MEMORANDUM

August 21, 2015

SUBJECT: Practicability of the Los Angeles – South Coast 2015 attainment of the 2006 24-

hour PM2.5 NAAQS

FROM: Michael Flagg, Air Quality Analysis Office, EPA Region 9

TO: Docket EPA-R09-OAR-2015-0204

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

Angeles – South Coast moderate nonattainment area to attain the 2006 PM2.5 24-hour NAAQS 

by the end of 2015, which requires data from the 2013-2015 time period. For this analysis, EPA 

obtained data from an Air Quality System (AQS) data query performed on August 21th, 2015 

(AMP 480 & AMP 355).

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, only one quarter of data had been entered into AQS for 2015 for most sites. For this 

analysis, EPA use certified data for 2013 and 2014, and preliminary data available from 2015.

For the 2006 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 in 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 DV2015 attains the NAAQS when: 

DV2015 = (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 percentiles for 2013 and 2014.

EPA computed these maximum allowed 2015 concentrations for all monitors in the area, and 

compared them to the estimated 2015 98th percentile. For each monitor, the 2015 98th percentile 

was estimated from the 2015 data available in AQS as of August 2015, 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 2015, and allowed 

for selection of the corresponding rank of the daily data available to use as the 98th percentile. It 
was also assumed that the concentrations measured during the remainder of 2015 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 future concentrations and 98

th percentiles could be higher, and attainment even 

more impracticable. 

EPA determined attainment at a monitoring site impracticable if the estimated 2015 98th

percentile was greater than the maximum allowed 2015 98th percentile concentration. EPA’s 

analysis showed that two sites (Rubidoux, AQS ID: 06-065-8001 & Mira Loma (Van Buren), 

AQS ID: 06-065-8005) had estimated 2015 98th percentiles greater than the maximum allowed 

2015 98th percentile concentration, which provides evidence that it is impracticable that the area 

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

EPA also considered the assumption that all sites in this analysis collected a minimum of 351 

daily samples (i.e. consistent with an everyday sampling frequency) in 2015 that would allow for

selection of the 8th highest recorded value as the 98th percentile for 2015. This assumption would 

allow for selection of the lowest 98th percentile value possible for 2015. Even under this 

assumption, both Rubidoux and Mira Loma (Van Buren) had estimated 2015 98th percentiles 

greater than the maximum allowed 2015 98th percentile.

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 Los Angeles-South 

Coast area.