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Comments for Consideration by the CASAC Ozone Review Panel on the

Reconsideration of the 2008 NAAQS		

Nationwide Ozone Trends and their Relevance to Determining a Policy Relevant

Background Concentration

Albert Hendler, URS Corporation (Sponsored by the American Petroleum Institute)

February 18, 2011

My comments deal with ozone design value trends and their relevance to estimating policy relevant

background (PRB). Over the last decade decreases in ozone precursor emissions taking place at the local,

regional, and national levels have reduced the number of monitors with design values above 84 ppb

from 524 in 1997‐1999 to 64 in 2007‐2009. The 2010 monitoring data in AQS is still partially incomplete

but it appears that only 47 monitoring sites will have 2008‐2010 design values above 84 ppb, according

to uncertified 2010 data retrieved from AQS Data Mart on February 11, 2011 (of the 47 monitors still

violating the 1997 standard, one is near Baltimore; two are in Tarrant Co., TX, near Fort Worth; and the

rest are in 14 southern and central California counties).  Significant progress toward attaining the 2008

ozone standard is also evident as 209 of the 321 counties that had 2006‐2008 design values above 75

ppb have 2008‐2010 design values equal to or below that level, based on the mostly complete but

uncertified data currently in AQS.  

The attached slides illustrate the design value changes that have taken place from 1999 to 2009. They

show that reductions have been achieved throughout most of the U.S. but that sites near the outer

boundary of the contiguous 48 states, which have generally cleaner air than those in the interior U.S.,

have been relatively stable.  The data suggest that a baseline design value of about 55‐60 ppb exists,

which may be more realistic of a PRB than the levels estimated by EPA.

Figures 1 and 2 show the contrast between the 1997‐1999 and 2007‐2009 design values. Red circles

represent sites with design values above 84 ppb, yellow circles represent sites with design values of 76‐

84 ppb, green circles represent sites with design values of 61‐75 ppb, and blue circles represent sites

with design values of 60 ppb or less. The sharpest contrast between the two figures is in the eastern

U.S., which was predominantly red in the first 3‐year period and predominantly green in the second 3‐

year period, 10 years later. That is, design values at most of the eastern sites have been reduced from

greater than 84 ppb to somewhere in the range of 61‐75 ppb.

The decreases in ozone design values are widespread geographically and across the entire range of

measured design values except for about the lowest 1‐2 percent of design values.  Cumulative

distribution curves for 1997‐1999 and 2007‐2009 shown in Figure 3 show design value decreases ranging

from 13‐17 ppb in the 90‐99 percentile range but the differences are smaller below the median.  At

about the 1‐2 percentile range, corresponding to design values of approximately 55‐60 ppb, the

cumulative distribution curves converge, indicating no apparent change in the lowest few percentile

range of design values.  Design values in this range have been comparatively nonresponsive to the

influences that have driven down the design values elsewhere.   
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Figure 4 shows the same data presented as a pair of frequency histograms.  Over time, the peak of the

distribution has shifted 20 ppb to the left and the distribution has become much narrower.  The number

of sites with design values above 80 ppb has decreased dramatically while the number of sites with

design values in the range of 55‐80 ppb has increased.  No change, however, was observed in the

number of sites with design values less than 55 ppb.

The monitoring sites where design values equal to or less than 60 ppb were measured in 2007‐2009 are

pinpointed in Figure 5. Sites with sub‐61 ppb design values were measured mostly near the California

coast and Pacific Northwest but also near the northern, eastern, and southern perimeters of the 48

contiguous states.  Figure 6 pinpoints the subset of these sites that have design values equal to or less

than 55 ppb.

   
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Figure 1. 1997-1999 Ozone Design Values

Legend

Numbers of Sites are in Parentheses

0 - 60 ppb (43)

61 - 75 ppb (177)

76 - 84 ppb (230)

> 84 ppb (524)

Figure 2. 2007-2009 Ozone Design Values

Legend

Numbers of Sites are in Parentheses

0 - 60 ppb (56)

61 - 75 ppb (684)

76 - 84 ppb (234)

> 84 ppb (64)
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Figure 3. Cumulative Distributions for 1997-1999 and 2007-2009 

Design Values for 665 Sites with Complete Data in Both Periods

1997-

1999

2007-

2009 Difference

N total 665 665 0

Minimum 41 40 -1

Median 87 73 -14

Maximum 147 118 -29

P1 (ppb) 51 52 1

P2 (ppb) 58 57 -1

P5 (ppb) 64 61 -3

P10 (ppb) 70 64 -6

P90 (ppb) 100 83 -17

P95 (ppb) 105 89 -16

P98 (ppb) 113 99 -14

30 40 50 60 70 80 90 100 110 120 130 140 150 P99 (ppb) 118 105 -13

0.1

0.5

1

2

5

10

20

30

40

50

60

70

80

90

95

98

99

99.5

99.9

Distributions converge 

between the 1 - 2 percentiles 

or approximately 55 - 60 ppb 

Percentiles

Design Value (ppb)

1997-1999

2007-2009
5

40 50 60 70 80 90 100 110 120

0

50

100

150

200

Number of Sites

Design Value (ppb)

 1997-1999

 2007-2009

Figure 4. Design Value Histograms for 1997-1999 and 2007-2009
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Figure 5. Sites with 2007-2009 Ozone Design Values Below 60 ppb

Hawaii and Puerto Rico are not shown

Figure 6. Sites with 2007-2009 Ozone Design Values Below 55 ppb

Hawaii and Puerto Rico are not shown