MEMORANDUM

SUBJECT:	Comparison of current 8-hr form and W126 form

FROM:	Jeffrey Herrick

TO:		Ozone NAAQS Review Docket (EPA-HQ-OAR-2005-0172)

DATE:	March 12, 2008

Table 1 considers the extent to which there is overlap between
county-level air quality measured in terms of the 8-hour average form of
the current secondary standard and that measured in terms of the 12-hour
W126, alternative cumulative, seasonal form. These comparisons were done
using 3-year averages for both forms, as well as using the 3-year
average current 8-hour form and the annual W126 county-level air quality
values. This is an update of Staff Paper assessment that used 2002-2004
county-level air quality data from the AQS sites and the subset of
CASTNET sites having the highest ozone levels for the counties in which
they are located. Since the completion of the Staff Paper, this analysis
has been updated using the more recent 3-year period of 2003 to 2005
data from AQS sites only and did not include rural CASTNET monitors.
Table 1 below was created from county-level 2003 to 2005 AQS data.  In
counties with multiple AQS monitors, data from the monitor with the
highest 3-year average of the current 8-hour form was compared.   The
top value in each box represents the number of counties meeting the
8-hour level based on 2003-2005 AQS data but exceeding the W126 level
based on a 3-year W126 average for the 2003-2005 period. The numbers in
parentheses indicate the range in the number of counties that exceed the
W126 level on an annual basis in each of the three years—2003, 2004,
2005– based on 1-year W126 values. The range indicates significant
interannual variability.

Table 1: Comparison of Number of Counties Exceeding Various W126 Levels
When Meeting Various Levels of the 8-Hr Standard for the 3-Year Period
2003–2005a

	Levels of 12-hr W126 (ppm-hrs)

8-Hour Level Met	>21	>15	>7

0.08 ppm	11

(7-27)	76

(23-173)	221

(244-382)

0.075 ppm	0

(0-7)	11

(3-33)	114

(49-134)

0.070 ppm	0

(0-1)	2

(1-6)	25

(13-36)

a The top value in each box represents the number of counties meeting
the 8-hour level based on 2003-2005 AQS data but exceeding the W126
level based on a 3-year W126 average for the 2003-2005 period. The
numbers in parentheses indicate the range in the number of counties that
exceed the W126 level on an annual basis in one of the three
years—2003, 2004, 2005– based on 1-year W126 values. The range
indicates significant interannual variability.



   

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