 152.2 12.7 62.2 5.2

150 1468.9 12.7 567.6 5.2

200 6771.0 21.7 2397.5 8.8

Coal 

EGU

0 232.8 5.0 74.8 6.9

25 970.7 6.0 74.9 6.9

65 434.3 3.8 75.8 6.9

100 222.1 3.8 77.3 6.9

150 517.2 3.8 1328.8 6.9

200 1572.6 6.1 4388.2 16.7

Ethanol 

plant

0 391.4 6.4 32.2 3.0

25 811.9 6.4 175.6 3.0

65 1735.2 7.4 1091.4 4.7

100 1746.1 15.6 1223.5 11.0

150 1871.6 13.7 1311.5 9.6

200 1109.3 6.0 777.3 4.2
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Table 5 - Results for Phase 1 screening modeling for NO2 and SO2 by Facility type, continued5

Facility Receptor 

Elevation

NO2 NAAQS (impacts in ppb) SO2 NAAQS (impacts in ppb)

Maximum 1-

hour impact

Impact at 50 km

(SIL 4 ppb)

Maximum 1-

hour impact

Impact at 50 km

(SIL 3 ppb)

Flare

0 4.0 0.3 207.2 16.4

25 17.2 0.3 897.8 16.4

65 95.8 0.7 5014.2 37.7

100 132.3 1.5 6924.3 77.3

150 150.8 1.2 7888.0 60.9

200 90.2 0.6 4718.9 29.9

Fuel oil 

turbine

0 78.3 3.2 22.0 0.9

25 212.4 3.2 64.1 0.9

65 770.3 5.8 275.6 1.4

100 808.5 10.8 231.8 3.5

150 1126.0 12.5 403.9 4.5

200 907.9 7.4 325.1 2.7

Landfill gas 

turbine

0 28.1 1.0 13.0 0.4

25 110.5 1.1 65.7 0.5

65 230.5 1.9 136.9 1.0

100 174.0 1.1 104.0 0.6

150 75.5 0.5 45.1 0.3

200 42.4 0.4 25.3 0.2

NG 

compressor

0 92.6 3.2 0.0 0.0

25 835.8 3.7 0.1 0.0

65 351.9 1.8 0.1 0.0

100 181.9 1.2 0.0 0.0

150 59.9 1.0 0.0 0.0

200 43.1 1.0 0.0 0.0

Pulp & 

paper plant

0 303.0 26.7 452.9 11.3

25 290.6 26.8 603.3 11.3

65 815.1 26.8 3438.1 18.0

100 9733.0 43.7 2718.0 12.0

150 10313.8 85.8 2542.2 21.8

200 8546.9 65.6 2235.4 18.4

 5 The screening runs have several layers of conservatism built into them. Firstly, the screening meteorology is 

designed to pick the meteorology that will result in the worst case concentrations for any facility configuration. 

Secondly, plume centerline concentrations have been calculated, giving the highest concentration possible from 

along a cross section of a plume. Thirdly, the plume centerline concentrations are occurring for each source 

simultaneously. While there is some physical possibility of the worse-case met occurring and directing a plume 

such that a receptor is along the plume centerline, the third case is generally not possible.
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impact of 1.4 ppb at 50 km, which is well below the SIL of 4 ppb. The pattern that emerges from this 

adjustment to emissions indicates that facilities with emissions from the taller stacks (e.g., the cement 

kiln and the coal EGU) are the only ones that still have some impacts above the applicable SIL at 50 km. 

Facilities with the lower release points have their maximum impacts in close proximity to the fence line 

so their impacts at 50 km are much lower after the adjustment to meet the NAAQS. Facilities with 

emissions concentrated at tall stacks do not see their maximum impacts until much farther downwind 

because the plumes need more time to impact the surface. Of course, this pattern is significantly 

influenced by including receptors that are close to stack height. When looking at only the elevated 

receptors, the emissions scaling necessitated to bring the elevated receptors to NAAQS levels replicates 

the pattern of maximum near field impacts for facilities with lower release heights.

The findings from the screening results is that most types of facilities in most terrain will have their 

maximum impacts relatively close to the source. When the emissions from these facilities are adjusted 

to meet the NAAQS, the far-field impacts are unequivocally less than the SIL such that no assessment of 

LRT would be necessary. While terrain is a complicating factor, the results also show that the only facility 

type that could pass a NAAQS assessment in the near-field but still have impacts above the applicable 

SIL at distances of 50 km or greater are those with very tall stacks.

4.2 Refined analyses for NO2 and SO2

Based on the findings from the initial screening analysis, the second phase of refined modeling, which 

uses 5 years of actual meteorology and computes NAAQS and increment appropriate modeled design 

values, focuses solely on those facility types with tall stacks. Both the coal EGU and the cement kiln fit 

this profile, but we focused only on the coal EGU because its SO2 emissions were concentrated at the 

tallest stack and its NO2 emissions were distributed to the two tallest stacks (150 m and 100 m stack 

heights), thereby providing the most insightful test case for refined modeling. Table 6 shows the specific 

emissions for each stack at the coal EGU scenario for NO2 and SO2.

Table 6 - Summary of NO2 and SO2 emissions for the coal EGU facility

Stack height (m) 1506 100 6 5 5

NO2 emissions (tpy) 1564 174 104 10.4 10.4

SO2 emissions (tpy) 4867 87 4.3 .7 .3

For the refined modeling analysis, we used four meteorological datasets consisting of 5 years of 

meteorological data from 2006 to 2010, reflecting National Weather Service (NWS) stations located at 

JFK airport in Ashland, WI (ASX), Somerset airport in Somerville, NJ (SMQ), Dalhart airport in Dalhart, TX 

(DHT), and Oakland airport in Oakland, CA (OAK). These datasets were selected based on prior usage of 

a large set of meteorological datasets regularly used by the EPA and are known to represent a range of 

meteorological conditions. These data sets also provide spatial variability in the refined modeling 

analyses. The airports are all ASOS sites, with 1-minute observations processed through AERMINUTE and 

AERMET using the beta u* adjustment option (U. S. EPA, 2014). The maximum 1-hour concentration (5-

year average), NAAQS specific design value (5-year average of 98th percentile and 99th percentile daily 1-

 6 The 150 m stack is above GEP height, but was modeled at this height anyway. 
18

hour maximum concentrations), annual average (5-year average of each annual average), and 24-hour 

and 3-hour increment levels (highest first and second values) for SO2. As noted above, a polar grid was 

used with 1-degree separation and 360 degrees (receptor distances and heights matched those used in 

the screening analysis). 

Table 7 - Results from the Phase 2 refined analysis for NO2

Met 

scenario

Receptor 

Elevation

Maximum 1-hr impact 

(ppb, 1st high)

98th percentile 1-hr impact 

(ppb, 8th high)

All receptors 50 km (SIL 4 ppb) All receptors 50 km (SIL 4 ppb)

ASX 0 546.81 1.50 366.15 0.94

25 505.18 1.66 344.43 1.04

65 373.36 1.34 270.46 0.97

100 301.35 1.27 217.27 0.85

150 311.18 1.39 217.01 0.99

200 286.22 1.60 191.64 1.15

DHT 0 357.80 1.38 235.82 0.75

25 340.30 1.45 210.89 0.76

65 269.71 1.32 129.94 0.77

100 216.93 1.28 118.30 0.73

150 199.68 1.35 128.86 0.80

200 239.83 1.50 127.65 0.96

OAK 0 417.98 1.62 318.30 1.16

25 397.27 1.75 302.32 1.17

65 312.91 1.53 239.16 1.15

100 250.09 1.52 191.94 1.12

150 245.54 1.56 146.20 1.14

200 212.50 1.91 138.62 1.19

SMQ 0 443.30 1.15 344.51 0.72

25 425.08 1.22 370.58 0.81

65 302.98 1.20 227.72 0.81

100 243.67 1.09 183.29 0.72

150 253.43 1.19 189.48 0.94

200 250.14 1.32 184.82 1.08

Plots summarizing the refined modeling analysis for NO2 and SO2 are presented in Appendix B and the 

results are summarized in Table 7 and Table 8. The most striking difference from the screening and 

refined analysis is the decrease in the maximum 1-hour values. For NO2, the maximum from the 

screening (with unscaled emissions) was 1572 ppb, while the maximum from the refined runs was 

around 546 ppb (again, with unscaled emissions). For SO2, the maximum concentrations decrease even 
19

Table 8 - Results from the Phase 2 refined analysis for SO2

Met 

scenario

Receptor 

Elevation

Maximum 1-hr impact 

(ppb, 1st high)

99th percentile 1-hr impact 

(ppb, 4th high)

All receptors 50 km (SIL 4 ppb) All receptors 50 km (SIL 4 ppb)

ASX 0 23.65 2.58 14.22 1.98

25 23.69 2.57 14.31 1.97

65 24.08 2.57 14.68 1.96

100 34.60 2.56 24.57 1.95

150 68.70 2.56 57.76 1.95

200 262.24 3.20 200.78 2.47

DHT 0 23.39 2.47 12.04 1.81

25 23.60 2.46 12.07 1.80

65 23.97 2.46 12.40 1.78

100 49.53 2.46 42.82 1.77

150 87.43 2.46 53.61 1.76

200 652.11 3.15 454.48 2.28

OAK 0 28.98 3.01 21.36 2.65

25 29.01 2.99 21.52 2.64

65 29.23 2.97 21.75 2.63

100 43.59 2.96 28.06 2.63

150 82.33 2.95 67.46 2.62

200 394.67 4.04 329.24 2.97

SMQ 0 25.57 2.08 16.24 1.63

25 25.63 2.07 16.35 1.63

65 25.75 2.06 16.47 1.62

100 34.85 2.05 24.03 1.62

150 49.22 2.04 38.58 1.62

200 177.58 2.40 139.80 2.12

further from 4388 ppb to 454 ppb. For NO2, the maximum 1-hour concentrations at 50 km from the 

refined modeling (with unscaled emissions) are now well below the 1-hour SIL at X to Y. However, the 

SO2 concentrations are still above the SIL at 50 km with the unscaled emissions, ranging from F to G. If 

the SO2 emissions are scaled such that the near-field results meet the NAAQS, then the impact at 50 km 

are below the 1-hour SIL at W to V. However, the maximum impacts in the near-field for SO2 are driven 

by results at the 150 and 200 m receptors, i.e., elevated receptors reflective of terrain features in vicinity 

of the facility. The elevated receptors were included to evaluate the potential impacts of terrain on the 

modeling results. However, realistically, a facility with a 150 m stack with large emissions is not likely to 

be built in the immediate vicinity of terrain near or above stack height, as the facility would not be able 

to pass a NAAQS compliance demonstration. Thus, it is somewhat unrealistic to consider these elevated 

receptors at the closest distances. If these elevated receptors are not considered in the near-field, then 
20

the EGU would pass the NAAQS in the near-field, but be above the SIL in the far-field. Thus, the nearfield concentrations would not indicate a NAAQS violation because there would be no receptors near 

stack height and that a source of this type could indeed have significant impacts at 50 km or greater and 

need an LRT assessment. For the other averaging times, the maximum concentrations or estimated 

increment contributions are well below the SILs and increments at 50 km, even with the unscaled 

emissions. If emissions were scaled to pass the 1-hour NAAQS, then the concentrations at 50 km for the 

other averaging periods would be even farther below the benchmarks being considered here. In general, 

these results show that the longer the averaging period, the less likely that there will be significant 

impacts at distances of 50 km and greater. 

4.3 Refined analyses for PM10 and PM25

Since the AIWG facilities did not include PM10 or PM2.5 emissions, we derived these emissions by 

scaling from the emission rates for SO2 and NO2 based on emission ratios of these pollutants for EGUs 

listed in the NEI. The ratios of PM10 and PM2.5 emissions to NOx emissions for all EGUs with NOx 

emissions greater than 40 tons were calculated. Similarly, emission ratios were computed for EGUs with 

SO2 emissions greater than 40 tons. PM emissions were significantly lower than NOx and SO2 emissions. 

The results are summarized in Table 7. On average, the PM10 emissions were about 22% of NOx and 

SO2 emissions, while PM2.5 emissions were around 19% of NOx and SO2 emissions. The facilities with 

the greatest PM10 and PM2.5 emission ratios resulted in PM10 and PM2.5 being 50% of NOx and 38% of 

SO2. Given how close PM10 and PM2.5 emission ratios were, this assessment focused on PM2.5 

emissions only, as the PM2.5 standard are more stringent than the PM10 standards. Since both PM10 

and PM25 would be modeled as inter pollutants, the model would treat each pollutant equally with 

respect to dispersion, so modeling both PM10 and PM25 with the approximately the same emission 

rates would result in roughly the same modeled concentrations. The average emission ratios (20%) were 

used to scale the PM emissions for the PM analysis. 

Table 9 - Summary of 2011 NEI emission data used in refined modeling analysis

NOx ratios

PM10 max PM10 min PM10 mean PM25 max PM25 min PM25 mean

8.0% 49.4% 22.9% 4.8% 49.2% 19.5%

SO2 ratios

PM10 max PM10 min PM10 mean PM25 max PM25 min PM25 mean 

3.1% 37.7% 22.6% 2.7% 37.6% 18.7%

Plots summarizing the refined modeling analyses for PM2.5 are presented in Appendix C. The results 

from the annual average are well below the annual SILs at 50 km. For the 24-hour concentrations, it is 

difficult to draw specific conclusions from the results, given the uncertainty in the applicable SIL. The 

maximum concentrations are certainly well below the increment and below some of the 24-hr Class I 

SILs that are available. For the NO2-scaled emissions, the NAAQS near-field concentrations are slightly 

above the NAAQS (16 ug/m3) and right at the lowest PM25 SIL option. The SO2-scaled emissions have 

slightly higher maximum impacts (20 ug/m3) but are in between the two lowest PM25 SIL options. For 

the NO2 case, when emissions are scaled down to meet the NAAQS, the 50 km impacts become solidly 

below the SIL, while the SO2 50 km impacts are still higher than the lowest PM25 SIL option. 
21

5. Conclusions 

The results from this analysis indicate that for most facility types, if the facility can confirm compliance 

with the short-term standards in the near-field, then there are not likely to be significant impacts at 50 

km. Thus, for most facilities that show compliance in the near-field, no evaluation of LRT would be 

needed. There are indications, however, that for a select class of facilities, mainly those that have very 

tall stacks (greater than 100 m), there is a possibility of having an impact that is significant with respect 

to the short-term NO2, SO2 and PM25 SILs. These types of facilities have their maximum impact much 

farther from the facility than most, which means a higher emissions rate would be acceptable. The 

results also indicate that terrain features can be important for these types of facilities, as elevated 

terrain, near stack height, can see higher plume impacts much closer to the source. When this occurs, 

the compliance with the short-term standard may be sufficient to decrease long-range impacts and 

eliminate the potential need for an LRT assessment. Conversely, elevated receptors in the far-field can 

increase the need for LRT assessments, as these receptors would experience impacts closer to the 

plume centerline. Given the GEP restrictions on stack heights, there are not likely to be many facilities 

with stacks tall enough to pass the near-field NAAQS requirement while still having significant impacts at 

50 km. 

6. Additional information 

Data for the analyses described in this TSD can be obtained by contacting:

Chris Owen, PhD

Office of Air Quality Planning and Standards, U. S. EPA

109 T.W. Alexander Dr.

RTP, NC 27711

919-541-5312

owen.chris@epa.gov
22

References 

Cimorelli, et al. (2005). AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General 

Model Formulation and Boundary Layer Characterization. J. App. Meterol, 682-693.

Snyder, E., & Thurman, J. (2012). AERMOD Implementation Workgroup NO2 & SO2 modeling. 10th 

Conference on Air Quality Modeling. U. S. EPA, RTP, NC.

U. S. EPA. (1996). 40 CFR Parts 51 and 52: Prevention of Significant Deterioration (PSD) and 

Nonattainment New Source Review (NSR). FR, Vol. 61, No. 142, July 23, 1996, 38249 - 38344.

U. S. EPA. (2003). Guideline on Air Quality Models. 40 CFR Part 51 Appendix W (68 FR 18440).

U. S. EPA. (2010a, June 29). Guidance Concerning the Implementation of the 1-hour NO2 NAAQS for the 

Prevention of Significant Deterioration Program.

U. S. EPA. (2010b, August 23). Guidance Concerning the Implementation of the 1-hour SO2 NAAQS for 

the Prevention of Significant Deterioation Program.

U. S. EPA. (2011, March). AERSCREEN User’s Guide. pp. EPA-454/B-11-001.

U. S. EPA. (2014, May). Addendm to User's Guide for the AERMOD Meteorological Preprocessor 

(AERMET), EPA document number EPA-454/B-03-002. Retrieved from RTP, NC.

U. S. EPA. (2014). Guidance for PM2.s Permit Modeling, EPA report number EPA-454/B-14-001. RTP, NC, 

May.

 
23

Appendix A 

Plots from NO2 and SO2 screening analysis

For all figures in this section, the NO2 SIL is shown in blue and the SO2 SIL is shown in red.
24

Figure 2 - Results from the screening analysis for the asphalt plant.
25

Figure 3 - Results from the screening analysis for the biomass plant
26

Figure 4 - Results from the screening analysis for the cement kiln
27

Figure 5 - Results from the screening analysis for the coal EGU
28

Figure 6 - Results from the screening analysis for the ethanol plant
29

Figure 7 - Results from the screening analysis for the flare
30

Figure 8 - Results from the screening analysis for the fuel oil turbine
31

Figure 9 - Results from the screening analysis for the landfill gas turbine
32

Figure 10 - Results from the screening analysis for the natural gas compressor station
33

Figure 11 - Results from the screening analysis for the pulp and paper plant
34

Appendix B 

Plots from NO2 and SO2 refined analysis

For all figures in this section, the NO2 SIL is shown in blue and the SO2 SIL is shown in red.
35

Figure 12 - Refined NO2 and SO2 results, ASX, max and design values, 1-hr NAAQS
36

Figure 13 - Refined NO2 and SO2 results, DHT, max and design values, 1-hr NAAQS
37

Figure 14 - Refined NO2 and SO2 results, OAK, max and design values, 1-hr NAAQS
38

Figure 15 - Refined NO2 and SO2 results, SMQ, max and design values, 1-hr NAAQS
39

Figure 16 - Refined SO2 results, ASX, 3 & 24-hr increment
40

Figure 17 - Refined SO2 results, DHT, 3 & 24-hr increment
41

Figure 18 - Refined SO2 results, OAK, 3 & 24-hr increment
42

Figure 19 - Refined SO2 results, SMQ, 3 & 24-hr increment
43

Figure 20 - Refined NO2 & SO2 results, ASX, annual NAAQS
44

Figure 21 - Refined NO2 & SO2 results, DHT, annual NAAQS
45

Figure 22 - Refined NO2 & SO2 results, OAK, annual NAAQS
46

Figure 23 - Refined NO2 & SO2 results, SMQ, annual NAAQS
47

Appendix C 

Plots from PM refined analysis

For all figures in this section, the PM25 increment is indicated on each plot. For the NO-scaled emissions, 

this is shown in blue and the SO2-scaled emissions are shown in red.
48

Figure 24 - Refined PM25 results, ASX, 24-hr annual increment
49

Figure 25 - Refined PM25 results, DHT, 24-hr annual increment
50

Figure 26 - Refined PM25 results, OAK, 24-hr annual increment
51

Figure 27 - Refined PM25 results, SMQ, 24-hr annual increment
52

Figure 28 - Refined PM25 results, ASX, 24-hr NAAQS
53

Figure 29 - Refined PM25 results, DHT, 24-hr NAAQS
54

Figure 30 - Refined PM25 results, OAK, 24-hr NAAQS
55

Figure 31 - Refined PM25 results, SMQ, 24-hr NAAQS
56
57

United States

Environmental Protection

Agency

Office of Air Quality Planning and Standards

Air Quality Analysis Division

Research Triangle Park, NC

Publication No. EPA- 454/B-15-003

[July, 2015]