In 2007, the EPA Administrator
proposed the use of a 3-month, 12-hour W126 exposure index as
a possible secondary O3 standard. 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 web site 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, 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.
It is important to note
that in the United States, the day length during the summer months
at all locations is greater than 12 hours. In some locations,
the day length is greater than 16 hours. The figure below illustrates
the day length 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 day length
is less than 12 hours is after the third week in September. The
day length 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 day length 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 acknowledges 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.
Adding 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 controlled 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 seedline 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 included 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. The requirement for an N100
index has been discussed further by 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.
Davis, D. D.; Orendovici,
T. (2006). Incidence of ozone symptoms on vegetation within a
National Wildlife Refuge in New Jersey, USA. Environmental Pollution.
Grulke, N. E.; Alonso,
R.; Nguyen, T.; Cascio, C.; Dobrowolski, W. (2004) Stomata open
at night in pole-sized and mature ponderosa pine: implications
for O3 exposure metrics. Tree Physiology 24, 1001-1010.
Heath R. L.; Lefohn A.
S.; Musselman R. C. (2009) Temporal processes that contribute
to nonlinearity in vegetation responses to ozone exposure and
dose. Atmospheric Environment. 43:2919-2928.
Lefohn, A. S.; Foley, J.
K. (1992) NCLAN results and their application to the standard-setting
process: protecting vegetation from surface ozone exposures.
J. Air Waste Manage. Assoc. 42: 1046-1052.
Lefohn, A.S.; Jackson,
W.; Shadwick, D.S.; Knudsen, H.P. (1997) Effect of surface ozone
exposures on vegetation grown in the southern Appalachian Mountains:
Identification of possible areas of concern. Atmospheric Environment
Massman, W. J. (2004) Toward
an ozone standard to protect vegetation based on effective dose:
a review of deposition resistance and a possible metric. Atmospheric
Environment. 38: 2323-2337.
Musselman, R. C.; Minnick,
T. J. (2000) Nocturnal stomatal conductance and ambient air quality
standards for ozone. Atmos. Environ. 34: 719-733.
Musselman R. C.; Lefohn
A. S.; Massman W. J.; Heath, R. L. (2006) A critical review and
analysis of the use of exposure- and flux-based ozone indices
for predicting vegetation effects. Atmospheric Environment. 40:1869-1888.
U.S. Environmental Protection
Agency (2007) Review of the National Ambient Air Quality Standards
for Ozone: Policy Assessment of Scientific and Technical Information
OAQPS Staff Paper. Research Triangle Park, NC: Office of Air
Quality and Planning and Standards, EPA-452/R-07-003. January.