

                               October 23, 2018

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 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
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REGION 8
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1595 Wynkoop Street
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DENVER, CO   80202-1129
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Phone 800-227-8917
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http://www.epa.gov/region08
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0
Ref: 8P-AR

MEMORANDUM							

SUBJECT: 	PM10 24-hour Design Concentration for Missoula Montana

FROM:	Ethan L. Brown
		Physical Scientist

TO: 		File					

The Montana Department of Environmental Quality (DEQ) has indicated that they would like to redesignate the Missoula County Partial PM10 nonattainment area (NAA) using the Limited Maintenance Plan (LMP) Option for Moderate PM10 Nonattainment Areas (NAA). According to the LMP memo, to qualify for the LMP option, "an area should meet the following applicability criteria. The area should be attaining the NAAQS and the average PM10 design value for the area, based upon the most recent 5 years of air quality data at all monitors in the area, should be at or below 40 μg/m[3] for the annual and 98 μg/m[3] for the 24-hr PM10 NAAQS with no violations at any monitor in the nonattainment area." Since the annual PM10 NAAQS has been revoked, it is no longer an appropriate metric, but the other two requirements must still be met.

The Missoula County Partial PM10 NAA currently has one PM10 monitoring site, Boyd Park, located in the southern portion of the town of Missoula. The most recent five years of Boyd Park PM10 data in the EPA air pollution database Air Quality System (AQS) show six exceedances, two in 2015, and four in 2017. All six values have been flagged (in AQS) as exceptional events (EEs) due to wildfire smoke. In addition to these exceedances, there are multiple values between 98 μg/m[3] and 155 μg/m[3] that are flagged as wildfire-smoke impacted. All of these flagged values are included in two Montana DEQ EE demonstrations, one for 2015-2016 24-hour PM10 events, and one for 2017 24-hour PM10 events, and have been approved by EPA.

The methods for calculating design values (or design concentrations) for PM10 are presented in a document entitled the "PM10 SIP Development Guideline", EPA-450/2-86-001, June 1987. These guidelines state that the design concentration for a particular year is determined by analyzing three years of monitoring data. Therefore, for a five-year period, one is able to calculate three separate 3-year design concentrations, the average of which will be compared to the 98 μg/m[3] threshold for LMP eligibility.

There are multiple methods for calculating PM10 design concentrations presented in the PM10 SIP Development Guideline. The "Table Look-up" method is the simplest method, and is adequate for this exercise. Using data from 2013 through 2017, the Boyd Park station has three valid 3-year design concentrations. These design concentrations (using the Table Look-up method) are 78, 78, and 86 μg/m[3] in 2015, 2016, and 2017 respectively. Therefore, the average design value from 2013 through 2017 at Boyd Park is 81 μg/m[3].
