MEMORANDUM

August 21 , 201 5

S UBJECT : Practicability of the Los Angeles

– South Coast 201 5 attainment of the 2006 24

-

hour PM 2.5 NAAQS

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

T O : Docket EPA

- R09

- OAR

- 201 5

- 0204

This memorandum explains the methodology used by EPA to judge the practicability f or the Los Angeles

– South Coast moderate nonattainment area to attain the 2006 PM 2.5 24

- hour NAAQS by the end of 2015 , which requires data from the 201 3

- 2015 time period

. For this analysis, EPA obtained data from an Air Quality System (AQS) data query performed on August 21 th, 201 5 ( 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 f or 2013 and 2014

, and preliminary data available from 20 15

.

For the 2006 24

- hour NAAQS of 3 5 μg/m 3

, the design value is the three

- year average of annual 98 th percentile PM 2.5 concentrations. Under the rounding conventions of 40 CFR part 50, appendix N, section 4.3 , daily PM 2.5 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 PM 2.5 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 C x is the 98th percentile concentration for year x, the 2015 design value D V 2015 attains the NAAQS when:

D V 2015 = (C 201 3 + C 201 4 + C 2015 )/3 ≤ 35 .4

which can be restated as:

C 2015 ≤ 3 * 3 5.4

– ( C 201 3 + C 201 4 )

C 2015 represents the maximum 2015 98th percentile concentration allowed for attainment, given the known 98th percentile s for 201 3 and 201 4

. EPA computed these maximum allowed 2015 concentrations for all monitors in the area , and compared them to the estimated 2015 98 th percentile

. For each monitor, the 201 5 98 th percentile was estimated from the 201 5 data available in AQS as of August 201 5 , together with two assumptions. It was assumed that the sampling r ate observed so far would continue throughout the year ; this yielded an estimate of the number of total samples expected for 201 5 , and allowed for selection of the corresponding rank of the daily data available to use as the 98 th percentile. Itwas also assumed that the concentrations measured during the remainder of 201 5 would be no

higher than those already recorded, so the 98 th percentile could be chosen from among the already recorded data. This is a conservative assumption for assessing the impractic ability of attainment, since future concentrations and 9 8 th percentiles could be higher, and attainment even more impracticable.

EPA determined attainment at a monitoring site impracticable if the estimated 2015 98 th percentile was greater than the maximu m allowed 201 5 98 th 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 98 th percentiles greater than the maximum allowed 2015 98 th percentile concentration, which provides evidence that it is impractica b l e that the area attain the 2006 24

- hour PM 2.5 NAAQS by the end of 2015

.

EPA also considered the assumption that all sites in this analysis collect ed a minimum of 351 daily samples ( i .e. consistent with an everyday sampling frequency) in 201 5 that would allow for select ion of the 8 th highest recorded value as the 98 th percentile for 201 5

. This assumption would allow for selection of the lowe st 98 th percentile value possible for 201 5

. Even under this assumption, both Rubidoux and Mira Loma (Van Buren) had estimated 2015 98 th percentiles greater than the maximum allowed 2015 98 th percentile.

Based on the analysis described in this memorandum, EPA concludes that attainment of the 2006 24

- hour PM 2.5 NAAQS by the end of 2015 would be impracticable for the Los Angeles

- South Coast area

.