December 15, 2014

MEMORANDUM

SUBJECT: Practicability of SJV 2015 attainment of the 2006 24-hour PM2.5 NAAQS

FROM: Elfego Felix, Air Quality Analysis Office

Scott Bohning, Air Quality Analysis Office

TO: San Joaquin Valley 2006 PM2.5 NAAQS Reclassification Docket EPA-R09-OAR2014-0636

This memorandum explains the methodology used by EPA to judge the practicability for the San 

Joaquin Valley (SJV) moderate nonattainment area to attain the 2006 PM2.5 24-hour NAAQS of 

35 μg/m3by December 31, 2015, which requires data from the 2013-2015 time period. For this 

analysis, EPA obtained data from the U.S. EPA, 2013 Design Value Reports, PM2.5 Detailed 

Information Updated 8/24/14, available at http://www.epa.gov/airtrends/values.html 

(“PM25_DesignValues_20112013_FINAL_08_28_14”), and from an Air Quality System (AQS)

data query performed on August 7th, 2014 (AQS Combined site values report, AMP355).

Per 40 CFR part 58.15, annual data certification for data collected from January 1 to December 

31 of the previous year is due on May 1 of the following year. At the time EPA conducted this 

analysis, no data had yet been collected for 2015, 2014 data had not been fully collected and 

certified, and 2013 data had been certified. For this analysis, EPA was thus able to use certified 

data for 2013, preliminary data available for 2014 as of early August 2014, and no data from 

2015.

For the 24-hour NAAQS of 35 μg/m3, the design value is the three-year average of annual 98th 

percentile PM2.5 concentrations. Under the rounding conventions of 40 CFR part 50, appendix N, 

section 4.3, daily PM2.5 values are reported to one decimal place; in computing the design value, 

intermediate calculation results are not rounded; and in the final result for comparison to the 

level of the NAAQS, decimals of 0.5 or higher are rounded up. Appendix N section 4.5, table 1 

specifies which recorded daily PM2.5 value rank is chosen as the 98th percentile; for example, if 

there are between 301 and 350 daily samples collected in a year, then the 7th highest value is the 

98th percentile. The fewer the number of samples collected, the higher the rank used (e.g. 1st 

high for between 1 and 50 samples). If Cx is the 98th percentile concentration for year x, the 

2015 design value D2015 attains the NAAQS when: 

D2015 = (C2013 + C2014 + C2015)/3 ≤ 35.4

which can be restated as:

C2015 ≤ 3 * 35.4 – (C2013 + C2014)

C2015 represents the maximum 2015 98th percentile concentration allowed for attainment, given 

the known 98th percentile for 2013 and an estimated 98th percentile for 2014.
EPA computed these maximum allowed 2015 concentrations for selected SJV monitors, and 

compared them to zero, the lowest possible concentration. (In practice, a concentration of zero is 

actually not possible since it would require no anthropogenic or natural emissions in the SJV 

area, and no transport of pollutants into the area from outside. Every area is expected to have 

some amount of background PM2.5.) For each monitor, the 2014 98th percentile was estimated 

from the 2014 data available in AQS as of early August 2014, together with two assumptions. It 

was assumed that the sampling rate observed so far would continue throughout the year; this 

yielded an estimate of the number of total samples expected for 2014, and allowed for selection 

of the corresponding rank of the daily data available to use as the 98th percentile. This rank was 

the 7th high for several sites, and the 3rd high for several others. It was also assumed that the 

concentrations measured during the remainder of 2014 would be no higher than those already 

recorded, so the 98th percentile could be chosen from among the already recorded data. This is a 

conservative assumption for assessing the impracticability of attainment, since in reality some 

concentrations could be higher, the chosen 98th percentile could be higher, and attainment even 

more impracticable. 

EPA deemed attainment at a monitoring site impracticable if the maximum allowed 2015 98th 

percentile concentration was less than zero (i.e. negative). EPA’s analysis showed that all 

monitoring sites examined — Bakersfield-California Ave, Bakersfield-Planz Rd., FresnoGarland, Fresno-Pacific, and Hanford — would require negative PM2.5 concentrations to attain 

the 2006 24-hour PM2.5 NAAQS by the end of 2015.

EPA considered the assumption that somehow all sites in this analysis could collect a minimum 

of 351 daily samples (i.e. consistent with an everyday sampling frequency) in 2014 that would 

allow for selection of the 8th highest recorded value as the 98th percentile for 2014. This is 

particularly an unrealistic assumption for the Bakersfield-Planz Rd. and Fresno-Pacific sites that, 

based on preliminary data as of early August 2014, have operated at a 1:3 sampling frequency 

(i.e. one sample is collected every three days) in 2014. This unrealistic assumption would allow 

for selection of the lowest 98th percentile value possible for 2014 and thus render the easiest

attainment scenario conceivable. Even under this assumption, all of the monitoring sites 

evaluated in this analysis, except Fresno-Pacific, would require a negative PM2.5 98th percentile 

concentration in 2015 to attain the 2006 24-hour PM2.5 NAAQS by the end of 2015 (FresnoPacific would require a 98th percentile concentration of 2.9 μg/m3

in 2015, far below expected 

background concentrations).

Based on the analysis described in this memorandum, EPA concludes that attainment of the 2006 

24-hour PM2.5 NAAQS by the end of 2015 would be impracticable for the SJV.