

                               January 16, 2020

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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: 	Kalispell and Libby Critical Design Value Calculations

FROM:	James Hou
		Environmental Engineer

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,

     "To qualify for the limited maintenance plan 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 :g/m3 for the annual and 98 :g/m3 for the 24-hr PM10 NAAQS with no violations at any monitor in the nonattainment area5 . If an area cannot meet this test it may still be able to qualify for the LMP option if the average design values of the site are less than their respective site-specific CDV.
     
     We believe it is appropriate to offer this second method of qualifying for the LMP because, based on the air quality data we have studied, we believe there are some monitoring sites with average design values above 40 :g/m3 or 98 :g/m3 , depending on the NAAQS in question, that have experienced little variability in the data over the years. When the CDV calculation was performed for these sites we discovered that their average design values are less than their CDVs, indicating that the areas have a very low probability (1 in 10) of exceeding the NAAQS in the future. We believe it is appropriate to provide these areas the opportunity to qualify for the LMP in this circumstance since the 40 :g/m3 or 98 :g/m3 criteria are based on a national analysis and don't take into account each local situation."

The following methodology can be found in Attachment A of the LMP Option memo, and is used to determine the site-specific CDV for a NAA

The equation set forth is:
                             CDV = NAAQS/(1+tc*CV)

Where:
      CDV = Critical Design Value in μg/m3
      NAAQS = 150 ug/m3 for the 24-hr PM10 standard,
      tc = critical t-value corresponding to a 10% probability,
      CV = coefficient of variation of the annual design values 

Kalispell Critical Design Value Calculation:

                                     Year
                         Design Concentration (ug/m3)
                                                                           2007
                                                                            102
                                                                           2008
                                                                            105
                                                                           2009
                                                                            115
                                                                           2010
                                                                             61
                                                                           2011
                                                                            108
                                                                           2012
                                                                            108
                                                                           2013
                                                                            108
                                                                           2014
                                                                             88
                                                                           2015
                                                                             89
                                                                           2016
                                                                             89
                                                                           2017
                                                                             88
                                                                           2018
                                                                             90
st dev
                                                                          14.76
mean
                                                                          95.92
CV
                                                                           0.15
NAAQS
                                                                         150.00
tc
                                                                           1.36
Critical Design Value
                                                                          124.0
Current Avg 3-yr Design Value
                                                                             89


Libby Critical Design Value Calculation

DV Year
Design Values (Table Look-up) 98-155 removed
                                                                           2007
                                                                             99
                                                                           2008
                                                                             89
                                                                           2009
                                                                             79
                                                                           2010
                                                                             87
                                                                           2011
                                                                             87
                                                                           2012
                                                                             90
                                                                           2013
                                                                             90
                                                                           2014
                                                                             90
                                                                           2015
                                                                             90
                                                                           2016
                                                                             90
                                                                           2017
                                                                             92
                                                                           2018
                                                                             95
st dev
                                                                           4.76
mean
                                                                          89.83
CV
                                                                           0.05
NAAQS
                                                                         150.00
tc
                                                                           1.36
Critical Design Value
                                                                          139.9
Current Avg 3-yr Design Value
                                                                           92.3



