Dr. Donna Lee Jones 

February 29, 2008

Page 2 of 2 

Attachment 

Letter from Dr. Joseph Scire, TRC, to Michael Palazzolo, Alcoa, February 21, 2008 
February 21, 2008 

Mr. Michael Palazzolo 

Alcoa 

201 Isabella Street at 7th Street Bridge 

Pittsburgh, PA 15212-5858 

Dear Mike: 

For modeling the aluminum reduction facilities, the use of the CALPUFF modeling 

system is my recommendation as the most appropriate technical approach. CALPUFF is 

a Guideline Model recommended by the U.S. EPA for regulatory applications involving 

for long range transport and on a case-by-case basis for near-field applications involving 

complex flow situations. It has been used for numerous near-field studies for aluminum 

smelters and other types of sources including power plants. 

The rationale for using CALPUFF rather than the AERMOD model, which is a Guideline 

Model recommended by the U.S. EPA for most types of near-field applications is that 

AERMOD was not designed for treating the special types of sources associated with 

aluminum smelters. In particular, AERMOD does not treat buoyant line sources, which 

is an important part of aluminum smelters. Attached is a paper that describes the issues 

related to properly modeling potroom emissions using buoyant line source techniques. 

The deficiencies of ISCST3 noted in the paper in its treatment of line sources as nonbuoyant volumes or as a series of point sources apply equally to AERMOD. The 

conclusion of the analysis is that it is not possible to properly treat buoyant line sources 

either as non-buoyant volume sources or point sources. In addition, AERMOD’s 

downwash methodology was not designed for very long buildings, and contains known 

deficiencies for structures such as the potroom buildings. 

AERMOD’s treatment of point source downwash using the PRIME model with the BPIP 

effective building dimensions is inadequate for situations involving large buildings such 

as the potrooms at aluminum facilities. The building downwash scheme and effective 

dimension calculations were developed and evaluated with wind tunnel and field data of 

structures with building length to width (L/Wb) ratios of up to 4, while a typical ratio at 

an aluminum facility might be of order 20. 
Mr. Michael Palazzolo 

February 21, 2008 

Page 2 

The BLP model (Schulman and Scire, 1980), was designed specifically for treating 

buoyant line source emissions and point source downwash associated with large 

buildings. BLP is a Guideline Model, as are AERMOD and CALPUFF. CALPUFF 

contains an implementation of the BLP buoyant line source algorithms as part of its 

formulation. In addition, CALPUFF contains a point source downwash model based on 

the algorithms in BLP as one of its options. The current EPA recommended application 

for BLP is for buoyant line sources. However, there are several reasons for 

recommending the use of CALPUFF rather than BLP for modeling aluminum facilities 

and other background sources. First, the BLP model distributed on the EPA web site 

contains errors introduced when modifications were made by EPA in 1999 to allow the 

use of ISCST3 meteorological data. The current official version of the model will not run 

without changes to either the code or the meteorological input data. Second, the BLP 

model framework is over 25 years old. It does not use current datasets or model features 

that are available in more recent models. It also contains algorithms such as the treatment 

of calm winds that are inconsistent with other Guideline models such as AERMOD 

(which doesn’t include calm hours in the calculations) and CALPUFF (which explicitly 

treats calm wind effects). In addition, there are no processors available that would allow 

the output of BLP to be combined with the output of other models for background 

sources such as AERMOD or CALPUFF. 

Because CALPUFF will treat both the special complex source configuration of an 

aluminum plant’s line and point sources in a manner consistent with BLP, but within a 

modern framework that includes interfaces to currently available meteorological datasets, 

and it allows the modeling of all the sources at a facility sources and their cumulative 

impact to be merged with postprocessors that are known and approved by EPA. The use 

of a single model framework (CALPUFF) for assessing the impact of all sources at the 

facility in a consistent manner is recommended. 

CALPUFF can be run in different modes, include one mode that uses AERMOD 

meteorological data files as input. Other modes include the use of ISCST3 or CTDM 

meteorological files. Although these modes of running the model are not the most 

refined in that three-dimension wind data are not used, they provide a screening level 

result that might be adequate for some purposes. The level of effort required to run 

CALPUFF using simplified meteorological data is no greater than running AERMOD or 

ISCST3. 

As presented below, CALPUFF has been demonstrated to produce line source impacts 

consistent with BLP, and in addition, it has been shown to produce concentration 
Mr. Michael Palazzolo 

February 21, 2008 

Page 3 

predictions of point and line sources that are equivalent or superior to the BLP 

predictions. AERMOD predictions have been demonstrated to significantly overpredict 

both the line source impacts as well as the point source impacts from an aluminum 

smelter using the datasets upon which the BLP model was evaluated (Scire and 

Schulman, 1981 attached) as part of the process of designating the model as a Guideline

model. As an example, Figure 1 shows the line source impacts of CALPUFF and 

AERMOD against the BLP model for an SF6 tracer study conducted in Arkansas. In this 

study, the tracer was released from the potroom building. As expected, the non-buoyant 

treatment of line sources in AERMOD significantly overpredicts the BLP results. As 

discussed in the paper, the BLP predictions match the observations very closely. Also 

shown in the figure is the strong match of the CALPUFF buoyant line source model with 

the BLP predictions. 

The second dataset from the 1981 paper for which CALPUFF, BLP and AERMOD were 

evaluated is an SO2 dataset from the Alcoa Tennessee smelter. In this study, SO2

measured at a monitor over a two-year period was used to evaluate the models. Most of 

the impacts during the periods with higher concentrations are due to the point sources 

rather than the line sources because SO2 emissions are much higher from the points. 

Figures 2 and 3 show the results of AERMOD, BLP and CALPUFF for 1977. The 

format of the meteorological data has been modified to allow the current BLP code on the 

EPA web site to run. Its results are shown along with the original (Version 1.1) version 

of the model dated 1980. The results indicate CALPUFF reproduce the BLP results well, 

although the performance of CALPUFF is slightly closer to the observations than BLP. 

AERMOD performance showed large overpredictions relative to the observations. 

On an annual average basis for 1977, the overprediction of AERMOD for the Alcoa 

Tennessee dataset were substantial, nearly a factor of 10, while both CALPUFF and BLP 

while conservative, were within a factor of two of the observations. 

 Observations – 13 μg/m3

 BLP (current code) – 19.6 μg/m3

 CALPUFF – 18.5 μg/m3 

AERMOD – 114.2 μg/m3

 Based on theoretical considerations involving the model formulation, and an analysis of 

the performance of AERMOD versus CALPUFF and BLP, the use of CALPUFF for 

assessing modeling aluminum facilities is recommended. 
Mr. Michael Palazzolo 

February 21, 2008 

Page 4 

If you have any questions, please contact me at (978) 656-3627 or by email at 

jscire@trcsolutions.com. 

Sincerely, 

TRC Environmental Corporation 

Joseph S. Scire, CCM 

Vice President, Atmospheric Studies Group 

Enc.
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BLP Results (Chi*u/Ql)

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BLP (X-axis) plotted against AERMOD (Y-axis)

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1:1 Line 2x Line

1/2 x Line

Figure 1. Comparison of AERMOD and CALPUFF predictions of line source impacts from the Arkadelphia 

Arkansas SF6 tracer study compared to BLP predictions. This study shows the impacts of the line sources 

alone. 
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Figure 2. Predictions of 3-hour average SO2 concentrations at the downwind Alcoa Tennessee monitor for 

1977 versus observations for CALPUFF, BLP and AERMOD. The BLP results based on Version 1.1 of the 

model as well as the current version of BLP on the U.S. EPA web site (with modifications to the 

meteorological file to allow it to run) are shown. CALPUFF closely matches BLP results while AERMOD 

significantly overpredicts the observed concentration measurements. 
0.01 0.05 0.1 0.2 0.5 1 2 5 10 20 30 40 50 60 70 80 90 95 98 99 99.8 99.9 99.99

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Figure 3. Predictions of 24-hour average SO2 concentrations at the downwind Alcoa Tennessee monitor for 

1977 versus observations for CALPUFF, BLP and AERMOD. The BLP results based on Version 1.1 of the 

model as well as the current version of BLP on the U.S. EPA web site (with modifications to the 

meteorological file to allow it to run) are shown. CALPUFF closely matches BLP results while AERMOD 

significantly overpredicts the observed concentration measurements.