Dr. D o nna Lee Jo nes Febr u a r y 29 , 20 08 Page 2 o f 2

Attachment

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

Mr. Michael Palazzolo

Alcoa

201 Isabella Street at 7th Street Bridge Pittsburgh, PA 15212-5858

Dear Mike:

For modeling the aluminum reduction faci lities, the use of the CALPUFF modeling system is my recommendation as the most a ppropriate technical a pproach. CALPUFF is

a Guideline Model recommended by the U.S. EPA for re gulatory applications involving for long range transport and on a case-by-case basis for near-field a pplications 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 type s of near-field applications is that AERMOD was not designed for treating the sp ecial types of source s 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 emissi ons using buoyant line source techniques. The deficiencies of ISCST3 noted in the pa per 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 trea t buoyant line sources either as non-buoyant volume sources or point sources. In addition, AERMOD’s downwash methodology was not designed for ve ry long buildings, and contains known

deficiencies for structures su ch as the potroom buildings.

AERMOD’s treatment of point source downw ash using the PRIME model with the BPIP effective building dimensions is inadequate fo r situations involving large buildings such as the potrooms at aluminum facilities. The building down wash scheme and effective dimension calculations were developed and ev aluated with wind tunne l and field data of

structures with buildi ng length to width (L/W b ) 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 buoyan t line source algorithms as part of its formulation. In addition, CALPUFF contai ns a point source dow nwash 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 th an BLP for modeling al uminum facilities and other background sources. First, the BL P model distributed on the EPA web site

contains errors introduced when modificati ons 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 ei ther the code or the meteorologi cal 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 al so 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 calc ulations) 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 consiste nt with BLP, but within a

modern framework that include s interfaces to currently avai lable meteorological datasets, and it allows the modeling of all the sources at a facility sources and their cumulative impact to be merged with postprocessors th at are known and approved by EPA. The use of a single model framework (CALPUFF) for a ssessing the impact of all sources at the facility in a consistent manner is recommended.

CALPUFF can be run in di fferent 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 mode s of running the model are not the most

refined in that three-dimension wind data ar e not used, they provi de a screening level result that might be adequate for some purpos es. The level of effort required to run CALPUFF using simplified mete orological data is no grea ter than running AERMOD or

ISCST3.

As presented below, CALPUFF has been de monstrated to produce line source impacts consistent with BLP, and in addition, it has been shown to pr oduce concentrationMr. 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 th e 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 SF 6 tracer study conducted in Arkansas. In this study, the tracer was released from the potr oom 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 st rong match of the CALPUFF bu oyant line source model with the BLP predictions.

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

evaluated is an SO 2 dataset from the Alcoa Tennessee smelter. In this study, SO 2 measured at a monitor over a two-year period was used to evaluate the models. Most of the impacts during the periods with higher c oncentrations are due to the point sources

rather than the line sources because SO 2 emissions are much higher from the points. Figures 2 and 3 show the results of AERMOD, BLP and CALP UFF for 1977. The format of the meteorological da ta 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 indicat e CALPUFF reproduce the BLP results well, although the performance of CALPUFF is slightly closer to the observations than BLP. AERMOD performance showed large overpre dictions relative to the observations.

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

Tennessee dataset were substant ial, nearly a factor of 10, while both CALPUFF and BLP while conservative, were within a fa ctor of two of the observations.

Observations – 13 μ g/m 3

BLP (current code) – 19.6 μ g/m 3

CALPUFF – 18.5 μ g/m 3

AERMOD – 114.2 μ g/m 3

Based on theoretical consider ations involving the model form ulation, and an analysis of the performance of AERMOD versus CALP UFF and BLP, the us e 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.0 50 100 150 200 250 300 350 400

BLP Results (Chi*u/Ql) 0 50 100 150 200 250 300 350 400CALPUFF/AERMOD Results (Chi*u/Ql)

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

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

## 1:1 Line 2x Line

1/2 x Line

Figure 1. Comparison of AERMOD a nd CALPUFF predictions of line sour ce impacts from the Arkadelphia

Arkansas SF 6 tracer study compared to BLP predictions. Th is study shows the impacts of the line sources

alone.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 10 20 30 40 50 60 70 80 90 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Cumulative Frequency (%)Concentrations (μg/m3) 3-Hour SO 2 Concentration

OBS77

CALPUFF77

BLP77 V1.1

BLP77

AERMOD77

Figure 2. Predictions of 3-hour average SO 2 concentrations at the downw ind Alcoa Tennessee monitor for

1977 versus observations for CALPU FF, BLP and AERMOD. The BLP re sults based on Version 1.1 of the

model as well as the current vers ion of BLP on the U.S. EPA web site (with modifications to the meteorological file to allow it to run) are show n. 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 10 20 30 40 50 60 70 80 90 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Cumulative Frequency (%)Concentrations (μg/m3) 24-Hour SO 2 Concentration

OBS77

CALPUFF77

BLP77 V1.1

BLP77

AERMOD77

Figure 3. Predictions of 24-hour average SO 2 concentrations at the downwind Alcoa Tennessee monitor for

1977 versus observations for CALPU FF, BLP and AERMOD. The BLP re sults based on Version 1.1 of the

model as well as the current vers ion of BLP on the U.S. EPA web site (with modifications to the meteorological file to allow it to run) are show n. CALPUFF closely matches BLP results while AERMOD significantly overpredicts the observed concentration measurements.