1 

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

Office of Air Quality Planning and Standards (OAQPS) 

Research Triangle Park, North Carolina 27711 

December 14, 2012 

MEMORANDUM 

SUBJECT: Evaluation of the Formula for Estimating the 98th Percentile Concentration for 

PM2.5 Seasonal Sampling 

FROM: Mark Schmidt, Air Quality Analysis Group, OAQPS/OAR /s/ 

TO: PM NAAQS Review Docket (EPA-HQ-OAR-2007-0492) 

The EPA proposed to eliminate the special procedures used to estimate 98th percentile 

concentrations for comparison to the primary and secondary 24-hour PM2.5 standards for 

monitoring sites with approved seasonal sampling schedules.(77 FR 39002, June 29, 2012).The 

proposal explained that with an approved seasonal sampling schedule, a site is typically required 

to sample during periods of the year when the highest concentrations are expected to occur, but 

less frequently during periods of the year when lower concentrations are expected to occur (77 

FR 39002, June 29, 2012). This type of sampling schedule generally leads to an unbalanced data 

record; that is, a data record with proportionally more ambient measurements (with respect to the 

total number of days in the sampling period) in the “high” season and proportionally fewer 

ambient measurements in the “low” season. 

In the last PM NAAQS review in 2006, the EPA revised section 4.5 of appendix N to 

include a special formula for computing annual 98th percentile values when a site operates on an 

approved seasonal sampling schedule. This special formula accounted for an unbalanced data 

record and was consistent with guidance documentation (US EPA, 1999), and, where 
2 

appropriate, with official OAQPS design value calculations (71 FR 61211, October 17, 2006). In 

cases where there is a balanced1

 (or near-balanced) data record, the special formula yields the 

same result as the regular procedure for calculating annual 98th percentile concentrations. 

As EPA noted in the proposal, there are very few PM2.5 FRM monitors that operated on 

an approved seasonal sampling schedule (only 15 sites out of approximately 1,000 total sites in 

2010) and that for almost half of those sites, the required collocated continuous instrument was a 

PM2.5 FEM (77 FR 39002, June 29, 2012). The proposal stated that for the 3-year period 2008 to 

2010, the annual 98th percentile concentrations calculated with the special formula at those 15 

sites were approximately five percent lower than if the regular procedure was used. 

The purpose of this memorandum is to update the above results using 2009-2011 data and 

to present the trend in the computed difference between the two methods when used to estimate 

annual 98th percentiles and 24-hour design values. The results show a narrowing of the 

difference between the two methods for identifying annual 98th percentiles. Moreover, there are 

no instances in the 2009-2011 timeframe (i.e., 0 out of 12 monitoring sites) for which the 

seasonal-formula 24-hour design value meets the NAAQS but the corresponding regular-formula 

design value shows a violation. These findings can likely be attributed, at least in part, to the 

increasing network-wide deployment of Federal Equivalent Monitors (FEMs). 

This evaluation of seasonal sampling looked at several issues: the extent of seasonal 

sampling recently and over time; the extent to which sites approved for seasonal sampling have 

had a collocated FEM; and the estimated difference between results from use of the special 

formula and those from the regular formula both for 98th percentiles and 24-hour design values. 

All data used in this evaluation were extracted from AQS on December 11, 2012. 

 

1 A balanced data record has the same proportion of ambient measurements (with respect to the total number of days 

in the sampling period) in the “high” season as in the “low” season.
3 

Figure 1 shows the number of sites, by year, approved for seasonal sampling (as 

determined via AQS metadata) over the entire PM2.5 regulatory timeframe (1999 through 2011). 

The number of sites approved for seasonal sampling ranged from around 18 to 20 for about the 

first ten years of the regulatory program, and then dropped dramatically during the most recent 

few years . In 2008, there were 20 sites (about two percent) with an approved seasonal sampling 

schedule In 2011, there were only 12 sites with an approved seasonal sampling schedule (about 

1.3 percent of the total). These data indicate a 40 percent reduction in the absolute number, and 

a 35 percent reduction in the annual proportion of total sites that have been approved for seasonal 

sampling over the most recent 4-year period. 

Figure 1. Number and percentage of sites with an approved seasonal sampling schedule, 1999-2011 

0

5

10

15

20

25

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Number of sites Percentage of total number of sites
4 

Figure 2 shows the number of sites, by year, approved for seasonal sampling, from 2008 

to 2011, for which there is a collocated continuous FEM. Note that continuous FEMs did not 

come on-line until 2008. As seen in the figure, the number of seasonal sampling sites with a 

collocated continuous FEM has increased from none in 2008 to five in 2011. 

Figure 2. Number of sites utilizing a seasonal sampling schedule and collocated with a continuous FEM, 2008-2011 

Table 1 (provided in Attachment 1) shows all the sites that used a seasonal sampling 

schedule from 1999 through 2011 and includes: the annual 98th percentiles identified as using the 

special seasonal sampling formula, corresponding annual 98th percentiles identified using the 

regular table-look-up procedure, 24-hour design values derived from seasonal-formula 98th

percentiles, corresponding 24-hour design values derived from regular-formula 98th percentiles, 

and estimates of differences between the two methods . For this table, only years that contained 

at least 48 samples were included, and 24-hour design values are only shown if there were three 

consecutive years that met the annual criterion. The 98th percentiles and design values that 

exceed the 24-hour NAAQS level of 35 μg/m3 are shaded. The differences between the results 
5 

of the two seasonal and regular methods are presented in two ways: 1) by concentration 

difference (in μg/m3) (e.g., P98seasonal - P98regular), and 2) by percentage difference (in %) (e.g., 

((P98seasonal / P98regular - 1) * 100). As seen in the summary at the bottom of table 1, for the 210 

site-year pairs, the seasonal-formula 98th percentiles were, on average, 3.9 μg/m3 (8.2 %) lower 

than the corresponding regular-formula 98th percentiles. For the 164 site-level design value 

pairs, the 24-hour design values were, on average, 4.0 μg/m3 (9.0 %) lower than the 

corresponding regular-formula design values. In the 1999-2011 timeframe, 21 of the 210 (10%) 

annual 98th percentile site-year pairs have a seasonal-formula 98th percentile that is equal or 

below the 24-hour NAAQS level but a corresponding regular-formula 98th percentile that 

exceeds it. In the same time period, 16 of the 164 (10%) design value pairs have a seasonalformula 24-hour design value that meets the NAAQS but a corresponding regular-formula design 

value that does not meet it. 

Table 2 (shown in Attachment 2) provides similar information as Table 1, but only for the 

most recent 3-year period (2009-2011). For this table, only sites that operated on a seasonal 

schedule for all three years are shown; all shown sites have valid 24-hour design values (per 

appendix N protocol) so the annual 98th percentiles are also considered valid. As in Table 1, 98th

percentiles and design values that exceed the 24-hour NAAQS level of 35 μg/m3 are shaded. 

98th percentiles that are bolded in this table represent years for which the site had a collocated 

continuous FEM. For the 36 site-years shown for 2009-2011, the seasonal-formula 98th

percentiles were, on average, 1.9 μg/m3 (5.3 %) lower than the corresponding regular-formula 

98th percentiles. For the 12 site-level design value pairs, the 24-hour design values were, on 

average, 1.8 μg/m3 (5.4 %) lower than the corresponding regular-formula. The differences noted 

for the 2009-2011 period (for both 98th percentiles and 24-hour design values) are roughly one 
6 

half the levels noted for the 1999-2011 timeframe. In the 2009-2011 results, there are two annual 

98th percentiles out of the 36 shown (6%) that have a seasonal-formula 98th percentile that is 

equal or below the 24-hour NAAQS level but a corresponding regular-formula 98th percentile 

that exceeds it. There are no instances in the 2009-2011 timeframe (i.e., 0 out of 12) for which 

the seasonal-formula 24-hour design value meets the NAAQS but the corresponding regularformula design value shows a violation. 

Figure 3 below plots the average annual percent difference, from 1999 through 2011 with 

a fitted polynomial trend line. It appears that the differences between the results of the two 

different methods for identifying annual 98th percentile concentrations began narrowing around 

2006. As discussed above, the difference in the results from the two methods in the most recent 

3-year period is noticeably smaller than the differences in the results between the two methods 

for 1999-2011. 

The recent reduction in difference between the results from the two different methods for 

identifying annual 98th percentiles as well as the reduction in the percentage of sites that had a 

change in NAAQS compliance status based on the two approaches (i.e., where the seasonal 

formula yielded a NAAQS metric that that met the 24-hour standard level), can likely be 

attributed, at least in part, to the 2006 monitoring requirements. As noted in the proposal, the 

Agency modified the monitoring requirement for areas with an FRM operating on a non-daily 

schedule such that, when the design values were within five percent of the 24-hour PM2.5

NAAQS, those areas would be required to increase the frequency of sampling to every day (40 

CFR 58.12(d)(1); 71 FR 61165, October 17, 2006; 71 FR 61249, October 17, 2006). As more 

FEMs are deployed, there will continue to be less need for seasonal sampling and thus, there will 

also be less need for a seasonal sampling adjustment. 
7 

Figure 3. Average percentage difference, by year, between 98th percentiles identified with the seasonal sampling 

procedure versus those identified with the regular table look-up procedure 

0%

2%

4%

6%

8%

10%

12%

14%

16%

18%

1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011

Percentage difference (%)
SITE Year

98th percentiles 

using special 

seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

98th percentiles 

using regular 

table look-up 

method, 1999-

2011 (μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

special seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

regular table 

look-up method, 

1999-2011 

(μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

021100027 2003 5.7 5.7 0 0.0%

060010007 2000 38.3 49.7 -11.4 -22.9%

060010007 2001 57.1 77.7 -20.6 -26.5%

060010007 2002 50.5 51.6 -1.1 -2.1% 49 60 -11 -18.3%

060010007 2003 24.2 27.1 -2.9 -10.7% 44 52 -8 -15.4%

060010007 2004 35.3 38.1 -2.8 -7.3% 37 39 -2 -5.1%

060010007 2005 28.7 29.2 -0.5 -1.7% 29 31 -2 -6.5%

060010007 2006 36.6 49.7 -13.1 -26.4% 34 39 -5 -12.8%

060010007 2007 39.2 49.2 -10 -20.3% 35 43 -8 -18.6%

060010007 2008 32.7 34.5 -1.8 -5.2% 36 44 -8 -18.2%

060010007 2009 30.7 33.6 -2.9 -8.6% 34 39 -5 -12.8%

060010007 2010 26.5 28.4 -1.9 -6.7% 30 32 -2 -6.3%

060010007 2011 27 27 0 0.0% 28 30 -2 -6.7%

060010009 2008 23.8 25.9 -2.1 -8.1%

060010009 2009 24.5 24.7 -0.2 -0.8%

060010009 2010 21.7 21.7 0 0.0% 23 24 -1 -4.2%

060010009 2011 28 28 0 0.0% 25 25 0 0.0%

060011001 1999 45.2 45.2 0 0.0%

060011001 2000 36.3 42.2 -5.9 -14.0%

060011001 2001 47 51 -4 -7.8% 43 46 -3 -6.5%

060011001 2002 41.5 44.2 -2.7 -6.1% 42 46 -4 -8.7%

060011001 2003 22.1 23.8 -1.7 -7.1% 37 40 -3 -7.5%

060011001 2004 33 37 -4 -10.8% 32 35 -3 -8.6%

060011001 2005 27.6 30.9 -3.3 -10.7% 28 31 -3 -9.7%

060011001 2006 30.4 39.7 -9.3 -23.4% 30 36 -6 -16.7%

060011001 2007 33 3 43 3 10 23 1% 30 38 8 21 1%

Table 1. Comparison of 98th percentiles (and associated 24-hour design values) derived via the special seasonal sampling formula versus 

those derived via the regular table look-up method, 1999-2011

060011001 2007 33.3 43.3 -10 -23.1% 30 38 -8 -21.1%

060011001 2008 22.7 27 -4.3 -15.9% 29 37 -8 -21.6%

060011001 2009 24.1 25.4 -1.3 -5.1% 27 32 -5 -15.6%

060011001 2010 22.5 22.5 0 0.0% 23 25 -2 -8.0%

060130002 1999 44.1 54.9 -10.8 -19.7%

060130002 2000 44.8 45.8 -1 -2.2%

060130002 2001 43.3 67.2 -23.9 -35.6% 44 56 -12 -21.4%

060130002 2002 57.3 63 -5.7 -9.0% 48 59 -11 -18.6%

060130002 2003 33.8 36.1 -2.3 -6.4% 45 55 -10 -18.2%

060130002 2004 38.1 42.5 -4.4 -10.4% 43 47 -4 -8.5%

060130002 2005 33.4 40.5 -7.1 -17.5% 35 40 -5 -12.5%

060130002 2006 33.6 38.5 -4.9 -12.7% 35 41 -6 -14.6%

060130002 2007 34.9 37.8 -2.9 -7.7% 34 39 -5 -12.8%

060130002 2008 35.2 31.1 4.1 13.2% 35 36 -1 -2.8%

060130002 2009 29.2 32.5 -3.3 -10.2% 33 34 -1 -2.9%

060130002 2010 26.8 30.9 -4.1 -13.3% 30 32 -2 -6.3%

060130002 2011 24.4 27.6 -3.2 -11.6% 27 30 -3 -10.0%

060195001 1999 59.2 83.2 -24 -28.8%

060195001 2000 72.5 72.5 0 0.0%

060195001 2001 71.5 76.2 -4.7 -6.2% 68 77 -9 -11.7%

060195001 2002 53.2 53.5 -0.3 -0.6% 66 67 -1 -1.5%

060195001 2003 48.1 48.4 -0.3 -0.6% 58 59 -1 -1.7%

060195001 2004 52.4 56 -3.6 -6.4% 51 53 -2 -3.8%

060195001 2005 63 67.1 -4.1 -6.1% 55 57 -2 -3.5%

060195001 2006 51.3 60.5 -9.2 -15.2% 56 61 -5 -8.2%

060195001 2007 60.9 61.2 -0.3 -0.5% 58 63 -5 -7.9%

060195001 2008 49 49.7 -0.7 -1.4% 54 57 -3 -5.3%

060195025 2000 64.8 65.1 -0.3 -0.5%

060195025 2001 61.5 72.1 -10.6 -14.7%

060195025 2002 71.9 73 -1.1 -1.5% 66 70 -4 -5.7%

060195025 2003 49.7 52 -2.3 -4.4% 61 66 -5 -7.6%

Evaluation of the PM2.5 Special Seasonal Sampling 98th Percentile Conncentration Identification Formula 

Attachment 1 

Page 1 of 4
SITE Year

98th percentiles 

using special 

seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

98th percentiles 

using regular 

table look-up 

method, 1999-

2011 (μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

special seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

regular table 

look-up method, 

1999-2011 

(μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

Table 1. Comparison of 98th percentiles (and associated 24-hour design values) derived via the special seasonal sampling formula versus 

those derived via the regular table look-up method, 1999-2011

060195025 2004 49.4 52.1 -2.7 -5.2% 57 59 -2 -3.4%

060195025 2005 71.2 74.1 -2.9 -3.9% 57 59 -2 -3.4%

060195025 2006 55 65 -10 -15.4% 59 64 -5 -7.8%

060195025 2007 57.4 57.9 -0.5 -0.9% 61 66 -5 -7.6%

060195025 2008 44.5 46.4 -1.9 -4.1% 52 56 -4 -7.1%

060195025 2009 48.2 52.3 -4.1 -7.8% 50 52 -2 -3.8%

060195025 2010 37 40.2 -3.2 -8.0% 43 46 -3 -6.5%

060195025 2011 59.6 67.5 -7.9 -11.7% 48 53 -5 -9.4%

060231002 2006 21.5 25.7 -4.2 -16.3%

060231002 2007 27.2 27.2 0 0.0%

060231002 2008 21.5 21.5 0 0.0% 23 25 -2 -8.0%

060231002 2009 22.1 23.2 -1.1 -4.7% 24 24 0 0.0%

060231002 2010 15.8 17.4 -1.6 -9.2% 20 21 -1 -4.8%

060231002 2011 18.7 18.7 0 0.0% 19 20 -1 -5.0%

060231004 2007 21.8 23.5 -1.7 -7.2%

060231004 2008 26 26.5 -0.5 -1.9%

060231004 2009 23 24.2 -1.2 -5.0% 24 25 -1 -4.0%

060231004 2010 17.7 21.2 -3.5 -16.5% 22 24 -2 -8.3%

060231004 2011 23.7 23.7 0 0.0% 21 23 -2 -8.7%

060290010 1999 95.3 97.5 -2.2 -2.3% 0

060290010 2000 93.9 102.5 -8.6 -8.4% 0

060290010 2001 95.9 96.3 -0.4 -0.4% 95 99 -4 -4.0%

060290010 2002 80.4 81.6 -1.2 -1.5% 90 93 -3 -3.2%

060290010 2003 51.9 57.1 -5.2 -9.1% 76 78 -2 -2.6%

060290010 2004 53.9 54.6 -0.7 -1.3% 62 64 -2 -3.1%

060290010 060290010 2005 74 9. 77 9. -3 -3 9%. 60 63 -3 -4 8%.

060290010 2006 64.4 75.2 -10.8 -14.4% 64 69 -5 -7.2%

060290010 2007 67.7 69.4 -1.7 -2.4% 69 74 -5 -6.8%

060290010 2008 60.8 60.9 -0.1 -0.2% 64 69 -5 -7.2%

060310004 1999 53 53 0 0.0%

060310004 2000 55.1 55.1 0 0.0%

060310004 2001 89.5 120.6 -31.1 -25.8% 66 76 -10 -13.2%

060310004 2002 65.1 77.4 -12.3 -15.9% 70 84 -14 -16.7%

060310004 2003 42.2 48.5 -6.3 -13.0% 66 82 -16 -19.5%

060310004 2004 49.4 49.6 -0.2 -0.4% 52 59 -7 -11.9%

060310004 2005 74.5 77.8 -3.3 -4.2% 55 59 -4 -6.8%

060310004 2006 50.1 63.8 -13.7 -21.5% 58 64 -6 -9.4%

060310004 2007 57.9 59.5 -1.6 -2.7% 61 67 -6 -9.0%

060310004 2008 47.9 47.9 0 0.0% 52 57 -5 -8.8%

060310004 2009 53.4 53.4 0 0.0% 53 54 -1 -1.9%

060310004 2010 46.8 47.2 -0.4 -0.8% 49 50 -1 -2.0%

060310004 2011 38.7 40.8 -2.1 -5.1% 46 47 -1 -2.1%

060472510 1999 91.9 91.9 0 0.0%

060472510 2000 60 68.4 -8.4 -12.3%

060472510 2001 49.3 49.3 0 0.0% 67 70 -3 -4.3%

060472510 2002 55.1 57.6 -2.5 -4.3% 55 58 -3 -5.2%

060472510 2003 44.2 44.7 -0.5 -1.1% 50 51 -1 -2.0%

060472510 2004 43 46.9 -3.9 -8.3% 47 50 -3 -6.0%

060472510 2005 48.3 48.6 -0.3 -0.6% 45 47 -2 -4.3%

060472510 2006 43.8 52.5 -8.7 -16.6% 45 49 -4 -8.2%

060472510 2007 52.7 53 -0.3 -0.6% 48 51 -3 -5.9%

060472510 2008 54 53.6 0.4 0.7% 50 53 -3 -5.7%

060472510 2009 45.2 49.8 -4.6 -9.2% 51 52 -1 -1.9%

060472510 2010 35.5 39.1 -3.6 -9.2% 45 48 -3 -6.3%

060472510 2011 35.4 38.5 -3.1 -8.1% 39 42 -3 -7.1%

060670006 1999 84 84 0 0.0%

Evaluation of the PM2.5 Special Seasonal Sampling 98th Percentile Conncentration Identification Formula 

Attachment 1 

Page 2 of 4
SITE Year

98th percentiles 

using special 

seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

98th percentiles 

using regular 

table look-up 

method, 1999-

2011 (μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

special seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

regular table 

look-up method, 

1999-2011 

(μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

Table 1. Comparison of 98th percentiles (and associated 24-hour design values) derived via the special seasonal sampling formula versus 

those derived via the regular table look-up method, 1999-2011

060670006 2000 84 81 3 3.7%

060670006 2001 38 38 0 0.0% 69 68 1 1.5%

060670006 2002 62 66 -4 -6.1% 61 62 -1 -1.6%

060670006 2003 43 49 -6 -12.2% 48 51 -3 -5.9%

060670006 2004 42 45 -3 -6.7% 49 53 -4 -7.5%

060670006 2005 49 67 -18 -26.9% 45 54 -9 -16.7%

060670006 2006 55 58 -3 -5.2% 49 57 -8 -14.0%

060674001 1999 71 71 0 0.0%

060674001 2000 50 58 -8 -13.8%

060674001 2001 28 42 -14 -33.3% 50 57 -7 -12.3%

060674001 2002 60 62 -2 -3.2% 46 54 -8 -14.8%

060674001 2003 36 37 -1 -2.7% 41 47 -6 -12.8%

060674001 2004 35 36 -1 -2.8% 44 45 -1 -2.2%

060674001 2005 42 46 -4 -8.7% 38 40 -2 -5.0%

060674001 2006 39 40 -1 -2.5% 39 41 -2 -4.9%

060750005 1999 48.3 55.2 -6.9 -12.5%

060750005 2000 35.3 39.7 -4.4 -11.1%

060750005 2001 51.3 55.4 -4.1 -7.4% 45 50 -5 -10.0%

060750005 2002 57.5 66.2 -8.7 -13.1% 48 54 -6 -11.1%

060750005 2003 33 36.3 -3.3 -9.1% 47 53 -6 -11.3%

060750005 2004 32.2 36.1 -3.9 -10.8% 41 46 -5 -10.9%

060750005 2005 32.6 39.4 -6.8 -17.3% 33 37 -4 -10.8%

060750005 2006 27.8 32.7 -4.9 -15.0% 31 36 -5 -13.9%

060750005 2007 27.4 35.7 -8.3 -23.2% 29 36 -7 -19.4%

060750005 2008 23.8 26.8 -3 -11.2% 26 32 -6 -18.8%

060811001 060811001 1999 53 4. 53 4. 0 0 0%.

060811001 2000 43 44 -1 -2.3%

060811001 2001 46.1 50.9 -4.8 -9.4% 48 49 -1 -2.0%

060811001 2002 36.3 37.8 -1.5 -4.0% 42 44 -2 -4.5%

060811001 2003 26.1 30.6 -4.5 -14.7% 36 40 -4 -10.0%

060811001 2004 27.9 29.1 -1.2 -4.1% 30 33 -3 -9.1%

060811001 2005 29.4 29.4 0 0.0% 28 30 -2 -6.7%

060811001 2006 30.9 34.8 -3.9 -11.2% 29 31 -2 -6.5%

060811001 2007 32.8 33 -0.2 -0.6% 31 32 -1 -3.1%

060811001 2008 24.3 26.8 -2.5 -9.3% 29 32 -3 -9.4%

060850004 1999 59.7 65.4 -5.7 -8.7%

060850004 2000 55.3 56.6 -1.3 -2.3%

060850004 2001 56.2 62.5 -6.3 -10.1% 57 62 -5 -8.1%

060850005 2002 46.7 52 -5.3 -10.2%

060850005 2003 37.4 40.1 -2.7 -6.7%

060850005 2004 39.8 42.7 -2.9 -6.8% 41 45 -4 -8.9%

060850005 2005 39.8 41.2 -1.4 -3.4% 39 41 -2 -4.9%

060850005 2006 36 42.8 -6.8 -15.9% 39 42 -3 -7.1%

060850005 2007 39.1 44.2 -5.1 -11.5% 38 43 -5 -11.6%

060850005 2008 32.4 36.3 -3.9 -10.7% 36 41 -5 -12.2%

060850005 2009 29.8 30.7 -0.9 -2.9% 34 37 -3 -8.1%

060850005 2010 29.2 33.3 -4.1 -12.3% 30 33 -3 -9.1%

060850005 2011 30.5 34.2 -3.7 -10.8% 30 33 -3 -9.1%

060852003 1999 63.2 66.9 -3.7 -5.5%

060852003 2000 54.5 60 -5.5 -9.2%

060852003 2001 49.7 57.2 -7.5 -13.1% 56 61 -5 -8.2%

060852003 2002 37.3 43.7 -6.4 -14.6% 47 54 -7 -13.0%

060852003 2003 32.5 37 -4.5 -12.2% 40 46 -6 -13.0%

060852003 2004 36.5 37.9 -1.4 -3.7% 35 40 -5 -12.5%

060852003 2005 38.7 40.9 -2.2 -5.4% 36 39 -3 -7.7%

060852003 2006 23.8 25.6 -1.8 -7.0% 33 35 -2 -5.7%

Evaluation of the PM2.5 Special Seasonal Sampling 98th Percentile Conncentration Identification Formula 

Attachment 1 

Page 3 of 4
SITE Year

98th percentiles 

using special 

seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

98th percentiles 

using regular 

table look-up 

method, 1999-

2011 (μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

special seasonal 

sampling 

formula, 1999-

2011 (μg/m3)

24-hour design 

values (shown 

on end year) 

using 98th 

percentiles 

derived by 

regular table 

look-up method, 

1999-2011 

(μg/m3)

Difference 

between the two 

estimates 

(μg/m3)

Difference 

between the two 

estimates as a 

percent of the 

regular method 

estimate (%)

Table 1. Comparison of 98th percentiles (and associated 24-hour design values) derived via the special seasonal sampling formula versus 

those derived via the regular table look-up method, 1999-2011

060950004 1999 52.3 52.3 0 0.0%

060950004 2000 44 60.1 -16.1 -26.8%

060950004 2001 56 78 -22 -28.2% 51 63 -12 -19.0%

060950004 2002 55.4 71.2 -15.8 -22.2% 52 70 -18 -25.7%

060950004 2003 25.1 27.3 -2.2 -8.1% 46 59 -13 -22.0%

060950004 2004 36.9 38.6 -1.7 -4.4% 39 46 -7 -15.2%

060950004 2005 35.6 41 -5.4 -13.2% 33 36 -3 -8.3%

060950004 2006 34.3 40.5 -6.2 -15.3% 36 40 -4 -10.0%

060950004 2007 38.6 40 -1.4 -3.5% 36 41 -5 -12.2%

060950004 2008 36.3 40.5 -4.2 -10.4% 36 40 -4 -10.0%

060950004 2009 33.5 37.1 -3.6 -9.7% 36 39 -3 -7.7%

060950004 2010 22.8 25.5 -2.7 -10.6% 31 34 -3 -8.8%

060970003 1999 44.5 44.5 0 0.0%

060970003 2000 36.8 39.5 -2.7 -6.8%

060970003 2001 41.4 41.9 -0.5 -1.2% 41 42 -1 -2.4%

060970003 2002 42.4 47.4 -5 -10.5% 40 43 -3 -7.0%

060970003 2003 29.8 30.8 -1 -3.2% 38 40 -2 -5.0%

060970003 2004 25.2 25.2 0 0.0% 32 34 -2 -5.9%

060970003 2005 29.7 32.6 -2.9 -8.9% 28 30 -2 -6.7%

060970003 2006 31.2 32.4 -1.2 -3.7% 29 30 -1 -3.3%

060970003 2007 30.2 30.5 -0.3 -1.0% 30 32 -2 -6.3%

060970003 2008 29.8 29.8 0 0.0% 30 31 -1 -3.2%

060970003 2009 25.2 25.9 -0.7 -2.7% 28 29 -1 -3.4%

060970003 2010 22.2 22.2 0 0.0% 26 26 0 0.0%

060970003 2011 23.4 23.4 0 0.0% 24 24 0 0.0%

230052003 230052003 1999 23 33 3. -10 3. -30 9%.

230052003 2000 21.1 21.1 0 0.0%

230052003 2001 28.3 28.9 -0.6 -2.1%

230052003 2002 25.8 25.9 -0.1 -0.4% 25 25 0 0.0%

230090103 1999 17.1 20.6 -3.5 -17.0%

230090103 2000 13.5 13.5 0 0.0%

230090103 2001 28.2 28.2 0 0.0% 20 21 -1 -4.8%

230090103 2002 25.9 43.3 -17.4 -40.2% 23 28 -5 -17.9%

230090103 2003 21.8 25.4 -3.6 -14.2% 25 32 -7 -21.9%

230090103 2004 18.7 22.6 -3.9 -17.3% 22 30 -8 -26.7%

230090103 2005 18.7 24.2 -5.5 -22.7% 20 24 -4 -16.7%

230090103 2006 19.9 19.9 0 0.0% 19 22 -3 -13.6%

230090103 2007 19.8 21.6 -1.8 -8.3% 19 22 -3 -13.6%

230090103 2008 18.6 18.6 0 0.0% 19 20 -1 -5.0%

230090103 2009 10.1 19.3 -9.2 -47.7% 16 20 -4 -20.0%

230090103 2010 16.2 18.7 -2.5 -13.4% 15 19 -4 -21.1%

230090103 2011 12.6 12.8 -0.2 -1.6% 13 17 -4 -23.5%

210 210 210 210 164 164 164 164

41.1 45.0 -3.9 -8.2% 41 45 -4.0 -9.0%

37.7 42.0 -2.7 -6.7% 39 43 -3 -7.9%

21 16

Note. 98th percentiles and design values that are shaded exceed the 24-hour NAAQS level of 35 μg/m3

average

count

median

Number of annual 98th percentiles that are above the 24-hour NAAQS level (35 μg/m3) using 

the regular formula but equal/below the NAAQS level using the seasonal formula

Number of DVs that violate the 24-hour NAAQS (35 μg/m3) 

using the regular formula but meet the NAAQS using the seasonal 

formula

Evaluation of the PM2.5 Special Seasonal Sampling 98th Percentile Conncentration Identification Formula 

Attachment 1 

Page 4 of 4
98th 2009 98th 2010 98th 2011 DV 2009-2011 98th 2009 98th 2010 98th 2011 DV 2009-2011

060010007 30.7 26.5 27.0 28 33.6 28.4 27.0 30

060010009 24.7 21.7 28.0 25 24.7 21.7 28.0 25

060130002 29.2 26.8 24.4 27 32.5 30.9 27.6 30

060195025 48.2 37.0 59.6 48 52.3 40.2 67.5 53

060231002 22.1 15.8 18.7 19 23.2 17.4 18.7 20

060231004 23.0 17.7 23.7 21 24.2 21.2 23.7 23

060310004 53.4 46.8 38.7 46 53.4 47.2 40.8 47

060472510 45.2 35.5 35.4 39 49.8 39.1 38.5 42

060850005 29.8 29.2 30.5 30 30.7 33.3 34.2 33

060950004 33.5 22.8 31.0 29 37.1 25.5 31.0 31

060970003 25.9 22.2 23.4 24 25.9 22.2 23.4 24

230090103 10.1 16.2 12.6 13 10.1 16.2 12.6 13

Notes: 

98th 2009 98th 2010 98th 2011 DV 2009-2011 98th 2009 98th 2010 98th 2011 DV 2009-2011

060010007 -2.9 -1.9 0.0 -2.0 -8.6% -6.7% 0.0% -6.7%

060010009 0.0 0.0 0.0 0.0 0.0% 0.0% 0.0% 0.0%

060130002 -3.3 -4.1 -3.2 -3.0 -10.2% -13.3% -11.6% -10.0%

060195025 -4.1 -3.2 -7.9 -5.0 -7.8% -8.0% -11.7% -9.4%

060231002 -1 1 -1 6 0 0 -1 0 -4 7% -9 2% 0 0% -5 0%

Table 2. Comparison of 98th percentiles (and associated 24-hour design values) derived via the special seasonal sampling 

formula versus those derived via the regular table look-up method, 2009-2011

Site

Difference in 98th percentiles and 24-hour design values 

between the two methods, in μg/m3 Difference in 98th percentiles and 24-hour design values 

between the two methods, as a % of the regular method value

1. 98th percentiles that are bolded represent years for which there was a collocated FEM at the site.

2. 98th percentiles and design values that are shaded exceed the 24-hour NAAQS level of 35 μg/m3.

Site

98th percentiles and 24-hour design values using special 

seasonal sampling formula, 2009-2011 (μg/m3)

98th percentiles and 24-hour design values using regular 

table look-up method, 2009-2011 (μg/m3)

060231002 -1.1 -1.6 0.0 -1.0 -4.7% -9.2% 0.0% -5.0%

060231004 -1.2 -3.5 0.0 -2.0 -5.0% -16.5% 0.0% -8.7%

060310004 0.0 -0.4 -2.1 -1.0 0.0% -0.8% -5.1% -2.1%

060472510 -4.6 -3.6 -3.1 -3.0 -9.2% -9.2% -8.1% -7.1%

060850005 -0.9 -4.1 -3.7 -3.0 -2.9% -12.3% -10.8% -9.1%

060950004 -3.6 -2.7 0.0 -2.0 -9.7% -10.6% 0.0% -6.5%

060970003 0.0 0.0 0.0 0.0 0.0% 0.0% 0.0% 0.0%

230090103 0.0 0.0 0.0 0.0 0.0% 0.0% 0.0% 0.0%

98th 2009 98th 2010 98th 2011 DV 2009-2011 98th 2009 98th 2010 98th 2011 DV 2009-2011

average -1.8 -2.1 -1.7 -1.8 -4.8% -7.2% -3.9% -5.4%

median -1.2 -2.3 0.0 -2.0 -4.9% -8.6% 0.0% -6.6%

-1.9 -5.3%

-1.6 -5.1%

-2.4 -6.9%

-2.9 -8.1%

2

0

Median difference in the 27 98th percentiles for which the 

sites didn't have a collocated FEM (μg/m3)

Median differnce in the 27 98th percentiles for which the 

sites didn't have a collocated FEM (percent)

Number of annual 98th percentiles that are above the 24-hour NAAQS level (35 

Number of design values that are above the 24-hour NAAQS level (35 μg/m3) 

Avg. difference in all (36) 98th percentiles (μg/m3) Avg. diff. in all 98th %iles (percent of regular value)

Median difference in all (36) 98th percentiles (μg/m3) Median diff. in all 98th %iles (percent of regular value)

Avg. difference in the 27 98th percentiles for which the sites 

didn't have a collocated FEM (μg/m3)

Avg. differnce in the 27 98th percentiles for which the 

sites didn't have a collocated FEM (percentage)

Evaluation of the PM2.5 seasonal sampling 98th percentile concentration identification procedure 

Attachment 2 

Page 1 of 1