The EPA Administrator proposed
the use of a 3-month, 12-hour W126 exposure index as a possible
secondary O3 standard. 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.
By selecting a 12-hour
window as originally suggested by the Agency, the EPA may have
contributed to the the problem reported of the inconsistency
of the W126 index. Adding to the accumulation period
issue, the W126 will be inconsistent when predicting ozone vegetation
effects due to the problem of ignoring the high hourly average
concentrations that were contained within the vegetation experiments
used to establish the exposure-response relationships. The requirement
for an additional metric that describes the number of hourly
average concentrations greater than or equal to 100 ppb (i.e.,
N100 index) has been discussed in the peer-reviewed literature.
Recently Davis and Orendovici (2006) reported that the incidence
of vegetation symptoms in the field was most related to the W126
index coupled with the N100 index. The requirement for an N100
index is discussed further by Musselman et al. (2006) and Lefohn
and Foley (1992) and an introduction to the N100 is provided
by clicking here.
References
Davis, D.D.; Orendovici,
T. (2006). Incidence of ozone symptoms on vegetation within a
National Wildlife Refuge in New Jersey, USA. Environmental Pollution.
143:555-564.
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.
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.
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.
