

                               November 17, 2017

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

MEMORANDUM							

SUBJECT: 	PM10 24-hour Design Concentration for Sheridan Wyoming

FROM:	Ethan L. Brown
		Physical Scientist

TO: 		File					

According to the Limited Maintenance Plan (LMP) Option for Moderate PM10 Nonattainment Areas (NAA) memo, in order 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 Since the annual PM10 NAAQS has been revoked, it is no longer an appropriate metric, but the other two requirements must still be met.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 Sheridan NAA currently has two PM10 monitoring sites, the Sheridan Police Station located in the north of town, and the Meadowlark Elementary School farther to the south. Looking at the most recent five years of data from these sites, the Sheridan nonattainment area has attained the NAAQS each year and there have been no violations. Therefore, the only remaining requirement is that the Sheridan NAA must have an average PM10 design value at or below 98 μg/m[3] to quality for the LMP option.

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 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 considering the availability of data in the Sheridan NAA, as well as the low concentrations observed over the past five years.

Using data from 2012 through 2016, the Sheridan Police station has three valid design concentrations. These design concentrations are 60, 57, and 72 μg/m[3] in 2014, 2015, and 2016 respectively. The Meadowlark School monitor was installed in July 2012, and therefore is missing the first two quarters of data in 2012. As a result, the Meadowlark School Site only has two valid design concentrations, one in 2015 and one in 2016. These design concentrations are 72 μg/m[3] for both years.

Considering each site separately, the average design concentration at the Police Station for 2012-2016 was 63 μg/m[3]. Ordinarily, design concentrations are calculated for each monitor separately, but because there are multiple design values for 2015 and 2016 (one from the Police Station and one from the Meadowlark site for each year), the highest design concentration for each year was also selected for averaging purposes. The resulting average PM10 design concentration is 68 μg/m[3] (the average of 60, 72, and 72 μg/m[3]) for the area. Both these calculations give average design concentrations over the most recent 5 years well below the 98 μg/m[3] threshold to quality for the LMP option.
