Mr. Shawn M. Garvin 

Regional Administrator 

USEPA Region 3 

1650 Arch Street 

Mail Code: 3RAOO 

Philadelphia, PA 19103-2029 

DearM~ 

I am writing to you regarding the 1-hour S02 Characterization Modeling for the area around 

the H.A. Wagner and Brandon Shores power plants air quality modeling protocol that was 

recently submitted to the Maryland Department of the Environment (MOE), Air and 

Radiation Management Administration (ARMA). This modeling protocol was prepared to 

describe the approach being taken to demonstrate that the H.A. Wagner (Wagner) power 

plant located in Anne Arundel County, Maryland would be in attainment of the 1-hour S02 

National Ambient Air Quality Standard (NAAQS). 

On March 20, 2015, EPA informed Maryland that the Wagner Power Plant would be part of 

the expedited round of designations under the 1-hour S02 NAAQS due to terms of the S02 

Consent Decree negotiated between the Sierra Club and EPA (Sierra Club v. McCarthy). 

The EPA intends to designate the Wagner Power Plant area as either 

unclassifiable/attainment, nonattainment or unclassifiable by July 2, 2016 after a review of 

available modeling or monitoring data to support the S02 concentration characterization. 

The model selected for this modeling application is the EPA American Meteorological 

Society/Environmental Protection Agency Regulatory Model (AERMOD) modeling system 

version 15181, including the American Meteorological Society/Environmental Protection 

Agency Regulatory Improvement Committee (AERMIC) meteorological (AERMET) nonregulatory default/beta ADJ_U* option (EPA,2015a). EPA has indicated support for this 

change as part of their July 29, 2015 Appendix W proposal. In addition, Roger Brode's 

(USEPA) Proposed Updates to AERMOD Modeling System presentation (EPA, 2015b) 

delivered at the 11th Modeling Conference on August 12, 2015 indicated that the ADJ_U* 

option be incorporated into the regulatory versions of AERMOD and AERMET in the notice 

of proposed rulemaking (NPRM). 

Section 3.2.2.b of Appendix W to 40 CFR Part 51, ("Revision to the Guideline on Air Quality 

Models", November 9, 2005), details the approach for approval of an alternative model. 

Specially, the request must meet one of the following three conditions: 

1800 Washington Boulevard I Baltimore. MD 21230 I 1-800·633-6101 I 410-537-3000 I TIY Users 1·800·735·2258 

www.mde.maryland.gov 
preferred model. 

The Wagner power plant request falls under condition 3. 

Appendix W (Section 3.2.2.e) states that for condition 3 in paragraph b of section 3.2.2 

of Appendix W to 40CFRPart 51, "an alternative refined model may be used provided that: 

1 . The model has received a scientific peer review; 

2. The model can be demonstrated to be applicable to the problem on a theoretical 

basis; 

3. The databases which are necessary to perform the analysis are available and 

adequate; 

4. Appropriate performance evaluations of the model have shown that the model is not 

biased toward underestimates; and, 

5. A protocol on methods and procedures to be followed has been established." 

These five (5) points are discussed below separately. 

Condition 1 : The model has received a scientific peer review 

EPA has acknowledged poor AERMOD performance during low wind-speed conditions 

(Robinson and Brode 2007). The proposed AERMET formulation changes to the friction 

velocity computation for low wind speeds are referenced in a Boundary-Layer Meteorology 

Qian and Venkatram (2011) peer-reviewed paper which demonstrated that the AERMOD 

meteorological preprocessor (AERMET) tends to grossly under-predict surface friction 

velocity (u*) under low wind-speed conditions (less than two meters per second). When 

simulating emission sources with AERMOD, the under-prediction of u* leads to 

inappropriately low mechanical mixing heights, consequently resulting in overly 

conservative (excessively high) ambient concentration estimations (EPA 2015c; Paine and 

Connors 2013; Qian and Venkatram 2011, Robert Paine 2015). 

Qian and Venkatram (2011) suggested a new method for calculating u* and showed results 

that support improved u* and model concentration predictions in the low wind-speed 

regime. EPA has incorporated this calculation methodology in AERMET as ADJ_U* (EPA 

2013), most recently in AERMET version 15181. The ADJ_ U* method is a processing 

option for calculating u* for low wind speeds during stable (nighttime) conditions (EPA 

2015a}. Several study results support the conclusion that the application of the ADJ_U* 

option significantly improves 

AERMOD performance for low wind-speed conditions while maintaining a conservatively 
theoretical basis 

There is no theoretical limitation to the application of the AERMET ADJ_U*- it is generally 

applicable. The current default algorithm in AERMET has been demonstrated to be faulty 

and needs to be replaced by the ADJ_U* approach. 

Condition 3: The databases which are necessary to perform the analysis are 

available and adequate 

The necessary data needed for implementing ADJ_U* within the AERMOD modeling 

system is routine meteorological data are already available and are sufficient for exercising 

this low wind option. The use of the Baltimore Washington International Thurgood Marshall 

Airport (BWI) National Weather Service (NWS) data is sufficient. There are no special 

database requirements for the use of the ADJ_U* option. 

Condition 4: Appropriate performance evaluations of the model have shown that the 

model is not biased toward underestimates 

There have been many model evaluation studies that illustrate improved performance of 

the AERMOD modeling system with the use of the ADJ_U* option. Most notably is the 

performance evaluation referenced in Appendix F of the AERMOD User's Guide (EPA 

2015c) that was conducted by EPA. This performance evaluation performed by EPA 

utilizes three evaluation databases: (1) Idaho Falls (NOAA Technical Memorandum ERL 

ARL-52, August 1974), (2) Oak Ridge (NOAA Technical Memorandum ERL ARL-61, 

August 1976), and (3) Lovett. The first two databases were part of the API-sponsored 

evaluation of AERMOD conducted by AECOM, that were submitted as part of API's public 

comments on EPA's 10th Conference on Air Quality Models held in March 2012. The two 

NOAA field studies were low-level, non-buoyant tracer releases with three arcs of samplers 

located at 1 00 m, 200 m, and 400 m from the release point. The Idaho Falls study was 

located in a flat terrain, while Oak Ridge was located in complex terrain. 

The third database utilized by EPA to evaluate the ADJ_U* option presented in Appendix F 

of the AERMOD User's Guide (EPA 2015c) was the Lovett database. Lovett is a historical 

AERMOD evaluation database and features a single 145 meter stack located within a few 

kilometers of complex terrain. The Lovett field study was a year-long 802 field program 

with monitors located on the primary terrain features to the north of the stack. 

The most representative field program that could be used to predict the performance of 

ADJ_U* for a modeling study at the Wagner power plant is the Lovett field program. 
In Attachment A (Modeling Analysis to Support Use of ADJ_U

 Option) of this letter is an 

additional analysis completed to demonstrate that the model using ADJ_U* is not biased 

toward underestimate. In addition, the modeling files used in this analysis to support the 

use of ADJ_U* are also enclosed. 

In the proposed revisions to the Guideline (EPA 2015d), EPA intends for the ADJ_U* option 

to be part of the regulatory default AERMOD modeling system. EPA made this proposal in 

the preamble to the proposed changes to the Guideline, referred to below as NPRM. 

Due to several initial comments from stakeholders, members of the EPA modeling group 

provided clarifications (EPA 2015e and 2015f) that reinforced EPA's intent to include 

ADJ_U* as a regulatory default option. These clarifications were provided during EPA's 

11th Conference on Air Quality Modeling and Public Hearing for the Proposed Revisions to 

the Guideline held on August 12-13, 2015 (2015 Conference). EPA's statements regarding 

the ADJ_U* option as presented in the NPRM and the 2015 Conference are provided 

below. 

From NPRM section IV.A.2., "Updates to EPA's AERMOD Modeling System" (EPA 2015g): 

"Based on studies presented and discussed at the Tenth Modeling Conference, and 

additional relevant research since 2010, the EPA and other researchers have 

conducted additional model evaluations and developed changes to the model 

formulation of the AERMOD modeling system to improve model performance in its 

regulatory applications. We propose the following updates to the AERMOD modeling 

system to address a number of technical concerns expressed by stakeholders: 

1. A proposed option incorporated in AERMET to adjust the surface friction 

velocity (u*) to address issues with AERMOD model over prediction under 

stable, low wind speed conditions. This proposed option is selected by the 

user with the METHOD STABLEBL ADJ_U* record in the AERMET Stage 

3 input file." 

As presented on the public record at the 2015 Conference by Tyler Fox in his presentation 

"Overview of Proposed Revisions to Appendix W" (EPA 2015e): 

"In the NPRM, EPA has proposed to incorporate specific updates to the regulatory 

version that are the subject of public review and comment and then would be 

codified as part of the final rule action, as appropriate. 
ltEPA has proposed in the NPRM that the ADJ_U* option (with or without BULKRN) 

be incorporated into the regulatory version of AERMET." 

It is clear that EPA, pending review and comments during the public comment period, 

intends to incorporate ADJ_U* as a regulatory default option. At this time, ADJ_U* remains 

a nondefault option and requires approval from EPA for use in modeling compliance 

demonstrations. According to statements at the 2015 Conference, the proposed revisions 

to the Guideline are expected to be finalized by the spring of 2016 (EPA 2015e). 

MDE/ARMA believes that these evaluations satisfy this condition. 

Condition 5: A protocol on methods and procedures to be followed has been 

established 

A modeling protocol addressing the 1-hour S02 characterization of the Wagner area was 

prepared by AECOM and submitted to EPA Region 3 in February 2016. Comments on the 

modeling protocol were received from EPA Region 3 in March 2016. MDE/ARMA has 

reviewed the modeling protocol prepared for utilizing ADJ_U* and EPA Region 3 comments 

and the implementation of the ADJ_U* in the application, and we believe that the 

comments have been adequately addressed. 

MDE/ARMA believes for the reasons described previously that the ADJ_U* option is 

justified for use in the 1-hour S02 characterization air quality modeling for the Wagner 

power plant. If there are any specific questions related to the technical aspects of this air 

modeling issue, please contact Mr. Michael Woodman of MDE/ARMA at (410) 537-3229. 

Sincerely, 

Ben rumbles 

Secretary 

cc: Tim Leon Guerrero, U.S. EPA Region Ill (e-mail) 

George (Tad) S. Aburn, Jr., MOE 

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(From page F-36 (EPA2015c)) 
EPA, 2015a. Addendum, User's Guide for the AERMOD Meteorological Preprocessor 

(AERMET). (EPA-454/B-03-002, November 2004). Office of Air Quality Planning and 

Standards, Research Triangle Park, NC. June 2015. 

http://www.epa.gov/ttn/scram/7thconf/aermod/aermet userguide.zip. 

EPA, 2015b. "Proposed Updates to AERMOD Modeling System." Roger W. Brode, U.S. 

EPA Office of Air Quality Planning and Standards (OAQPS), Air Quality Assessment 

Division (AQAD), Air Quality Modeling Group (AQMG). Presentation at EPA 11th Modeling 

Conference, Research Triangle Park, NC. August 12, 2015. Accessed March 14, 2016. 

https://www.epa.gov/ttn/scram/11 thmodconf/presentations/1-

5 Proposed Updates AERMOD System.pdf 

Robinson, R. and R. Brode. 2007. "AERMOD Implementation Workgroup." Presentation at 

EPA Regional/State/Local Modelers Workshop. Virginia Beach, VA. May 15-17,2007. 

Accessed March 14, 2016. 

http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2007/presentations/ 

Tuesday%20-%20May%2015%202007 I AERMOD Implementation Workgroup. pdf 

Qian, W. and A. Venkatram. 2011. "Performance of Steady-State Dispersion Models Under 

Low Wind-Speed Conditions." Boundary-Layer Meteorology (2011) 138:475-491 DOl 

10.1 007/s1 0546-010-9565-1. Published online December 3, 2010. Accessed March 14, 

2016. 

http://link.springer.com/article/1 0.1 007%2Fs1 0546-010-9565-1 

EPA. 2015c. "Addendum: User's Guide for the AMS/EPA Regulatory Model- AERMOD 

(EPA-454/B-03-001, September 2004." Office of Air Quality Planning and Standards, Air 

Quality Assessment Division. June 2015. Accessed March 14, 2016. 

http://www.epa.gov/ttn/scram/models/aermod/aermod userguide.zip. 

Paine, B. and J. Connors. 2013. "AERMOD Low Wind Speed Issues: Review of New Model 

Release." AECOM. EPA 2013 Modeling Workshop Presentation. April23, 2013. Accessed 

March 14, 2016. 

http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2013/Files/Presentatio 

ns/Tuesday/1 05-Review of AERMOD Low Wind Speed Options Paine.pdf 

Robert Paine, Olga Samani, Mary Kaplan, Eladio Knipping & Naresh Kumar (2015) 

Evaluation of low wind modeling approaches tor two tall-stack databases, Journal of the Air 

& Waste Management Association, 65:11, 1341-1353, DOl: 

0.1080/10962247.2015.1085924 Accessed on March 14,2016. 

http://www.tandfonline.com/doi/abs/1 0.1080/10962247.2015.1085924 
ations!Tuesday/1 04

Brode AERMOD System Update 

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EPA. 2014. 'Webinar: AERMOD Modeling System Update." Office of Air Quality Planning and 

Standards, Air Quality Modeling Group. January 14, 2014. Accessed March 24, 2016. 

https://www3.epa.qov/ttn/scram/webinar/AERMOD 13350 Update/AERMOD System Update 

Webinar 01-14-2014 FINAL.pdf 

NOAA Technical Memorandum ERL ARL-52. Diffusion Under Low Wind Speed, Inversion 

Conditions. Sagendorf, J.F., C. Dickson. Air Resources Laboratory, Idaho Falls, Idaho. 

August 197 4. 

NOAA Technical Memorandum ERL ARL-61. Diffusion Under Low Wind Speed Conditions 

near Oak Ridge, Tennessee. Wilson, R.B., G. Start, C. Dickson, N. Ricks. Air Resources 

Laboratory, Idaho Falls, Idaho. August 1976. 

EPA. 2015d. "40CFR Part 51, Revision to the Guideline on Air Quality Models: Enhancements 

to the AERMOD Dispersion Modeling System and Incorporation of Approaches To Address 

Ozone and Fine Particulate Matter; Proposed Rule." Federal Register. Vol. 80 No.145. July 

29. 2015. Accessed March 14, 2016. 

http://www.epa.gov/ttn/scram/11 thmodconf/EPA-HQ-OAR-2015-031 0-0001.pdf. 

EPA. 2015e. "Overview of Proposed Revisions to Appendix W ." Tyler Fox, Office of Air Quality 

Planning and Standards. 11th Modeling Conference, Research Triangle Park, NC. August 12, 

2015. Accessed March 14, 2016. 

https://www .epa.gov/ttn/scram/11 thmodconf/presentations/1-3 Overview 11 thMC.pdf 

EPA. 2015f. "Proposed Updates to AERMOD Modeling System." Roger W. Brode, Office of Air 

Quality Planning and Standards, Air Quality Assessment Division, Air Quality Modeling Group. 

11th Modeling Conference, Research Triangle Park, NC. August 12, 2015. Accessed March 

14, 2016. 

https://www .epa.gov/ttn/scram/11 thmodconf/presentations/1-

5 Proposed Updates AERMOD System.pdf 

EPA. 2015g. "40 CFR Part 51 , Revision to the Guideline on Air Quality Models: Enhancements 

to the AERMOD Dispersion Modeling System and Incorporation of Approaches To Address 

Ozone and Fine Particulate Matter; Proposed Rule." Federal Register. Vol. 80. 

No. 145. July 29, 2015. Accessed March 14, 2016. 

http://www.epa.gov/ttn/scram/11 thmodconf/EPA-HQ-OAR-2015-031 0-0001.pdf.