In 2007, the EPA Administrator
proposed the use of a 3-month, 12-hour W126 exposure index as
a possible secondary O3 standard. Although the EPA Administrator
recommended the W126 as the secondary ozone standard,
based on advice from the White
House (Washington
Post, April 8, 2008; Page D02), the EPA Administrator made the
secondary ozone standard the same as the primary 8-hour average
standard (0.075 ppm). On September 16, 2009, the EPA announced
it would reconsider the 2008 national ambient air quality standards
(NAAQS) for ground-level ozone for both human health and environmental
effects. The Agency planned to propose any needed revisions to
the ozone standards by December 2009 and issue a final decision
by August 2010. On January 7, 2010, the EPA announced on its
website its proposal to strengthen the national ambient air quality
standards for ground-level ozone. The EPA's proposal decreased
the 8-hour “primary” ozone standard level, designed
to protect public health, to a level within the range of 0.060-0.070
parts per million (ppm). EPA proposed to establish a distinct
cumulative, seasonal “secondary” standard, referred
to as the W126 index, which was designed to protect
sensitive vegetation and ecosystems, including forests, parks,
wildlife refuges, and wilderness areas. EPA proposed to set the
level of the W126 secondary standard within the range
of 7-15 ppm-hours. The accumulation period of the proposed W126
standard was 12 hours. On August 20, 2010 the Agency announced
that it would delay its final announcement to on or around the
end of October. In early November, the EPA announced that it
would reach a final decision on the ozone standards by December
31, 2010. On December 8, the EPA announced that it would delay
its final decision on the ozone standards until July 2011. EPA
announced on July 26 that it would not make a decision on the
ozone standards by its previously announced deadline of July
29. On September 2, 2011, President Obama requested that the
EPA withdraw its proposed revisions to the ozone standards and
deferred to the normal cycle of evaluating the current state
of science associated with ozone and its effects on human health
and vegetation.
It is important to note
that in the United States the daylength during the summer months
at all locations is greater than 12 hours. In some locations,
the daylength is greater than 16 hours. The figure below illustrates
the daylength at latitudes that cover the area from Montana to
southern Florida during the period from April 1 to October 29.
Note that the only time during this period that the daylength
is less than 12 hours is after the third week in September. The
daylength during June in Montana is greater than 16 hours and
in southern Florida is 13.5 hours.
European scientists use
a time window that is dependent upon location and time of year.
For low-elevation sites, the 0600-2059h window is often used.
What is the scientific justification for using a cumulative 12
hours, while the actual daylength during the summer months is
greater than that value?
An extensive review of
the literature reported that a large number of species had varying
degrees of nocturnal stomatal conductance (Musselman and Minnick,
2000). Although EPA (2007) acknowledged that uptake of O3 during
the nighttime may be important, the Agency states on page 8-17,
"…staff concludes that
it remains unclear to what extent nocturnal uptake contributes
to the vegetation effects of yield loss, biomass loss or visible
foliar injury. Due to the many species- and site-specific variables
that influence the potential for and significance of nocturnal
uptake, staff concludes that additional research needs to be
done before considering whether this component of vegetation
exposure should be addressed with a different averaging time."
Nocturnal O3 flux depends
on the level of turbulence that intermittently occurs at night.
Massman (2004) suggested that nocturnal stomatal O3 uptake accounted
for about 15% of the cumulative daily effective O3 dose that
was related to predicted injury. Similarly, Grulke et al. (2004)
showed that the stomatal conductance at night for Ponderosa pine
in the San Bernardino National Forest (CA) ranged from one tenth
to one fourth that of maximum daytime gas exchange. Heath et
al. (2009) discuss the importance of nighttime ozone exposures
associated with changes in the detoxification potential as a
function of the time of day. Lee et al. (2022) note that in some
circumstances, there may not always be concordance between diurnal
patterns of O3 concentrations and those for stomatal conductance.
The authors note that high-elevation sites often have O3 concentrations
that remain high at night and even peak at these times and consequently,
a 12-h O3 exposure metric may not produce optimal predictions
at high elevations and for some species with meaningful nocturnal
stomatal conductance and flux. Goumenaki et al. (2021) noted
that ascorbic acid (AA) content and/or redox state was subject
to day/night control. The investigators reported that plants
exposed to equivalent O3 fluxes administered during daytime versus
nighttime exhibited a significant decline in biomass in both
cases, and the losses were greater in plants subjected to equivalent
O3 flux at night. Thus, nighttime O3 exposures appeared to be
important. The change in diurnal detoxification is very important
when considering the use of flux-based indices (see Musselman
et al., 2006; Lefohn et al., 2018). In addition, it appears that
cumulative O3 exposures over a 24-hour period may be important.
In addition to the concern
whether the accumulation period should be 24 hours versus 12
hours for the W126 exposure index for assessing vegetation
effects, it is important to address the exposure regime patterns
used in the crop and forest seedling experiments in the 1980s
and 1990s. The experimental exposure protocols used to introduce
enhanced ozone concentrations into the chambers resulted in numerous
hourly average concentrations greater than or equal to 100 ppb
for some of the crops and tree seedling species (but not all)
in the experiments. While frequent occurrences under ambient
conditions of hourly average concentrations greater than or equal
to 100 ppb were prevelant in the 1980s and 1990s, this is not
the case today. Thus, some of the exposure regimes used in the
NCLAN and forest seedling experiments do not match the ambient
exposure regimes currently experienced in the United States.
Lefohn and Foley (1992) and Lefohn et al. (1997) noted the frequent
occurrences of hourly average concentrations greater than or
equal to 100 ppb in the NCLAN and forest seedling experiments
and suggested that an additional exposure metric that described
the number of hourly average concentrations greater than or equal
to 100 ppb (i.e., N100 index) be coupled with the W126 metric
for assessing vegetation effects if the exposure-response relationships
were based on some of the NCLAN and forest seedling experiments
performed in the 1980s and 1990s. Lee et al. (2022) reported
in their analysis of tree seedling data that the most sensitive
species in their analysis experienced a biomass loss of 5% at
a W126 of 2.5–9.2 ppm-hrs and that the N100 values ranged
from 0 to 7 at those exposures. The seedling O3 exposure studies
for western and eastern tree species in the Lee et al. (2022)
analysis were conducted from 1988 to 1995 at the U.S. Environmental
Protection Agency research laboratory in Corvallis, Oregon, Michigan
Technological University’s Ford Forestry Center in Alberta,
Michigan and by researchers from Appalachian State University
at Great Smoky Mountains National Park near Gatlinburg, Tennessee.
The requirement for an N100 index has been discussed in Musselman
et al. (2006) and Davis and Orendovici (2006). Additional discussion
of the N100 metric coupled with the W126 index is provided by
clicking here. During the reconsideration of the
O3 standard that took place during the 2022-2023 period, the
EPA suggested that if a secondary standard in the form of the
W126 index were to be considered by the Administrator, it should
also be accompanied by an additional metric that focuses on control
of the high hourly O3 concentrations. In August 2023, the EPA
Administrator made the decision to initiate a new review of the
ozone NAAQS, which meant that the entire ozone rulemaking process
would begin once again and take several years to complete.
References
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