1

Draft Technical Support Document

Mississippi

Area Designations for the 2010 SO2 Primary National Ambient Air Quality Standard

Summary

Pursuant to section 107(d) of the Clean Air Act (CAA), the U.S. Environmental Protection 

Agency (EPA, or the Agency) must designate areas as either “unclassifiable,” “attainment,” or 

“nonattainment” for the 2010 one-hour sulfur dioxide (SO2) primary national ambient air quality 

standard (NAAQS). The CAA defines a nonattainment area as one that does not meet the 

NAAQS or that contributes to a violation in a nearby area. An attainment area is defined as any 

area other than a nonattainment area that meets the NAAQS. Unclassifiable areas are defined as 

those that cannot be classified on the basis of available information as meeting or not meeting the 

NAAQS.

Mississippi submitted updated recommendations on September 18, 2015, ahead of a July 2, 

2016, deadline for the EPA to designate certain areas. This deadline established by the U.S. 

District Court for the Northern District of California is the first of three deadlines established by 

the court for the EPA to complete area designations for the 2010 SO2 NAAQS. Table 1 below 

lists Mississippi’s recommendations and identifies the county in Mississippi that the EPA intends 

to designate by July 2, 2016, based on an assessment and characterization of air quality through

ambient air quality data, air dispersion modeling, other evidence and supporting information, or a 

combination of the above.

Table 1. Mississippi’s Recommended and the EPA’s Intended Designations

Area

Mississippi’s

Recommended 

Area Definition

Mississippi’s 

Recommended 

Designation

EPA’s Intended 

Area Definition

EPA’s Intended 

Designation

Lamar 

County, 

Mississippi Lamar County

Unclassifiable/

Attainment

Same as state’s 

Recommendation

Same as state’s 

Recommendation

Background

On June 3, 2010, the EPA revised the primary (health based) SO2 NAAQS by establishing a new 

one-hour standard at a level of 75 parts per billion (ppb) which is attained when the three-year 

average of the 99th percentile of one-hour daily maximum concentrations does not exceed 75 

ppb. This NAAQS was published in the Federal Register on June 22, 2010 (75 FR 35520) and is 

codified at 40 CFR 50.17. The EPA determined this is the level necessary to protect public health 

with an adequate margin of safety, especially for children, the elderly and those with asthma. 

These groups are particularly susceptible to the health effects associated with breathing SO2. The 

two prior primary standards of 140 ppb evaluated over 24 hours, and 30 ppb evaluated over an 
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entire year, codified at 40 CFR 50.4, remain applicable.

1 However, the EPA is not currently 

designating areas on the basis of either of these two primary standards. Similarly, the secondary 

standard for SO2, set at 500 ppb evaluated over 3 hours has not been revised, and the EPA is also 

not currently designating areas on the basis of the secondary standard.

General Approach and Schedule

Section 107(d) of the CAA requires that not later than one year after promulgation of a new or 

revised NAAQS, state governors must submit their recommendations for designations and 

boundaries to the EPA. Section 107(d) also requires the EPA to provide notification to states no 

less than 120 days prior to promulgating an initial area designation that is a modification of a 

state’s recommendation. If a state does not submit designation recommendations, the EPA will 

promulgate the designations that it deems appropriate. If a state or tribe disagrees with the EPA’s 

intended designations, they are given an opportunity within the 120 day period to demonstrate 

why any proposed modification is inappropriate.

On August 5, 2013, the EPA published a final rule establishing air quality designations for 29

areas in the United States for the 2010 SO2 NAAQS, based on recorded air quality monitoring 

data from 2009 - 2011 showing violations of the NAAQS (78 FR 47191). In that rulemaking, the 

EPA committed to address, in separate future actions, the designations for all other areas for 

which the Agency was not yet prepared to issue designations.

Following the initial August 5, 2013 designations, three lawsuits were filed against the EPA in 

different U.S. District Courts, alleging the agency had failed to perform a nondiscretionary duty 

under the CAA by not designating all portions of the country by the June 2013 deadline. In an 

effort intended to resolve the litigation in one of those cases, plaintiffs Sierra Club and the 

Natural Resources Defense Council and the EPA filed a proposed consent decree with the U.S. 

District Court for the Northern District of California. On March 2, 2015, the court entered the 

consent decree and issued an enforceable order for the EPA to complete the area designations

according to the court-ordered schedule.

According to the court-ordered schedule, the EPA must complete the remaining designations by 

three specific deadlines. By no later than July 2, 2016 (16 months from the court’s order), the 

EPA must designate two groups of areas: (1) areas that have newly monitored violations of the 

2010 SO2 NAAQS and (2) areas that contain any stationary sources that had not been announced 

as of March 2, 2015 for retirement and that according to the EPA’s Air Markets Database 

emitted in 2012 either (i) more than 16,000 tons of SO2 or (ii) more than 2,600 tons of SO2 with

an annual average emission rate of at least 0.45 pounds of SO2 per one million British thermal 

units (lbs SO2/mmBTU). Specifically, a stationary source with a coal-fired unit that as of January 

1, 2010 had a capacity of over 5 megawatts and otherwise meets the emissions criteria, is 

excluded from the July 2, 2016 deadline if it had announced through a company public 

 

1 40 CFR 50.4(e) provides that the two prior primary NAAQS will no longer apply to an area one year after its 

designation under the 2010 NAAQS, except that for areas designated nonattainment under the prior NAAQS as of 

August 22, 2010, and areas not meeting the requirements of a state implementation plan (SIP) Call under the prior 

NAAQS, the prior NAAQS will apply until that area submits and the EPA approves a SIP providing for attainment 

of the 2010 NAAQS. Lamar County, Mississippi is not such an area.
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announcement, public utilities commission filing, consent decree, public legal settlement, final 

state or federal permit filing, or other similar means of communication, by March 2, 2015, that it 

will cease burning coal at that unit.

The last two deadlines for completing remaining designations are December 31, 2017, and 

December 31, 2020. The EPA has separately promulgated requirements for states and other air 

agencies to provide additional monitoring or modeling information on a timetable consistent with 

these designation deadlines. We expect this information to become available in time to help 

inform these subsequent designations. These requirements were promulgated on August 21, 2015 

(80 FR 51052), in a rule known as the SO2 Data Requirements Rule (DRR).

Updated designations guidance was issued by the EPA through a March 20, 2015 memorandum 

from Stephen D. Page, Director, U.S. EPA, Office of Air Quality Planning and Standards, to Air 

Division Directors, U.S. EPA Regions I-X. This memorandum supersedes earlier designation 

guidance for the 2010 SO2 NAAQS, issued on March 24, 2011, and it identifies factors that the 

EPA intends to evaluate in determining whether areas are in violation of the 2010 SO2 NAAQS. 

The guidance also contains the factors the EPA intends to evaluate in determining the boundaries 

for all remaining areas in the country, consistent with the court’s order and schedule. These 

factors include: 1) Air quality characterization via ambient monitoring or dispersion modeling 

results; 2) Emissions-related data; 3) Meteorology; 4) Geography and topography; and 5) 

Jurisdictional boundaries. This guidance was supplemented by two technical assistance 

documents intended to assist states and other interested parties in their efforts to characterize air 

quality through air dispersion modeling or ambient air quality monitoring for sources that emit 

SO2. Notably, the EPA released its most recent versions of documents titled, “SO2 NAAQS 

Designations Modeling Technical Assistance Document” (Modeling TAD) and “SO2 NAAQS 

Designations Source-Oriented Monitoring Technical Assistance Document” (Monitoring TAD) 

in December 2013.

Based on ambient air quality data collected between 2012 and 2014, no monitored violations of 

the 2010 SO2 NAAQS have been recorded in any undesignated part of the state.2 However, there 

is one source in the state meeting the emissions criteria of the consent decree for which the EPA 

must complete designations by July 2, 2016. In this draft technical support document, the EPA 

discusses its review and technical analysis of Mississippi’s updated recommendations for the 

area that we must designate. The EPA also discusses any intended modifications from the state’s 

recommendation based on all available data before us.

 

2 For designations based on ambient air quality monitoring data that violates the 2010 SO2 NAAQS, the consent 

decree directs the EPA to evaluate data collected between 2013 and 2015. Absent complete, quality assured and 

certified data for 2015, the analyses of applicable areas for the EPA’s intended designations will be informed by data 

collected between 2012 and 2014. States with monitors that have recorded a violation of the 2010 SO2 NAAQS 

during these years have the option of submitting complete, quality assured and certified data for calendar year 2015 

by April 19, 2016 to the EPA for evaluation. If after our review, the ambient air quality data for the area indicates 

that no violation of the NAAQS occurred between 2013 and 2015, the consent decree does not obligate the EPA to 

complete the designation. Instead, we may designate the area and all other previously undesignated areas in the state 

on a schedule consistent with the prescribed timing of the court order, i.e., by December 31, 2017, or December 31, 

2020. 
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The following are definitions of important terms used in this document: 

1. 2010 SO2 NAAQS – The primary NAAQS for SO2 promulgated in 2010. This NAAQS is 

75 ppb, based on the three year average of the 99th percentile of the annual distribution 

of daily maximum one-hour average concentrations. See 40 CFR 50.17. 

2. Design Value - a statistic computed according to the data handling procedures of the 

NAAQS (in 40 CFR part 50 Appendix T) that, by comparison to the level of the NAAQS, 

indicates whether the area is violating the NAAQS.

3. Designated nonattainment area – an area which the EPA has determined has violated the 

2010 SO2 NAAQS or contributed to a violation in a nearby area. A nonattainment 

designation reflects considerations of state recommendations and all of the information 

discussed in this document. The EPA’s decision is based on all available information 

including the most recent 3 years of air quality monitoring data, available modeling 

analysis, and any other relevant information.

4. Designated unclassifiable area – an area which the EPA cannot determine based on all 

available information whether or not it meets the 2010 SO2 NAAQS.

5. Designated unclassifiable/attainment area – an area which the EPA has determined to 

have sufficient evidence to find either is attaining or is likely to be attaining the NAAQS. 

The EPA’s decision is based on all available information including the most recent 3 

years of air quality monitoring data, available modeling analysis, and any other relevant 

information.

6. Modeled violation – a violation based on air dispersion modeling.

7. Recommended attainment area – an area a state or tribe has recommended that the EPA 

designate as attainment. 

8. Recommended nonattainment area – an area a state or tribe has recommended that the 

EPA designate as nonattainment.

9. Recommended unclassifiable area – an area a state or tribe has recommended that the 

EPA designate as unclassifiable.

10. Recommended unclassifiable/attainment area – an area a state or tribe has recommended 

that the EPA designate as unclassifiable/attainment.

11. Violating monitor – an ambient air monitor meeting all methods, quality assurance and 

siting criteria and requirements whose valid design value exceeds 75 ppb, based on data 

analysis conducted in accordance with Appendix T of 40 CFR part 50. 
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Technical Analysis for the Lamar County, Mississippi Area

Introduction

Lamar County, Mississippi contains a stationary source that according to the EPA’s Air Markets 

Database emitted in 2012 either more than 16,000 tons of SO2 or more than 2,600 tons of SO2

and had an annual average emission rate of at least 0.45 lbs SO2/mmBTU. As of March 2, 2015, 

this stationary source had not met the specific requirements for being “announced for 

retirement.” Specifically, in 2012, the R.D. Morrow, Sr. Generating Plant (R.D. Morrow Plant) 

emitted 3,948 tons of SO2, and had an emissions rate of 0.64 lbs SO2/mmBTU. Pursuant to the

March 2, 2015 court-ordered schedule, the EPA must designate the area surrounding the facility

by July 2, 2016.

In its submission, Mississippi recommended that the area surrounding the R.D. Morrow Plant, 

specifically the entirety of Lamar County (Lamar County Area), be designated as 

unclassifiable/attainment based on an assessment and characterization of air quality from the 

facility and other nearby sources which may have a potential impact in the area of analysis where 

maximum concentrations of SO2 are expected. This assessment and characterization was 

performed using air dispersion modeling software, i.e., AERMOD, analyzing actual emissions.

After careful review of the state’s assessment, supporting documentation, and all available data, 

the EPA agrees that the Lamar County Area is attaining the standard, and intends to designate 

Lamar County as unclassifiable/attainment.

The R.D. Morrow Plant owned and operated by South Mississippi Electric Power Association 

(SMEPA),

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is located in central Mississippi in the eastern portion of Lamar County. As seen in 

Figure 1 below, the facility is located approximately 8.5 kilometers (km) northeast of Purvis,

Mississippi or approximately 12 km southwest of Hattiesburg. Also included in the figure are 

nearby emitters of SO2, the state’s recommended area for the unclassifiable/attainment

designation, and the EPA’s intended unclassifiable/attainment designation for the area.

 

3 The modeling analysis submitted by state of Mississippi was performed by Trinity Consultants at the request of 

SMEPA. Throughout this document the EPA will refer to the state of Mississippi when discussing modeling 

information provided to support their designation recommendation. 
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Figure 1. The EPA’s intended designation for Lamar County, Mississippi. 

The SMEPA characterization of the area surrounding R.D. Morrow Plant included the 

consideration of other nearby sources utilizing emission inventory data from state of Mississippi.

The discussion and analysis that follows below will reference the state’s use of the Modeling 

TAD, the EPA’s assessment of the state’s modeling in accordance with the Modeling TAD, and 

the factors for evaluation contained in the EPA’s March 20, 2015 guidance, as appropriate.

Detailed Assessment

Model Selection and Modeling Components

The EPA’s Modeling TAD notes that for area designations under the 2010 SO2 NAAQS, the 

AERMOD modeling system should be used, unless use of an alternative model can be justified. 

In some instances the recommended model may be a model other than AERMOD, such as the 

BLP model for buoyant line sources. The AERMOD modeling system contains the following 

components:
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 AERMOD: the dispersion model

 AERMAP: the terrain processor for AERMOD

 AERMET: the meteorological data processor for AERMOD

 BPIPPRIME: the building input processor 

 AERMINUTE: a pre-processor to AERMET incorporating 1-minute automated surface 

observation system (ASOS) wind data

 AERSURFACE: the surface characteristics processor for AERMET

 AERSCREEN: a screening version of AERMOD

The state used AERMOD version 14134, and a discussion of the individual components will be 

referenced in the corresponding discussion that follows as appropriate.

Modeling Parameter: Rural or Urban Dispersion

The EPA’s recommended procedure for characterizing an area by prevalent land use is based on 

evaluating the dispersion environment within 3 km of the facility. According to the EPA’s 

modeling guidelines, rural dispersion coefficients are to be used in the dispersion modeling 

analysis if more than 50 percent of the area within a 3 km radius of the facility is classified as 

rural. Conversely, if more than 50 percent of the area is urban, urban dispersion coefficients 

should be used in the modeling analysis. The state analyzed the land use types within a 3 

kilometer radius from the center of the R.D. Morrow Plant as shown in Figure 2. Based on the 

visual inspection of the land use types, the state determined that is was most appropriate to run 

the model with rural dispersion coefficients.
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Figure 2. Land Use Map for area around the R.D. Morrow Plant. Source: 1-Hour SO2 NAAQS 

Designation Modeling Report prepared by Trinity Consultants for the South Mississippi Electric 

Power Association, August 2015.

Modeling Parameter: Area of Analysis (Receptor Grid)

The EPA believes that a reasonable first step towards characterization of air quality in the area 

surrounding the R.D. Morrow Plant is to determine the extent of the area of analysis, i.e., 

receptor grid. Considerations presented in the Modeling TAD include but are not limited to: the 

location of the SO2 emission sources or facilities considered for modeling; the extent of 

significant concentration gradients of nearby sources; and sufficient receptor coverage and 

density to adequately capture and resolve the model predicted maximum SO2 concentrations. For 
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the Lamar County Area, the state has included one other emitter of SO2 within 10 km of the R.D. 

Morrow Plant in any direction. The state considered SO2 sources within 50 km of the R.D. 

Morrow Plant, showing one source with significant emissions within a 10 km radius, and one 

source between 10 and 50 km from R.D. Morrow with more than 40 tons per year (tpy)

emissions. The emissions from Georgia Pacific Leaf River Cellulose, LLC, the smaller source 

located approximately 33 km from the R.D. Morrow Plant, were small relative to its distance 

(65.38 tpy in 2013). The state therefore determined that SO2 emissions from sources within a 10 

km radius around the R.D. Morrow Plant would effectively capture any predicted impacts to air 

quality considering the size of the source, leaving any smaller source impacts to be accounted for 

in the background analysis. In addition to the R.D. Morrow Plant, the other emitter of SO2

included in the area of analysis is Rain CII Carbon, LLC (Rain CII Carbon). The grid receptor 

spacing for the area of analysis chosen by the state is as follows:

 25-meter intervals along the fence line

 100-meter intervals out to a distance of 2.5 km

 500-meter intervals out to a distance of 10 km

The receptor network contained 8,561 receptors, and the network covered the northeastern 

portion of Lamar County in Mississippi.

Figure 3 included in the state’s recommendation, show the state’s chosen area of analysis 

surrounding the R.D. Morrow Plant to represent the Lamar County Area, as well as receptor grid 

for the area of analysis.

Consistent with the Modeling TAD, receptors for the purposes of this designation effort were 

placed only in areas where it would also be feasible to place a monitor to record ambient 

impacts. The impacts of the area’s geography and topography will be discussed later within this 

document.
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Figure 3: Area of Analysis and Receptor Grid for the Lamar County Area of Analysis. Source: 1-

Hour SO2 NAAQS Designation Modeling Report prepared by Trinity Consultants for the South 

Mississippi Electric Power Association, August 2015.

Modeling Parameter: Source Characterization

The state characterized the sources within the area of analysis in accordance with the best 

practices outlined in the Modeling TAD. Specifically, the state used actual stack heights in 

conjunction with actual emissions. The state also adequately characterized the sources’ building 

layouts and locations, as well as the stack parameters, e.g., exit temperature, exit velocity, 

location, and diameter. The AERMOD component BPIPPRIME was used to assist in addressing

building downwash for the R.D. Morrow Plant.

Modeling Parameter: Emissions

The EPA’s Modeling TAD notes that for the purposes of modeling to characterize air quality for 

use in designations, the recommended approach is to use the most recent 3 years of actual 

emissions data and concurrent meteorological data. However, the TAD does provide for the 

flexibility of using allowable emissions in the form of the most recently permitted, (referred to as 

potential to emit (PTE) or allowable) emissions rate.

The EPA believes that continuous emissions monitoring systems (CEMS) data provide

acceptable historical emissions information when it is available, and that these data are available 

for many electric generating units. In the absence of CEMS data, the EPA’s Modeling TAD 

highly encourages the use of AERMOD’s hourly varying emissions keyword HOUREMIS, or 

through the use of AERMOD’s variable emissions factors keyword EMISFACT. When choosing 
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one of these methods, the EPA believes that detailed throughput, operating schedules, and 

emissions information from the impacted source(s) should be used.

In certain instances, states and other interested parties may find that it is more advantageous or 

simpler to use PTE rates as part of their modeling runs. Specifically, a facility may have recently 

adopted a new federally-enforceable emissions limit, been subject to a federally- enforceable 

consent decree, or implemented other federally enforceable mechanisms and control 

technologies to limit SO2 emissions to a level that indicates compliance with the NAAQS. These 

new limits or conditions may be used in the application of AERMOD. In these cases, the 

Modeling TAD notes that the existing SO2 emissions inventories used for permitting or SIP 

planning demonstrations should contain the necessary emissions information for designationsrelated modeling. In the event that these short-term emissions are not readily available, they may 

be calculated using the methodology in Table 8-1 of Appendix W to 40 CFR Part 51 titled, 

“Guideline on Air Quality Models.” 

As previously noted, the state included R.D. Morrow Plant and one other emitter of SO2 within 

10 km in the area of analysis. This distance and these facilities were selected because the state 

believes that this area of analysis adequately represents the area where maximum concentrations 

of SO2 are expected and adequately includes the sources which might contribute to those 

concentrations. SMEPA performed an analysis considering nearby source SO2 emissions and 

distance from the R.D. Morrow Plant to determine their potential to cause or contribute to 

violation of the SO2 NAAQS. Emissions from the Rain CII Carbon in Lamar County, were 

included in the modeling analysis due to its close proximity to the R.D. Morrow Plant 

(approximately 3.85 km) and 2014 actual emissions of 498.63 tpy. Additionally, there are no 

other sources in Lamar County with SO2 emissions above 0.10 tpy, according to the 2011 NEI

v2(including the I. H. Bass Jr. Memorial Airport, Hunt Southland Refining Company, Merit 

Health Wesley Medical Center and Freedom Strip Airport). The Georgia Pacific Leaf River 

Cellulose plant located in Perry County, MS (two counties east of Lamar County) was not 

considered in the modeling analysis due to its distance from R.D. Morrow (approximately 33 

km) and Lamar county border (28 km) relative to its SO2 actual emissions. No other sources 

beyond 10 km were determined by the state to have the potential to cause significant 

concentration gradient impacts within the area of analysis. The facilities in the area of analysis 

and their associated annual actual SO2 emissions between 2012 and 2014 are summarized in 

Table 2 below.
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Table 2: Actual SO2 Emissions Between 2012 – 2014 from Facilities in the Lamar County, 

Mississippi Area of Analysis4

Facility Name

SO2 Emissions (tpy)

2012 2013 2014

R.D. Morrow 

Unit 1 – AA-001 1,500 1,128 938.3

R.D. Morrow 

Unit 2 – AA-002 2,448 2,043 1,271

Rain CII Carbon 717.22 718.3 498.63

Total Emissions 

From All 

Facilities in the 

State’s Area of 

Analysis 4,665 3,889 2,708

For R.D. Morrow Units 1 and 2 in the area of analysis, the state used actual emissions from the 

most recent 3-year data set, i.e., 2012 – 2014. These emissions data were obtained from CEMS 

data provided by SMEPA. This data was incorporated in the dispersion modeling using the 

hourly varying emissions options (AERMOD keyword: HOUREMIS) to represent the variability 

in emissions accurately.

For Rain CII Carbon in the area of analysis, the state used actual emissions from the most recent 

3-year data set, i.e., 2012 – 2014. The Rain CII Carbon facility does not have hourly CEMS data 

available, so these emissions data were calculated from production rates and SO2 emission 

factors obtained from stack-testing. The details of the emissions calculations are provided in 

Appendix C of the modeling report submitted by Mississippi. The calculations show that 

emissions were derived from annual production rates and annual hours of operation. The EPA 

requested that Mississippi provide more temporally resolved emissions data, as is recommended 

in the Modeling TAD. In response to the EPA’s request, Mississippi provided monthly varying 

emissions rates calculated from monthly production rates and indicated this was the most 

resolved data available. Mississippi also explained that Rain CII Carbon’s coking process is done 

in a rotary kiln unit that is operated continuously for long stretches at a time and is not a batch 

process, which helps support the contention that the emissions from the unit would be relatively 

consistent. The level of variation in the monthly data is relatively small and indicates that 

modeling using the emissions from the annual production rates is acceptable. Also due to limited 

information being available for the Rain CII Carbon facility, potential impacts of building 

downwash were not included in the modeling of the facility. While this introduces some level of 

uncertainty in the modeling results near the Rain CII Carbon facility, the potential impacts from 

building downwash would occur near the Rain CII Carbon facility and the maximum combined 

impacts from the R.D. Morrow and Rain CII Carbon (148.04 μg/m3) are located near the R.D. 

Morrow Plant and are significantly below the 1-hour SO2 NAAQS of 196.5 μg/m3.

 

4 Data provided in the SMEPA modeling report and submitted to the EPA by Mississippi was provided by the state, 

and is consistent with emissions the state submitted to the Emissions Inventory System (EIS) gateway pursuant to 40 

CFR Part 51, Subpart A. The EIS gateway can be accessed via: http://www3.epa.gov/ttnchie1/eis/gateway/.
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Modeling Parameter: Meteorology and Surface Characteristics

The most recent 3 years of meteorological data (concurrent with the most recent 3 years of 

emissions data) should be used in designations efforts. As noted in the Modeling TAD, the 

selection of data should be based on spatial and climatological (temporal) representativeness. 

The representativeness of the data are based on: 1) the proximity of the meteorological 

monitoring site to the area under consideration, 2) the complexity of terrain, 3) the exposure of 

the meteorological site, and 4) the period of time during which data are collected. Sources of 

meteorological data include National Weather Service (NWS) stations, site-specific or onsite 

data, and other sources such as universities, Federal Aviation Administration, and military 

stations.

For the Lamar County Area of analysis, surface meteorology from the U.S. National Climatic 

Data Center (NCDC) for Bobby L Chain Municipal Airport Station in Hattiesburg, Mississippi, 

approximately 26 km to the southeast, and coincident upper air observations from the NWS 

station in Jackson, Mississippi, approximately 140 km to the northwest were selected as best 

representative of meteorological conditions within the area of analysis.

The state used AERSURFACE version 13016 using data from the Bobby L Chain Municipal 

Airport Station in Hattiesburg, Mississippi (located at [31.269483, -89.256108]) to estimate the 

surface characteristics of the area of analysis. The state estimated values for 12 spatial sectors out 

to 1 km at a monthly temporal resolution for average conditions. The state also estimated values 

for albedo (the fraction of solar energy reflected from the earth back into space), the Bowen ratio 

(the method generally used to calculate heat lost or heat gained in a substance), and the surface 

roughness (sometimes referred to as “Zo”). In Figure 4, generated by the EPA, the location of the 

Bobby L Chain Municipal Airport Station is shown relative to the R.D. Morrow Plant and other 

sources in the area of analysis.
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Figure 4: Lamar County Area of Analysis and the Bobby L Chain Municipal Airport Station in 

Hattiesburg, Mississippi

The 3-year surface wind rose for Hattiesburg, Mississippi generated by the EPA is depicted in 

Figure 5. In Figure 5, the frequency and magnitude of wind speed and direction are defined in 

terms of from where the wind is blowing. The wind rose shows that the winds blow 

predominately from South-Southeast and North-Northwest directions, mostly at mid-level wind 

speeds of 4-17 knots.
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Figure 5: Hattiesburg, Mississippi Cumulative Annual Wind Rose for Years 2012 – 2014

WRPLOT View - Lakes Environmental Software

WIND ROSE PLOT:

2012-2014 Wind Rose

Bobby L. Chain Airport Station (KHGB, WMO 724560)

COMMENTS:

Created by Rick Gillam

EPA Region 4

11/17/15

COMPANY NAME:

MODELER:

DATE:

11/17/2015

PROJECT NO.:

NORTH

SOUTH

WEST EAST

2%

4%

6%

8%

10%

WIND SPEED 

(Knots)

 >= 22

 17 - 21

 11 - 17

 7 - 11

 4 - 7

 1 - 4

Calms: 4.21%

TOTAL COUNT:

26232 hrs.

CALM WINDS:

4.21%

DATA PERIOD:

Start Date: 1/1/2012 - 00:00

End Date: 12/31/2014 - 23:00

AVG. WIND SPEED:

4.88 Knots

DISPLAY:

 Wind Speed

Direction (blowing from)
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Meteorological data from the above surface and upper air stations were used in generating 

AERMOD-ready files with the AERMET processor. The output meteorological data created by 

the AERMET processor is suitable for being applied with AERMOD input files for AERMOD 

modeling runs. The Mississippi Department of Environmental Quality provided preprocessed 

meteorological files for the AERMOD modeling. Integrated surface hourly data was obtained 

from the National Climatic Data Center (NCDC) in DS3505 format. Radiosonde observations 

were obtained from the NOAA/ESRL Radiosonde Database. The state followed the methodology 

and settings presented in the document titled: “MS AERMOD Ready Met Files – Supporting 

Documentation, July 01, 2014,” as attached in Appendix B of the modeling report provided by 

Mississippi, in the processing of the raw meteorological data into an AERMOD-ready format, 

and used AERSURFACE to best represent surface characteristics. 

Hourly surface meteorological data records are read by AERMET, and include all the necessary 

elements for data processing. However, wind data taken at hourly intervals may not always 

portray wind conditions for the entire hour, which can be variable in nature. Hourly wind data 

may also be overly prone to indicate calm conditions, which are not modeled by AERMOD. In 

order to better represent actual wind conditions at the meteorological tower, wind data of one 

minute duration was provided from the same instrument tower, but in a different formatted file to 

be processed by a separate preprocessor, AERMINUTE. These data were subsequently 

integrated into the AERMET processing to produce final hourly wind records of AERMODready meteorological data that better estimate actual hourly average conditions and that are less 

prone to over-report calm wind conditions. This allows AERMOD to apply more hours of 

meteorology to modeled inputs, and therefore produce a more complete set of concentration 

estimates. As a guard against excessively high concentrations that could be produced by 

AERMOD in very light wind conditions, the state set a minimum threshold of 0.5 meters per 

second in processing meteorological data for use in AERMOD. This approach is consistent with 

a March 2013 EPA memo titled, “Use of ASOS meteorological data in AERMOD dispersion 

Modeling.” In setting this threshold, no wind speeds lower than this value would be used for 

determining concentrations. This threshold was specifically applied to the one minute wind data. 

Modeling Parameter: Geography and Terrain

The terrain in the area of analysis is best described as generally flat and without complex terrain. 

To account for these minor terrain changes, the AERMAP terrain program within AERMOD was 

used to specify terrain elevations for all the receptors. The source of the elevation data 

incorporated into the model is from the United States Geological Survey National Elevation 

Database.

Modeling Parameter: Background Concentrations of SO2

The Modeling TAD offers two mechanisms for characterizing background concentrations of SO2

that are ultimately added to the modeled design values: 1) a “first tier” approach, based on 

monitored design values, or 2) a temporally varying approach, based on the 99th percentile 
17

monitored concentrations by hour of day and season or month. For the Lamar County Area, the 

state chose the temporally varying approach and used the most recent data (2014) from the 

Jackson monitoring station. The background concentration for this area of analysis was 

determined by the state to be 36.65 micrograms per cubic meter (μg/m3), or 13.99 ppb,

5

and that 

value was incorporated into the final AERMOD results. 

Summary of Modeling Results

The AERMOD modeling parameters for the Lamar County Area of analysis are summarized 

below in Table 3.

Table 3: AERMOD Modeling Parameters for the Lamar County Area of Analysis

Lamar County Area of Analysis

AERMOD Version 14134

Dispersion Characteristics Rural

Modeled Sources 2

Modeled Stacks 3

Modeled Structures unavailable

Modeled Fencelines 2

Total receptors 8,561

Emissions Type Actual

Emissions Years 2012-2014

Meteorology Years 2012-2014

Surface Meteorology Station Hattiesburg, Mississippi

Upper Air Meteorology Station Jackson, Mississippi

Methodology for Calculating 

Background SO2 Concentration Temporal Varying

Calculated Background SO2

Concentration 36.65 μg/m3

The results presented below in Table 4 show the magnitude and geographic location of the 

highest predicted modeled concentration based on actual emissions.

 

5 The conversion factor for SO2 (at the standard conditions applied in the ambient SO2 reference method) is 1 ppb = 

approximately 2.62 μg/m3.
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Table 4: Maximum Predicted 99th Percentile 1-Hour SO2 Concentration in the Lamar County

Area of Analysis Based on Actual Emissions

Averaging Period Data Period

Receptor Location SO2 Concentration (μg/m3

)

UTM/Latitude UTM/Longitude

Modeled (including 

background) NAAQS

99th Percentile 

1-Hour Average 2012-2014 272900 3456400 148.04 196.5*

*Equivalent to the 2010 SO2 NAAQS set at 75 ppb

The state’s modeling indicates that the predicted 99th percentile 1-hour average concentration 

within the chosen modeling domain is 148.04 μg/m3, or 56.49 ppb. This modeled concentration 

includes the background concentration of SO2, and is based on actual emissions from the 

facilities. Figure 6 below was included as part of the state’s recommendation, and indicates that 

the predicted value occurred directly east of R.D. Morrow Plant. The state’s receptor grid is also 

shown in the Figure.

Figure 6: Maximum Predicted 99th Percentile 1-Hour SO2 Concentrations in the

Lamar County Area of Analysis Based on Actual Emissions. Source: 1-Hour SO2 NAAQS 

Designation Modeling Report prepared by Trinity Consultants for the South Mississippi Electric 

Power Association, August 2015.
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Jurisdictional Boundaries:

Once the geographic area of analysis associated with the R.D. Morrow Plant, other nearby 

sources, and background concentration is determined, existing jurisdictional boundaries are 

considered for the purpose of informing our intended unclassifiable/attainment area, specifically 

with respect to clearly defined legal boundaries. The state requested the entirety of Lamar 

County, which includes the area immediately surrounding the R.D. Morrow Plant, be designated 

as unclassifiable/attainment. This area has no former nonattainment areas for the 2010 SO2

NAAQS, and as previously discussed, emissions from the R.D. Morrow Plant and Rain CII 

Carbon were included in the modeling analysis for the Lamar County Area, indicating attainment 

with the NAAQS. Additionally, there are no other sources in Lamar County with SO2 emissions 

above 0.10 tpy, according to the 2011 NEI v2 (these include the I. H. Bass Jr. Memorial Airport, 

Hunt Southland Refining Company, Merit Health Wesley Medical Center and Freedom Strip 

Airport). 

Furthermore, the only facility within 50 km of the R.D. Morrow Plant with emissions above 40 

tpy is Georgia Pacific Leaf River Cellulose, LLC. This facility is located in Perry County (two 

counties away from Lamar County), and is approximately 33 km east of the R.D. Morrow Plant 

and 28 km from the Lamar County border. Its 2014 SO2 emissions were 48.55 tpy, and in 

conjunction with the distance from Lamar County, the EPA does not believe that emissions from 

this facility are likely to cause or contribute to a violation of the NAAQS in Lamar County. 

As a result, the EPA’s intended unclassifiable/attainment area, consisting of the entirety of 

Lamar County, is comprised of clearly defined legal boundaries, and we find these boundaries to 

be a suitably clear basis for defining our intended unclassifiable/attainment area.

Other Relevant Information

The EPA received no additional information regarding the R.D. Morrow Plant or its surrounding 

area.

Conclusion

Based on the modeling results provided to the state from SMEPA, including background levels 

of SO2 and SO2 emissions within Lamar County, the state concluded that the entirety of Lamar 

County should be designated unclassifiable/attainment for the 2010 SO2 NAAQS.

After careful evaluation of the state’s recommendation and supporting information, as well as all 

available relevant information, the EPA intends to designate the Lamar County Area as 

unclassifiable/attainment for the 2010 SO2 NAAQS. Specifically, the boundaries are comprised 

of the entirety of Lamar County.

At this time, our intended designations for Mississippi only applies to this area. Consistent with 

the conditions in the March 2, 2015 court-ordered schedule, the EPA will evaluate and designate 

all remaining undesignated areas in Mississippi by either December 31, 2017, or December 31, 

2020.