UNITED STATES ENVIRONMENTAL PROTECnON AGENCY

RESEARCH TRIANGLE PARK, NC 2n"

{FEB 2 6 2lltD'

OFFICE OF

AIR QUAlm' PlANNING

AND STAND....ROS

MEMORANDUM

SUBJECT:

FROM:

TO:

Model Clearinghouse Review of Modeling Procedures for Demonstrating

Compliance with PMu NAAQS

Tyler Fox, Leader

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Air Quality Modelin G;:tup, 39- I

Erik Snyder, Lead Regional Modeler

Air Planning Section (6PD·L), EPA Region 6

Jeff Robinson, Chief

Air Permits Section (6PD-R), EPA Region 6

INTRODUCTION

In response to your memorandum ofFebruary 4,2010. the Model Clearinghouse has reviewed

Region 6'5 position on the proposed modeling demonstration of compliance with the PM2.S

National Ambient Air Quality Standard (NAAQS) by Nucor Corporation for the proposed pig

iron plant in 51. James Parish, LA. The memorandum specifically requested a review ofthe

selection ofthe background monitoring site and the proJX>sed methodology for calculating the

total ambient air quality concentration combining the monitored background and modeled PMl.s

concentrations.

Nucor Corporation originally submitted a Title V and Prevention ofSignificant Deterioration

(PSD) pennit application for a proposed pig iron plant to be located in 51. James Parish,

Louisiana in May 2008. Following several application amendments, the facility submitted a

modeling analysis in August 2009, which relied on the PMlo surrogate JX>licy regarding PMl.S

compliance that was in effect at the time. In light ofEPA's current position as to how states

should implement the surrogate policy in their pennitting process, as reflected in our response to

the petition regarding the Louisville Gas and Electric (LG&E) Title V pennit, EPA Region 6

urged the Louisiana Department ofEnvironmental Quality (LDEQ) to either justify using PM10

as an appropriate surrogate for PMl.Sor request Nucor to conduct a PMl.S modeling analysis. A

dispersion modeling protocol for demonstrating compliance with PMz.s NAAQS was submitted

to LDEQ and EPA for review in October 2009. Upon review EPA Region 6 and LDEQ

requested that additional years and monitors be evaluated for establishing a representative

background monitor for PM;uIntemel Addreu (VAL) • http://www..,..gov

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On November 19, 2009 Nucor Corporation submitted a revised modeling protocol for the 

assessment of PM2.5 impacts associated with the proposed pig iron plant to be located in the St. 

James Parish in Louisiana. The revised protocol proposed the use of the maximum modeled 

annual average and maximum 8th highest 24-hour average PM2.5 concentration to be added to the 

representative background concentration and compared with the PM2.5 NAAQS. Nucor 

proposed to calculate the background concentration by averaging the 98th percentile from years 

2007-2008 because it believes 2006 is not representative of normal background air quality 

conditions for Louisiana. 

MODEL CLEARINGHOUSE RESPONSE 

Based on the information presented in the Region 6 request, the Model Clearinghouse concurs 

with Region 6’s position regarding the selection of the Bayou Plaquemine monitoring site for 

determining background PM2.5 concentrations, and on the requirement to include 2006 

monitoring data. While we concur with the application of procedures consistent with Appendix 

N to 40 CFR Part 50 for determining background monitored concentrations for PM2.5 NAAQS 

analyses, in order to be consistent with those procedures the 2006 monitoring data must be 

considered unless specific PM2.5 observations were requested to be excluded for comparison with 

the relevant NAAQS by the LDEQ and approved by the EPA pursuant to the requirements 

established under 40 CFR § 50.14 to address exceptional events. In the absence of an 

exceptional events declaration, the 2006 data must be considered. In any case, the monitored 

background PM2.5 concentration included in a cumulative impact assessment should be based on 

3 years of monitoring data, to be consistent with the form of the NAAQS. 

Given the generic issues associated with PM2.5 dispersion modeling and the specifics of this case, 

the Model Clearinghouse cannot endorse aspects of the modeling protocol presented by Nucor 

and approved by EPA Region 6 for the proposed pig iron plant. Specifically, we cannot concur 

with the use of the highest of the 8th highest (98th percentile) modeled 24-hour impacts from the 

5-year meteorological record for the modeled component of the cumulative impact assessment. 

Our position is based on a concern that cumulative PM2.5 impacts estimated based on this 

approach may not be protective of the 24-hour PM2.5 NAAQS, due to the method proposed for 

combining the 98th percentile monitored background concentration with the 98th percentile 

modeled concentration, in the form of the highest 8th high (H8H) value over five years of 

modeling. Combining the 98th percentile monitored value with the 98th percentile modeled 

concentrations for a cumulative impact assessment would result in a value that is below the 98th

percentile of the combined cumulative distribution and would therefore not be protective of the 

NAAQS. The Model Clearinghouse recommends use of the average of the 1st highest modeled 

24-hour impacts over 5 years as the modeled contribution to the cumulative NAAQS compliance 

analysis. It should also be noted that the use of a 3-year average for monitored design values to 

determine attainment of the NAAQS does not preempt the Appendix W requirement for use of 5 

years of National Weather Service (NWS) data, and the 5-year average of modeled impacts 

serves as an unbiased estimate of the 3-year average for purposes of modeling demonstrations of 

compliance with the NAAQS. 
3

EPA will be following up this Model Clearinghouse memorandum with additional clarification 

regarding appropriate modeling procedures for demonstrating compliance with PM2.5 NAAQS, 

as well as issues associated with use of the PM10 surrogate policy in light of recent EPA 

regulatory actions and proposals. 

cc: Richard Wayland, C304-02 

Bill Harnett, C504-01 

Raj Rao, C504-01 

Roger Brode, C439-01 

Bret Anderson, C439-01 

Dan deRoeck, C504-01 

EPA Regional Modeling Contacts 
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

REGION 6

1445 ROSS AVENUE, SUITE 1200

DALLAS, TX 75202-2733

FEB 0 4 2010'

MEMORANDUM

SUBJECT:

FROM:

TO:

Protocol for Modeling of 24-Hour and Annual Impacts Under the New PM2.

5

NAAQS

Erik Snyder, Lead Regional Modeler

Air Planning Section (6PD-L)

Jeff Robinson, Chief ~:J>f Air Permits Section (6PD-R) '-'--is\l

Tyler Fox, Leader

Air Quality Modeling Group (C439-1)

Office of Air Quality Planning and Standards

This memorandum seeks the review and concurrence from the EPA Model Clearinghouse

on Region 6's acceptance and approval of modeling methodologies for PM2.5 impacts associated

with the proposed Prevention of Significant Deterioration (PSD) permit for the Nucor

Corporation pig iron facility in St. James Parish, Louisiana. These techniques will be used to

determine compliance with both the 24-hour and annual averaging periods for the new PM25

national ambient air quality standards (NAAQS). Since the petition was granted on the

Louisville Gas and Electric (LG&E) facility in August 2009 there has been a lack offormal

guidance on how to address PM2.5 modeling analyses for PSD permitting actions. Without the

benefit of formal written guidance from the Office ofAir Quality Planning and Standards, the

U.S. Environmental Protection Agency (EPA) Region 6 approved the PM25 NAAQS analysis

techniques discussed below in an email to the Louisiana Department of Environmental Quality

(LDEQ) on January 12, 2010.

BACKGROUND

Nucor Corporation originally submitted a Title V and PSD permit application for a

proposed pig iron plant to be located in St. James Parish, Louisiana, in May 2008. Through

several application amendments, a permit proposal (October 15, 2008), and iterations of

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2

modeling the facility submitted their latest modeling in August 2009. After review ofthe

last submittal the Louisiana Department ofEnvironmental Quality (LDEEQ) was ready to

make the draft permit available for a public comment period in late September. In September,

EPA Region 6 discussed the recent LG&E petition and urged LDEQ to either justify using

the PMIO modeling as a surrogate for PM25 or request Nucor to conduct a PM25 modeling

analysis. The LDEQ proposed the draft Title V and PSD permit September 2,2009. Then on

September 25, 2009, LDEQ canceled the public hearing and public comment period, and

requested Nucor to submit a PM2.5 modeling protocol. A protocol was submitted to LDEQ and

EPA for review in October 2009. Upon review, EPA Region 6 and LDEQ requested additional

years and monitors be evaluated for establishing a representative background monitor for PM2.5

On November 19, 2009, Nucor Corporation submitted a revised modeling protocol (with

two supplements in early December) for the modeling ofPM2.5 impacts. The revised protocol

proposed the use ofthe maximum annual average and maximum Sth highest 24-hour average

PM25 concentration to be added to the representative background concentration and compared

with the PM2.5NAAQS. Nucor proposed to calculate the background concentration by

averaging the 9Sth percentile from years 2007-200S because it believes 2006 is not representative

of normal background air quality conditions for Louisiana.

BACKGROUND MONITOR SELECTION

To calculate the total air quality concentration for comparison against an NAAQS, it is

necessary to consider background monitored concentrations to characterize the influence of

emissions from naturally occurring sources and sources not identified explicitly in the modeling

analysis. Under Section S.2 ofAppendix W, a series of general guidelines are offered to aid in

the selection of background monitoring sites. However, due to the unique character ofPM2.5

being a mixture of both primary and secondary particulate matter, monitor site selection is

complicated by the necessity to insure that the monitoring site selected adequately characterizes

the secondary components ofPM2.5but is not burdened by impacts of direct PM2.5 from the

source or sources explicitly modeling in the NAAQS analysis. Therefore, careful consideration

must be given to the representativeness ofthe monitoring site selected.

In discussions with the state of Louisiana and Nucor representatives, EPA Region 6

outlined the following for selection ofthe background PM2.5 monitor and concentration for the

PM2.5 dispersion modeling analyses.

I. Use the Geismar monitor (St. Gabriel Agricultural Exp. Station) in Iberville Parish as the

background monitor unless justification can be provided to use another monitor.

2. To determine the background concentration, use a three year period (2006 - 200S). Use

the highest annual average over this period. Use the highest 24-hour average

concentration ifthe sampling period is one every six days and the second highest 24-hour

average concentration ifthe sampling period is one every 3 days. The 9Sth percentile can

be used ifthe sampling frequency is daily.
3

The proposed pig iron plant site is located in St. James Parish and is in a rural agricultural

environment along the Mississippi River. There are some industrial facilities located along the

river, but the Nucor site is not in a heavily industrialized area. The Geismar monitor, initially

recommended by EPA Region 6, is sited in an industrial environment. This monitor is located

approximately 25 Ian from the Nucor site. Nucor believed that this area is not representative of

the proposed pig iron facility site and that an alternate monitor should be used for assigning

background PM2.5 concentrations. Nucor recommended that the Bayou Plaquemine monitor in

the Iberville Parish be used as the background monitor. This monitor is in a rural agricultural

environment that is similar to the Nucor site. Although the monitor is located 47 Ian from the

site, it is located within the study area. In order to avoid the potential for "double counting"

impacts from sources of primary PM that are to be explicitly modeled in the NAAQS analysis

but yet select a monitor which adequately characterizes the regional secondary component of

PM, EPA Region 6 concurred with the proposed alternative site.

Constructing the 98th Percentile Monitored Background Value

In the modeling protocol, Nucor recommended that the development ofrepresentative

background PM2.5 concentrations be limited to the years 2007 and 2008. Although a three-year

period is required to construct design value concentrations for PM2.5 under Appendix N to

40 CFR Part 50, Nucor argued that monitoring data for PM25 was unusually elevated at many

LDEQ monitors in the state during 2006, suggesting that unusual outside factors created an

anomaly during this year.

Nucor supplemented their protocol with additional information in December 2009 which

identified events which they believed influenced the 2006 monitor values and would make this

year unrepresentative for defining background concentrations. Nucor contended that Louisiana

experienced unusual drought conditions in 2006, and that the number oflocal and regional

wildfires was significantly higher than average. Monitored concentrations ofPM2.5 during 2006

resulted in a 98" percentile background concentration more than 50% higher than any other year

under consideration. Nucor argued that PM2.5monitoring data during 2006 should be considered

a naturally caused anomaly that is not representative of background conditions in the State of

Louisiana and excluded from consideration for construction.

However, Nucor offered no definitive analysis including such things as receptor

modeling of chemically speciated PM2.5data, satellite imagery, aerosol optical depths, or back

trajectory analyses to support the arguments presented in the November and December 2009

submittals to EPA. Neither has the state of Louisiana formally requested an exceptional events

declaration on 2006 PM25 monitoring data to exclude such data from consideration. In the

absence of any such analysis, EPA Region 6 required that the 2006 monitoring year data for the

Bayou Plaquemine monitor be included for calculating the 3-year average ofthe 98th percentile

PM2.

5values for the background concentration.

Calculating the Total Air Quality Concentration

As described in Section 8.2 of Appendix W, background monitored concentrations are an

essential element to calculating the total air quality concentration for comparison with the
4

NAAQS under review. How to combine the monitored background data with modeled impacts

from the source(s) under review is an important question which is technically challenging given

the unique form ofthe new 24-hour PM25 NAAQS.

Under Section 8.3.1.2 (a) of Appendix W, 5-years ofrepresentative meteorological data

are recommended to ensure that the worst case meteorological conditions are adequately

represented in the modeling results. Since the New Source Review (NSR) implementation rule

for the new PM25 24-hour NAAQS was only promulgated in May 2008 and the PMI 0 surrogate

policy was in effect until the June 2009 administrative stay ofthe "grandfathering provision"

related to the surrogate policy, full modeling guidance detailing how model results from the

5-years recommended by Appendix W should be combined with the background monitoring data

to calculate the total air quality concentration was not available at the time ofsubmission ofthe

Nucor protocol.

Since each ofthe years ofthe 5-year meteorological record should be complete, the air

quality model will calculate 365 independent 24-hour concentrations for each meteorological

year. The 8th highest concentration value in the 365-day distribution ofmodeled impacts

corresponds to the 98th percentile value under Appendix N to 40 Code ofFederal Regulations

(CFR) Part 50. A possible methodology to combine modeled impacts with the background

concentrations to calculate a total air quality concentration consistent with the form ofthe

24-hour PM2.5 NAAQS would be to average each ofthe 8th highest concentrations from the

5-years modeled and combine this value with the background monitored value. In summary, this

procedure would combine the average 98th percentile modeled impact with the 98th percentile

monitor concentration to form the total air quality concentration, which we believe would be

consistent with how design value concentrations are determined under Appendix N to

40 CFR Part 50.

Nucor, in its protocol, proposed to use the highest ofthe 8th high modeled impacts from

the 5-year meteorological record. This value would be added to the background monitored value

described in the previous section to calculate the total air quality concentration and then

compared to the 24-hour PM2.5 NAAQS. EPA Region 6 believes that this approach is consistent

with the form ofPM2.5NAAQS, albeit it should be slightly more conservative since it would use

the highest ofthe 8th high concentrations rather than an average ofsuch. EPA Region 6 concurs

with the use ofthis methodology to combine modeled and monitored concentrations to compare

to the PM25 NAAQS.

For demonstrating compliance with the annual PM2.5NAAQS, Nucor will model annual

concentrations of PM2.5 to be added to the monitored annual design value (2006-2008 data).

Nucor will utilize the maximum modeled annual average PM25. The resulting cumulative annual

concentration will then be compared to the annual PM2.5 NAAQS.

Requested Actions Items

Please review the background monitoring site selection, and background concentration

calculation methodology. Please also review the methodology of calculating the total air quality

concentration which combines the 3-year average ofthe 98th percentile monitoring value with the
5

highest of 8th high modeled concentration from the 5-year meteorological record. Please also

review the methodology for demonstrating compliance with the annual PM25 NAAQS.

Please call Erik Snyder at (214) 665-7305 if you have any questions or require additional

information.