UNITED STATES ENVIRONMENTAL PROTECTION AGENCY REGION 8 1595 Wynkoop Street DENVER, CO 80202-1129 Phone 800-227-8917 http://www.epa.gov/region08 

## JUN 222007
Ref: 8P-AR

Mr. Larry Zink Montana Sulphur & Chemical Company P.O. Box 31118 Billings, MT 59107

Re: Results of"Subjectto" Visibility Modeling for Best Available Retrofit Technology (BART)

Dear Mr. Zink:
On June 19, 2006, EPA received a letter from the MontanaDepartment ofEnvironmental Quality that stated it was withdrawing its efforts to meet the requirements ofthe Regional Haze Rule (RHR) specified at 40 CFR51.308. On September 27, 2006, EPA Region 8 sent a letter notifying you that we would be conducting modeling to determine which sources in Montanaare "subject to" the Best Available Retrofit Technology (BART)requirements in the RHR as part of
our effort to address the requirements ofthat rule. Since it was determined that Montana Sulphur & Chemical Company (MSCC)was BART eligible, we informed youthat MSCC would be included in the modeling and asked youto certify the datathat would be used for this modeling.

EPA Region 8, withthe assistance ofthe Western Regional Air Partnership (WRAP), has
nowcompleted the visibility modelingfor determiningthose Montana sources subjectto BART. Based on the modeling, EPA Region 8 believes that your facility is not subject to BART.

In modeling MSCC's visibility impacts, EPA Region 8 followed the approach for detem1ining ifa source is subject to BART, including the specifics onhowthe modeling should be perfom1ed, set out in the final RHR and EPA's "Guidelines for Best Available Retrofit Technology Determinations" (BART Guidelines). See 70 FR 39104,July 6, 2005. The BART Guidelines can be found at Appendix Yto 40 CFR Part 51. EPA Region 8's visibility modeling also generally followed the protocol outlined in the document "Draft CaipuffBART Modeling Protocol for Federal Mandatory Class 1Areas" (MontanaDepartment ofEnvironmental Quality, September 2006). An electroniccopy ofthis protocol is located on EPA Region 8's website at http://www.epa.gov/region8/air/mtre£!ionalhaze.html. Where necessary, we made slight variations to the protocol, such as instances where meteorological data was not available for a certain site.
States (or EPA) have several options for determining whether a source is subjectto BART, including the use ofdispersion modeling to determine whether an individual source may be reasonably anticipated to cause orcontribute to any visibility impairment in any Class I area.

### 2 When dispersion modeling is used, part ofthe process ofdetennining whether a source is subject to BARTis the selection ofa deciview threshold against which to measure the visibility impact ofa source. For its analysis ofMontana sources, EPA Region 8 has established a threshold of 0.5 deciviews as the appropriate threshold based on the number and distribution ofsources impacting Class I areas in and around Montana. Ifdispersion modeling predicts that a source will have visibility impacts of0.5 deciviews or more at a Class I areaon the 98th percentile of values, EPA Region 8 considers that source to be subject to BART. Further information regarding this approach to determining whether a source is subject to BART canbe found in section lILA ofthe BART Guidelines.

For MSCC, the modeling results show a maximum98th percentile impact within a Class I
area of.30 deciviews, which does not exceed the threshold of0.5 deciviews. Since MSCC does not exceedthe 0.5 deciviewthreshold, EPA Region 8 believes that MSCC is not subject to BART.

A summary ofthe BART modeling results is enclosed. The enclosure provides a summary ofthe 98th percentile predictions for the three years modeled for MSCC at all federal Class I**a~eas** in the vicinity ofthe plant. More detailed modeling results, including the electronic CALPUFF/CALMETfiles, are available onthe WRAP website at http://pah.cert.ucr.edulaqrnl308Ibart.shtml.
While the modeling results indicate that MSCC is not subject to the BARTrequirements ofthe RHR, it is possible that emissioncontrols at MSCC will be needed in orderto meet the requirements for reasonable progress and the long-tenn strategy (see 40 CFR51.308(d)(1) and 40 CFR51.308(d)(3)). In orderto address reasonable progress andthe long-term strategy requirements, EPA Region 8 plans on analyzing contributions from BART sources, point sources not subject to BART, area sources, and fire.

We appreciate yourcooperation in providing us with the information necessary to conductthis analysis. Ifyouhave any questions regarding the modeling results, please contact Kevin Golden ofmy staffat (303) 312-6442. For questions related to development ofthe regional haze Federal Implementation Plan, please contact Laurel Dygowski at (303) 312-6144. Sincerely,

Stephen S. Tuber Assistant Regional Administrator Office ofPartnerships and Regulatory Assistance

Enclosure

@Printedon RecycledPaperI Source = Montana Sulfur: SOx = 3,259 TPY; NOx = 13 TPY Minimum Max 99 1n 98 1n 98th Class I Area Distance Delta-dv Delta-dv Delta-dv # Days 98th Percentile by Year 3-Year (km) (dv) (dv) (dv) > 0.5 dv 2001 2002 2003 Average 

### I noab I North Absaroka Wilderness 143 0.51 0.29 0.20 1 0.19 0.22 0.20 0.21 yell Yellowstone NP 151 0.34 0.20 0.16 0 0.14 0.17 0.15 0.15 wash Washakie Wilderness 176 0.32 0.18 0.13 0 0.13 0.16 0.10 0.13 ulbe UL Bend 187 0.75 0.37 0.28 6 0.30 0.23 0.30 0.28 teto Teton Wilderness 205 0.18 0.09 0.07 0 0.05 0.08 0.06 0.06 gamo 

### I Gates ofthe Mountains WA 278 0.69 0.20 0.12 1 0.12 0.19 0.07 0.13 redr Red Rock Lakes 282 0.24 0.09 0.05 0 0.04 0.09 0.04 0.06