# Attachment A Modeling Analysis to Support Use ofADJ_U* Option This AERMOD modeling analysis was completed to demonstrate that use non-regulatory ADJ_U*is applicable in this instance. 1-hour 

### S02 modeling completed using AERMOD with regulatory default options and variable hourly emissions resulted in modeled 4th high 1-hour 

### S02 concentrations nearby the Wagner power plant (Wagner) area and approximately 35 km to the northwest of the Wagner area in Baltimore County (Figure IA).

Figure 1 A. Modeled 4th High 1-hour 

### S02 Concentrations Within each of these areas of modeled 4th high 1-hour 

### S02 concentrations, one or two of the highest reading receptors were chosen and closely looked at. The areas to the northwest of the Wagner area were assigned the letters A- H to represent the various modeled 4th high 1-hour 

### S02 concentration areas. The group A-G receptors are shown in Figure 2A and Figure 3A shows the receptor group H. The receptors that will be further analyzed are represented by red dots. Areas of modeled 4th high 1-hour 

### S02 concentrations located in the Wagner area are represented by receptor group's 1-N as seen in Figure 4A. A-1



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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option

Figure 2A. Receptor Groups A - G

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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option

Figure 3A. Receptor Group H

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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option

Figure 4A. Receptor Groups I - N A-4 

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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option Within the areas A-Na total of 18 receptors were further analyzed. For each of these receptors the elevation, Julian day, hour, surface friction velocity (U*) and wind speed without and with the adjustment to U* are in Table lA. Based on Table IA, the receptors in groups A-Hare all located in areas of complex terrain and the greatest distance from the Wagner area. In addition, hours of the maximum concentrations at these receptors are all between the hours when stable conditions are expected and winds are light. The application of the ADJ_U* option reduces the frequency of low surface friction velocity (U*) values that are known to result in over-predictions of modeled concentrations with AERMOD. Receptors in groups I- N are in the immediate Wagner area and not located in complex terrain. The surface friction velocities are similar during the hours when unstable conditions are expected. In addition, the data in Table lA and the fact that the modeled 4th 

### high 1-hour S02 

### concentrations in Baltimore County are located at such a far distance from the Wagner area calls the model's applicability into question. The Baltimore Washington International Thurgood Marshall (BWI) Airport wind rose (Figure 5A) shows that the prevailing wind direction is from the west and west northwest. Based on the wind rose the frequency of strong winds needed to cause the high concentrations of 1-hour S02 

### to affect an area 35 kilometers away is not supported. Figure 6A shows the modeling run that was completed using the exact same input parameters, except the non-regulatory ADJ_U* option was used. This particular scenario still resulted in 4th 

### high 1-hour S02 modeled concentrations in the immediate Wagner area but the high concentrations approximately 35 kilometers to the northwest of the Wagner area in Baltimore County were no longer present.

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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option Table IA. Surface Friction Velocity (U*) With and Without the Adjustment Without ADJ U* With ADJ U* Surface Friction Wind Speed Surface Friction Wind Speed

Velocity (U*) (Ws) Velocity (U*) (Ws) Julian Group 

### x y Elevation Day Hour (m/s) (m/s) (m/s) (m/s) A 344732.83 4366325.66 212.8 247 20 0.081 2.25 0.15 2.25 A 344982.83 4365825.66 209.6 247 20 0.081 2.25 0.15 2.25 B 343232.83 4368325.66 213.7 247 20 0.081 2.25 0 . 15 2.25 B 343232.83 4368075.66 210.3 55 22 0.026 0.79 0.095 0.79 

## c 347982.83 4367575 .66 209.1 19 5 0.036 1.08 0.095 1.08 

## c 347953.75 4367592.5 208 .34 19 5 0 .036 1.08 0.095 1.08 D 348232.83 4367825.66 205.3 60 22 0.031 0.86 0.097 0.86 E 347482.83 4368825.66 217.5 19 5 0.036 1.08 0.095 1.08 E 347232.83 4369325.66 219.5 273 19 0.050 1.37 0.095 1.37 F 346232.83 4370075.66 219.4 19 5 0.036 1.08 0.095 1.08 G 348232.83 4365075 .66 201.2 128 20 0.069 1.94 0.126 1.94 H 349953.75 4349092 .5 159.13 273 20 0.047 1.30 0.094 1.30 I 365075 4337890 9.39 358 13 0.118 1.55 0.118 1.55 J 369375 4336940 7.69 363 13 0.179 1.99 0 . 179 1 .99 K 368075 4334890 9 . 16 61 16 0.165 2.04 0.165 2.04 L 369075 4335390 7.84 64 10 0.094 0.69 0.094 0.69 M 368075 4336140 6.51 33 13 0.118 1.30 0.118 1.30 N 367275 4336840 14.04 274 6 0.148 3.00 0.187 3.00
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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option

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### Figure 5A. BWI Airport Wind Rose A-7

# Attachment A Modeling Analysis to Support Use of ADJ_U* Option

Figure 6. Non-Regulatory Default ADJ_U* 4th 

### High I-Hour Modeling Results

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### Copyrighted map removed

### • 

# Attachment A Modeling Analysis to Support Use ofADJ_U* Option The completed modeling analysis demonstrates that the use of the regulatory default options and non-regulatory default ADJ_U* option results in very similar modeled 4th 

### high I-hour 

### S02 

### concentrations. The only difference between the two is that the modeled 4th 

### high I-hour 

### S02 

### concentrations 35 kilometers away in Baltimore County are no longer present using the nonregulatory default ADJ_U* option. As previously mentioned, the 

### 4th high 1-hour 

### S02 

### modeled concentrations are not highly probable based on the low wind speeds observed during the hours in question based on the available meteorological data. In addition, this same conclusion was reached by Raven Power based on the comments they submitted. In those comments, Raven Power says "it is impossible for the plume to travel that distance within the model's 1-hour averaging time" (Raven Power Comments on EPA's Proposed 

### S02 

### Non-Attainment Designation for H.A. Wagner Power Plant, March 3I, 20I6). 

### In conclusion, Maryland should be granted approval to use the non-regulatory default ADJ_U* option in this particular instance.

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# Attachment A Modeling Analysis to Support Use ofADJ_U* Option References Raven Power Comments on EPA's Proposed S02 Non-Attainment Designation for H.A. Wagner Power Plant, Baltimore, MD, Submitted by Raven Power, A Subsidiary of Talen Energy, March 31, 2016. Docket#EPA-HQ-OAR-2014-0464.

A-10 

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