Document ID: EPA-HQ-OAR-2007-0225-0010
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2007-08-02T04:00Z

Chapter 7: Discussion of Ozone Secondary Standard

Exposures to O3 have been associated with a wide array of vegetation and
ecosystem effects in the published literature.  These effects include
those that damage or impair the intended use of the plant or ecosystem. 
Such effects are considered adverse to the public welfare and can
include: reduced plant growth, visible foliar (leaf) injury, reduced
plant vigor (e.g., increased susceptibility to harsh weather, disease,
insect pest infestation, and competition), reduced crop yields, and
changes in ecosystems and associated ecosystem services.

Vegetation effects research has shown that seasonal air quality indices
that cumulate peak-weighted hourly O3 concentrations are the best
candidates for relating exposure to plant growth effects.  On the basis
of this research, as well as other information considered in this review
(e.g., policy-relevant background (PRB) levels), the Staff Paper
concluded that the cumulative, seasonal index referred to as “W126”
is the most appropriate index for relating vegetation response to
ambient O3 exposures.  Based on additional conclusions regarding
appropriate diurnal and seasonal exposure windows, the Staff Paper
concluded that it was appropriate for the Administrator to consider a
cumulative seasonal secondary standard, expressed as an index of the
annual sum of weighted hourly concentrations (using the W126 form), set
at a level in the range of 7 to 21 ppm-hours.  The index would be
cumulated over the 12-hour daylight window (8:00 a.m. to 8:00 p.m.)
during the consecutive 3 month period during the O3 season with the
maximum index value (hereafter referred to as the 12-hour, maximum
3-month W126). 

The Staff Paper also considered 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 Staff Paper assessment used 2002-2004 county-level
air quality data from the AQS sites and the subset of CASTNET sites
having the highest O3 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.  Results from the
more recent (2003-2005) 3-year average comparisons (see Table 1 below)
showed that after meeting the current 3-year average form of the
0.08-ppm, 8-hour average standard, the number of counties not meeting a
3-year average W126 form ranged from 11 at the upper level of the
proposed W126 range (21 ppm-hours) to 76 counties (W126 of 15 ppm-hours-
representing the upper bound of the CASAC recommended range), to 221
counties at the lower end of the proposed W126 range (7 ppm-hours).  The
degree of overlap is greater when levels within the proposed range
(0.070-0.075 ppm) for a revised 8-hour average standard are met,
specifically the number of counties still exceeding a W126 form of a
standard range from 0 at a W126 level of 21 ppm-hours to 25 at a W126
level of 7 ppm-hours.  The Staff Paper notes that when individual years
are compared (e.g., using the annual W126 level) significant variability
occurs between years in the degree of overlap between the numbers of
counties meeting various levels of the 8-hour and W126 forms and,
therefore, cautions that the degree of protection for vegetation
provided by an 8-hour average form in terms of cumulative, seasonal
exposures would not be expected to be consistent on a year to year
basis.

The Staff Paper also identified additional aspects of this analysis that
would suggest caution should be used in interpreting these results. 
First, due to the lack of more complete monitor coverage in many rural
areas, the Staff Paper concluded that this analysis may not be an
accurate reflection of the situation in non-monitored, rural counties. 
Because of the lack of monitoring in rural areas where important
vegetation and ecosystems are located, it remains uncertain as to the
extent to which air quality improvements designed to reduce 8-hour O3
average concentrations would reduce O3 exposures measured by a seasonal,
cumulative W126 index.  The Staff Paper indicated this to be an
important consideration because:  (1) the biological database stresses
the importance of cumulative, seasonal exposures in determining plant
response; (2) plants have not been specifically tested for the
importance of daily maximum 8-hour O3 concentrations in relation to
plant response; and (3) the effects of attainment of a 8-hour standard
in upwind urban areas on rural air quality distributions cannot be
characterized with confidence due to the lack of monitoring data in
rural and remote areas.

In addition, though within the range of 8-hour average levels being
proposed the numbers of counties exceeding mid- to low levels of W126
are greatly reduced, many of these counties contain areas of national
public interest.  For example, at the 8-hour level of 0.075 ppm, 12
counties would still exceed the W126 level of 15 ppm-hours.  Most of
these counties contain high elevation, rural or remote sites where O3
air quality distributions tend to be flatter and the potential for
disconnect between 8-hour average and cumulative, seasonal forms,
greater.  Therefore, the Staff Paper notes that additional rural high
elevation areas important for vegetation that are not currently
monitored likely experience similar O3 exposure patterns.  These factors
are important considerations in determining whether the current 8-hour
form can appropriately provide requisite protection for vegetation.

Table 7.1.  Comparison of counties above various levels of the W126 when
meeting various levels of the 8-hr standard.  The W126 and 8-hr metrics
are 3-year averages from 2003-2005.

	Levels of 12-hour W126 (ppm-hrs)

8-hr level met	W126 > 21	W126 > 15	W126 > 7

8-hr < 0.084	11	76	221

8-hr < 0.080	1	29	189

8-hr < 0.075	0	12	114

8-hr < 0.070	0	2	25

8-hr < 0.065	0	0	4

Due to time and resource limitations, EPA did not calculate the costs
and monetized benefits of a separate secondary standard.  Consideration
to these costs and benefits will be provided in the final RIA.