January 26, 2011 

MEMORANDUM

SUBJECT: Clean Air Act Section 110(k)(6) Error Correction for Ravalli County, Montana 

FROM: Richard Payton, EPA Region 8 Air Permitting, Monitoring and Modeling Unit 

TO: Docket EPA-HQ-OAR-2007-0562, Air Quality Designations for the 2006 24-Hour 

Fine Particle (PM2.5) National Ambient Air Quality Standards 

In the November 13, 2009 Federal Register Notice (74 FR 58688), EPA promulgated 

nationwide designations for most areas for the 2006 24-Hour PM2.5 NAAQS. In that notice, EPA 

designated Ravalli County, Montana as “unclassifiable.” The “unclassifiable” designation was 

assigned because at the time of promulgation, preliminary calculations of design values (DV) for the 

period from 2006 through 2008 indicated that Ravalli County had incomplete data and therefore EPA 

could not make a determination whether or not the area attained the NAAQS. Subsequent review of 

the preliminary design value calculations showed that a software error inappropriately labeled the 

Ravalli data as “incomplete” when acceptable data substitution techniques were available to generate 

a complete data set for Ravalli County. After completing the data substitution routines and 

recalculating the design values, EPA has determined that Ravalli County was attaining the 2006 24-

hour PM2.5 national ambient air quality standard using 2006-2008 data and continues to meet these 

standards using the most current 2007 to 2009 data. See, for example, 

http://www.epa.gov/airtrends/pdfs/dv_pm25_2006_2008rev102809.xls and 

http://www.epa.gov/airtrends/pdfs/PM2.5_DesignValues_20072009_Final.xls. 

EPA regulations provide for data substitution under appropriate circumstances. 40 CFR Part 

50, Appendix N indicates in 4.2(b) provides that 

(b) The use of less than complete data is subject to the approval of EPA which may consider 

factors such as monitoring site closures/moves, monitoring diligence, and nearby 

concentrations in determining whether to use such data for comparisons to the NAAQS. 

The EPA document "Guideline on Data Handling Conventions for the PM NAAQS" (EPA-454/R-99-

008, April 1999) contains guidance on substitution methods to allow incomplete data showing 

attainment to be considered complete for the purposes of determining the attainment status of an area. 

Provided the data pass three qualifying tests, EPA may substitute either available data from 

collocated monitors at the same site, or may substitute the highest monitored value from a given 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

REGION 8 

1595 Wynkoop Street 

DENVER, CO 80202-1129

Phone 800-227-8917 

http://www.epa.gov/region08

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calendar quarter considered over the three years addressed by the design value calculation. The three

qualifying tests for the data are: 

1) the monitor must have collected at least 50% of the scheduled data for incomplete quarters 

2) it must be shown that emissions and meteorology for the substituting data are comparable 

to the emissions and meteorology for the missing data 

3) the incomplete data must be showing attainment 

In the Ravalli monitor 2006-2008 data set, data for the 2nd quarter 2006 and 3rd quarter 2007 

are incomplete. For 2nd quarter 2006, 15 samples were scheduled (1 in 6 sampling schedule), but 

only 11 samples were collected. Thus 11/15 = 73% of required samples were collected. This is less 

than the required 75%, but greater than 50%, so the 2006 data are incomplete, but they pass the 1st

qualifying test for substitution. For 3rd quarter 2007, 31 samples were scheduled but only 19 

scheduled samples (plus 2 creditable make-up samples) were collected. (Note: the monitor was 

converted to a 1 in 3 sampling schedule on January 1, 2007). Thus 19/31 = 61% of scheduled 

samples were collected, which should be increased to 21/31 = 68% by considering the 2 creditable 

make-up samples. Third quarter 2007 also passes the 1st qualifying test for data substitution. 

Considering the 2nd test, the quarterly mean for the incomplete data from 2nd quarter 2006 

equals 5.5 μg/m3

. The 2nd quarter means for 2007 and 2008, with complete data, are each 4.5 μg/m3. 

While emissions and meteorology were apparently slightly higher in the incomplete 2nd quarter of 

2006, the difference of less than 1 μg/m3

 in the three quarterly means, nearly complete data in 2006 

(73%) and overall very low PM2.5 (max 2nd quarter value in 2006-2008 is 10.4 μg/m3), satisfy 

qualifying test 2 that emissions and meteorology for substituting data are comparable to emissions 

and meteorology for the missing data. For 3rd quarter 2007, EPA concurred on 4 forest fire 

exceptional event data flags, resulting in a 3rd quarter PM2.5 mean of 11 μg/m3

. The 3rd quarters for 

2006 and 2008 had mean values of 20 and 8.6 μg/m3, respectively. The 2007 quarterly mean with 

incomplete data falls between the quarterly means for the other 2 years with complete quarterly data. 

Therefore, emissions and meteorology (with the exception of exceptional event wildfire emissions) 

are comparable, and the 2007 data also pass the 2nd qualifying test. 

The design value calculated using the incomplete data for Ravalli County is 30 μg/m3 

compared to the 2006 PM2.5 24-hour standard level of 35 μg/m3, and the incomplete data show an 

attaining value, and thus pass the 3rd qualifying test for substitution. 

Given that the Ravalli County 2006-2008 data pass all the qualifying tests to allow data 

substitution, 2 substitution methods are provided by EPA-454/R-99-008: 

1) collocated data from other PM2.5 monitors at the same site may be substituted for missing 

data from the same day 

2) the maximum value recorded in the same quarter as that which is incomplete, considered 

over the 3 year period under consideration may be substituted for missing days, and the 

resulting 98th percentile values can be averaged to a new design value. 
A continuous Federal Equivalent Method (FEM) monitor was first collocated with the Federal 

Reference Method (FRM) monitor which had incomplete data on July 1, 2008; therefore, no 

collocated data are available for the earlier incomplete quarters in 2006 and 2007. Substitution 

method 1 is not available. 

For substitution method 2, the highest 2nd quarter value in 2006-2008 was the 10.4 μg/m3 

recorded on May 18, 2007; the highest 3rd quarter value in 2006-2008 (with the exception of EPA 

concurred exceptional events data) is the 34.8 μg/m3 recorded on August 19, 2008. 

Prior to substitution, the 98th percentile values and resulting design value for the monitor are: 

Raw, Incomplete Data: 

2006 27.8 (2nd max, 54 samples) 

2007 27.8 (2nd max, 96 samples) 

2008 34.7 (3rd max, 104 samples) 

DV 30 μg/m3

After substituting 10.4 μg/m3 for missing 2nd quarter 2006 samples and 34.8 μg/m3 for missing 

3

rd quarter 2007 samples, these values are revised as follows: 

With Substitution, Complete Data: 

2006 27.8 (2nd max, 58 samples) 

2007 34.8 (3rd max, 107 samples, counting 2 creditable samples and the 2 substituted 

samples for the missing days preceding these two creditable samples) 

2008 34.7 (3rd max, 104 samples) 

DV 32 μg/m3

The substitutions above use the highest value recorded in applicable quarters of 2006 through 

2008 by the primary FRM monitor. The highest 24-hour value recorded by the secondary collocated 

continuous FEM monitor in 2008 could instead be used for the 3rd quarter substitution. The 

continuous FEM was installed at the beginning of 3rd quarter 2008. It had a high 3rd quarter 2008 

value of 36.8 μg/m3

. If the 36.8 μg/m3 from the collocated monitor is used for the data substitution 

for missing data in the 3rd quarter of 2007, the resulting values are: 

With Substitution of max POC 3 Continuous Data: 

2006 27.8 (2nd max, 58 samples) 

2007 36.8 (3rd max, 107 samples, counting 2 creditable samples and the 2 substituted 

samples for the missing days preceding these two creditable samples) 

2008 34.7 (3rd max, 104 samples) 

DV 33 μg/m3

Finally, because the continuous FEM has operated in Ravalli County since July of 2008, EPA 

has concluded that its data properly should be combined with the primary 1 in 3 day FRM at the same 

site. The resulting 2008 98th percentile value (the 5th maximum from 224 creditable days of data) is 
thereby reduced from the 34.7 μg/m3 shown above for the primary monitor to 32 μg/m3

. The 

resulting decrease in design value for each of the 3 cases above is (34.7 - 32) / 3 = 0.9 μg/m3. The 

ultimate result of combining the two collocated data sets from 2008 is a reduction in the attaining 

2006-2008 design value shown for all the design value calculations considered. The actual final 

design value of 29 μg/m3, reflecting the first substitution method above and the combination of data 

from the primary FRM and secondary FEM in 2008 is shown at 

http://www.epa.gov/airtrends/pdfs/dv_pm25_2006_2008rev102809.xls. 

Given that the Ravalli monitor was eligible for data substitution using the 3 qualifying tests of 

EPA-454/R-99-008, and has an attaining design value both with the available incomplete data and 

with complete data as achieved through data substitution in accordance with EPA guidance, EPA has 

determined that the draft 2006-2008 design value spreadsheet available when the designations 

rulemaking was signed was incorrect in showing Ravalli County as having attaining but incomplete 

(after substitution) data. The final spreadsheet at 

http://www.epa.gov/airtrends/pdfs/dv_pm25_2006_2008rev102809.xls was corrected to show Ravalli 

county as attainment and complete (with substitution) subsequent to the Administrator’s signature of 

the rulemaking. Accordingly, the initial designation for the 2006 PM2.5 NAAQS for the Ravalli area 

should have been “unclassifiable/attainment” rather than “unclassifiable.” Rulemaking will be 

undertaken to correct Ravalli County, Montana’s designation to Unclassifiable/Attainment.