Lefohn and Benedict (1982) proposed
that the higher hourly average concentrations
should be given greater weight than the mid- and low-level values
when assessing crop growth reduction. The experimental results
reported by Musselman et al. (1983) and Hogsett et al. (1985)
illustrated the importance of these higher hourly average ozone
concentrations in affecting vegetation. Confirming the results
reported by Musselman et al. (1983) and Hogsett et al. (1985),
using the identical SUM06 exposure index values, Yun and Laurence
(1999) reported that the exposure regime that contained the higher
hourly average concentrations resulted in greater vegetation
effects than the regime that experienced the lower hourly average
concentrations.
An important question was raised
in the 1990s regarding whether the high hourly average concentrations
should have greater weighting than lower values for assessing
the potential effects of surface ozone on vegetation. Tonneijck
and Bugter (1991), Krupa et al. (1993, 1994, 1995), Gruenhage
and Jaeger (1994), Tonneijck (1994), and Legge et al. (1995)
published papers questioning the current understanding of the
peer-review literature that higher hourly average ozone concentrations
are more important than the mid- and lower-level values in eliciting
a plant response. Krupa et al. (1995) suggest that only hourly
average concentrations in the range of 50 to 87 ppb are important
for assessing vegetation effects and conclude that concentrations
> 90 ppb "appeared to be of little importance."
These authors have modified their previous statement by explaining
"While this overall conclusion does not negate the importance
of peak hourly O3 concentrations if they occur at the right time
of the day, it is important to note that as ambient hourly O3
concentrations reach peak concentrations, their frequency of
occurrences decline and so do the properties of atmospheric (O3
flux) and plant (uptake) conductance" [Legge et al., 1995].
However, while Legge et al. (1995) agree that higher concentrations
might be important given the right conditions, they conclude
that concentrations near or below background levels (i.e.,
35-60 ppb) are the best predictors of plant response for European
crops.
Several independent analyses appear
to contradict the conclusion reached by Krupa et al. (1994) and
Legge et al. (1995) that mid-level concentrations (values between
0.050 ppm and 0.087 ppm [Krupa et al., 1994, 1995] or between
0.035 ppm and 0.060 ppm for European crops [Legge et al., 1995])
are more important than the higher hourly average concentrations
(e.g., values greater than 0.060 for European crops and 0.087
ppm for U.S. crops). Using the EPA's National Crop Loss Assessment
Network (NCLAN) data, Musselman et al. (1994) reported that hourly
average O3 concentrations above 0.087 ppm appear to be more important
contributors to crop losses than hourly average concentrations
below 0.087 ppm. Similar to these findings, Lefohn et al. (1994),
using the results of an Auburn University intensive field research
study assessing the effects of O3 on two loblolly pine half-sibling
families (Lefohn et al., 1992), pointed out that the hourly average
concentrations above 0.087 ppm appeared to play a more important
role in determining growth reductions than mid-level values.
Similarly, Lefohn et al. (1994) commented on the inconsistency
in applying the Krupa et al. hypothesis when actual ambient data
were applied.
With conflicting results, who is
correct? In order to answer this question, one has to carefully
critique the series of papers that challenge the current thinking
that higher hourly average concentrations are more important
than the mid and lower values. The works by Tonneijck and Bugter
(1991) and Tonneijck (1994) were not designed to test the importance
of peak versus mid or low levels of ozone. Tonneijck and Bugter
(1991) concluded that O3 injury on tobacco Bel-W3 neither appeared
to be an adequate indication of the concentration of ambient
O3 nor an adequate indicator for determining the risk of O3 injury
to other plant species or to vegetation as a whole. After reporting
a poor relationship between ambient ozone and tobacco response,
Tonneijck (1994) used data obtained from the Dutch monitoring
network in the period 1979-1983 to explain injury response in
tobacco Bel-W3 and bean. The author concluded that the results
did not support the overall concept that higher concentrations
of O3 were more important than lower values in eliciting a response
because the higher concentrations did not necessarily cause the
greater effects. The work reported by Tonneijck (1994) is difficult
to substantiate because (1) latent variables, particularly climate,
were important and were excluded in their analysis of unplanned
data and (2) the power of their statistical approach was weak.
Krupa et al. (1993) concluded that
the best predictors of foliar injury on tobacco Bel-W3 were exposure
indices that focused on the mid levels of ozone. The conclusions
reached by Krupa et al. (1993) are difficult to substantiate
primarily because the best regression model relating weekly foliar
injury scores to various exposure indices is not interpretable,
i.e., the coefficient of the SUM60 or N60 (i.e., sum or count
of hourly concentrations > 0.06 ppm) is negative and not directionally
correct, a clear indication of problems of near linear dependency
among indices such that little or no distinction of the relative
influence of the exposure indices on injury response can be made.
The apparent problem of near collinearity among the regressor
variables used in stepwise regression provide problems in validating
the authors' conclusions.
In view of the discussion that had
emerged in the scientific literature regarding the importance
of high concentrations versus mid-level and lower values, the
Canadian Vegetation Objective Working Group (VOWG) thoroughly
evaluated the work described by Krupa et al. (1994, 1995) and
Legge et al. (1995). The Canadian working group (1997) concluded
that there was little support for using an exposure index that
focused on the mid-level versus the higher concentrations. The
findings were based on the following:
- Cumulative exposure indices that focused on the higher
hourly average concentration performed considerably better in
exposure-response models than the index proposed by Krupa et
al. (1995), which focused on the mid-level values.
- Inaccurate use by Krupa et al. (1994, 1995) of some of
the NCLAN data were identified.
- The exposure index that focused on the mid-level concentrations
predicted greater losses to vegetation at remote ambient O3 monitoring
sites in Canada (where losses were not observed) than those that
occurred at sites which experienced much higher O3 exposures
and where documented O3 effects on vegetation have occurred.
The exposure index predicted much greater vegetation losses at
remote northern areas of Ontario (i.e., Experimental Lakes Area),
where crop effects have not been documented. Similarly, high
losses were predicted for remote areas in Cormack, Newfoundland,
and Vegreville, Alberta.
In continuing its review, the Working
Group noted that Legge et al. (1995) have pointed out that although
mid-range concentrations are important, if high concentrations
were to occur during the time of day when plants were most sensitive,
then the higher concentrations would also be important. However,
based on its observation that the exposure index that focused
on the mid-level concentrations predicted greater losses at remote
ambient O3 monitoring sites than those sites which experienced
much higher O3 exposures where effects had been observed, the
Working Group concluded that the results reported by Legge et
al. (1995) were difficult to rationalize. The Working Group (1997)
concluded that there was sufficient evidence that cumulative
exposure indices that weight the higher hourly average concentrations
more than the mid levels should be used for developing exposure-response
relationships.
EPA (1996a) has evaluated the results
reported by Tonneijck and Bugter (1991), Krupa et al. (1993,
1994, 1995), and Tonneijck (1994). In the Ozone Criteria Document,
which summarizes the effects of ozone on humans and vegetation,
the EPA (1996a) reviewed the statistical procedures reported
by Krupa et al. (1994 and 1995) and concluded that the peak-weighted
cumulative exposure indices were appropriate for developing exposure-response
relationships to predict O3 vegetation effects (EPA 1996a, 1996b,
1997).
Based on atmospheric measurement of deposition
and diurnal patterns of O3 and gas exchange at a natural grassland
ecosystem (see Gruenhage et al., 1994), Gruenhage and Jaeger
(1994) proposed an ambient O3 exposure potential for characterizing
O3 uptake. The micrometeorological study by the authors is not
an effects study and no plant response data were reported. Their
conclusions were based on a micrometeorological study of O3 flux
observations above a natural grassland in Germany. A mathematical
model describing O3 flux to a meadow was developed and potential
injury to the grassland ecosystem was estimated based on their
observations. Effects data were not used by Gruenhage and Jaeger
(1994) in their model. Gruenhage and Jaeger (1994), using their
atmospheric exposure potential approach, concluded that mid-level
hourly average concentrations (0.05-0.09 ppm) were more important
than the higher concentrations (> 0.09 ppm) for the grassland
vegetation grown, in 1990 and 1991, at their site. Gruenhage
and Jaeger have since combined their work with others in the
application of their observations (Krupa et al., 1994, 1995)
and Legge et al. (1995).
It is inappropriate to use the meteorological
observations made at the German grassland site for the purpose
of deriving a model that predicts the potential effect on vegetation
for every hourly average ozone concentration anywhere in the
world at any time. Not every plant species behaves in the same
manner as the German grassland ecosystem did in 1991 and 1992.
In addition, every model has sets of key assumptions that may
or not be correct on a universal basis. Similarly, not every
agricultural rural site in the world has the same exposure characteristics
as those found for the site in Germany. As noted by Musselman
et al. (1994), hourly average concentrations greater than or
equal to 0.10 ppm occur frequently at many rural agricultural
and forested sites in the United States, in contrast with that
observed at the grassland research site in Germany. Second, unlike
the O3 exposure dynamics reported by Gruenhage and Jaeger (1994),
Musselman et al. (1994) found that for most rural agricultural
and forested sites in the United States the higher hourly average
concentrations occurred during the 0900-1559h daylight window.
The authors pointed out that more than 50% of the hourly average
concentrations greater than or equal to 0.10 ppm occurred during
this potentially biologically important period.
Another observation implicit in
Gruenhage and Jaeger (1994) was that the higher hourly average
concentrations did not contribute as much as the mid- level ones
when determining the cumulative atmospheric potential. This is
not surprising. One would expect that the contribution of higher
hourly average concentrations to any cumulative-type index (i.e.,
cumulative atmospheric potential, SUM06, or W126) would be minimal
in relation to the mid-level values when the number of higher
hourly average concentrations is small. However, this observation
should not be generalized because the relative role of hourly
average concentrations in the final summation of the atmospheric
exposure potential is dependent upon the distribution of the
hourly average concentrations, which will vary from site to site
and from year to year. Hourly average concentrations greater
than or equal to 0.10 ppm will have a greater contribution to
the cumulative sum of the atmospheric exposure potential values
at many rural sites in the United States than at the rural grassland
ecosystem site in Germany. It is inappropriate to use the relative
roles of the high- and mid-level hourly average concentrations
in a cumulative sum of either (1) an atmospheric exposure potential
or (2) exposure indices, such as the W126 or SUM06, to examine
the concept that the highest hourly average concentrations have
a greater potential to elicit an adverse effect on vegetation
than the lower values. The concept of higher concentrations being
more important than the mid and lower levels is biologically
based and is invariant to the mathematical distribution of the
hourly average concentrations or the time of day of occurrence
at a specific site.
The papers by Krupa et al. (1993,
1994, 1995), Tonneijck and Bugter (1991), and Tonneijck (1994),
Gruenhage and Jaeger (1994), and Legge et al. (1995) have raised
some interesting questions about relating ozone exposure and
dose for the purpose of predicting effects on vegetation. In
my own papers, I have stated that exposure is not a perfect surrogate
for dose and have cited examples where field studies have confirmed
inconsistent relationships. There always will be examples when
the exposure is high, but the uptake is low, and the vegetation
effects are minimal. However, while a set of physical observations
obtained at a grassland ecosystem for two years can be used in
a model for the purposes of developing a concept, one should
not assume that the collected meteorological and carbon dioxide
uptake data are representative of all physical sites and plant
species. For example, Gruenhage et al. (1994)
found that the maximum deposition velocity for ozone occurring
at their site in Germany was around 1100h in the morning. The
maximum hourly average concentration of ozone occurred near 1600h.
Thus, as observed by Gruenhage et al. (1994), the maximum hourly
average concentrations occurred out of phase with the maximum
uptake of ozone and thus, they concluded that the higher highly
average concentrations are not that important for affecting vegetation.
However, for
maize and beech,
alfalfa,
and
ponderosa pine
grown elsewhere, periods of uptake during the day are quite
different than the observations of Gruenhage et al. (1994). Note
that unlike the findings reported by Gruenhage and Jaeger (1994),
some plants appear to be sensitive to ozone exposure during the
part of the day when high hourly average concentrations occur
at many sites in the United States (e.g., 0900-1600h). In addition,
work by Musselman and Minnick (2000) report that stomates can
be open at night and therefore, the potential exists for noctural
ozone injury and damage to plants, which has been reported by
Winner et al. (1989), Matyssek et al. (1995), and Lee and Hogsett
(1999).
To perform the experiment correctly,
the following data are needed from the same site: (1) micrometeorological
information, (2) uptake rates unique to vegetation species, (3)
quantified defense estimates as a function of time of
day, and (4) documented vegetation effects. By performing a series
of experiments across many geographical areas under different
conditions, one may be able to develop a generalized approach
that incorporates hourly average concentrations with atmospheric
conditions and biological uptake for predicting vegetation effects.
At this time, when one takes a closer look at the set of papers
that hypothesize that midrange hourly average ozone concentrations
(e.g., 50 to 90 ppb) are more important than the high range for
vegetation effects purposes, the evidence simply is not there.
Lee and Hogsett (1999), after reviewing the analyses by Krupa
and co-workers, concluded that "evidence suggesting that
plants show greater response to mid-range hourly average O3 concentrations
(0.05-0.09 ppm) than to high concentrations (>0.09 ppm) cannot
be justified because of flaws in the statistical approaches or
limitations in design and inference due to data sets with few
exceedances of 0.10 ppm."
The data are not available to extrapolate
the grassland findings in Germany to other sites as the authors
have done in the papers with Krupa et al. (1994, 1995) and Legge
et al. (1995). It is desirable to develop a linkage between O3
exposure and actual O3 uptake by vegetation so that a dependable
standard or critical level can be established to protect vegetation.
However, given the variation in a plant's daily and developmental
sensitivity, it appears, at this time, that models that link
O3 exposure with vegetation uptake, quantifiable defense mechanisms,
and effects are not yet dependable enough to be useful for the
standard-setting process. The current flux models that are linked
with effects, ignore defense mechanisms and assume that plants
have no immune systems. This assumption is incorrect.
Based on the research results published
in the peer-review literature, the U.S. EPA (2006) currently
concludes that higher hourly average concentrations have greater
potential than the mid-level concentrations to elicit an adverse
vegetation effect. This does not mean that mid- and lower hourly
average concentrations are not important, but rather that a disproportionate
weighting should be given to the higher values. Gruenhage et
al. (1999) note that "at the present time, for plants other
than wheat, the data base is too small to derive meaningful and
reliable effective dose-response relationships."
Over the next several years, research
will continue to attempt to develop dose uptake models that predict
vegetation effects. However, just as important, will be the attempts
to link the more numerous measurements of ozone exposure in the
ambient air with dose uptake, quantifiable defense mechanisms,
and plant effects. Research is progressing nicely in this evolving
field. Papers by Musselman and Massman (1999), Massman et al.
(2000), and Musselman et al. (2006) summarize current efforts
to develop a dose-response model that would allow for the establishment
of a standard to protect vegetation from ozone. The work by Massman
et al. (2000) is particularly intriguing because it develops
a model that relates exposure and dose and stresses the importance
of defense mechanisms that vary as a function of time of day.
It is the change in the defense component as a function of time
of day that may explain the biologically based observation that
the higher hourly average concentrations should be weighted greater
than the mid- and lower- values in predicting vegetation damage
from ozone. Massman et al. (2000) and Massman (2004) stress that
the product of the overlapping mathematical relationships of
conductance, concentration, and defense mechanisms results in
a much different picture of potential impact to vegetation than
just the use of conductance and concentration in predicting vegetation
effects.
As indicated above, Gruenhage et al.
(1994) found that the the maximum hourly average concentrations
occurred out of phase with the maximum uptake of ozone. However,
as pointed out by Massman et al. (2000), it is important to quantify
the relationship among concentration exposure, ozone uptake,
and the ability of defense mechanisms to neutralize some
of the ozone update as a function of time of day. Thus, although
the maximum hourly average concentrations occurred out of phase
with the maximum uptake of ozone as reported by Gruenhage et
al. (1994), the defense mechanisms or repair mechanisms, varying
as a function of time of day, may actually define when vegetation
is most sensitive to ozone. As indicated above, biological evidence
developed under both experimental conditions and ambient conditions
indicate that, in general, the higher hourly average concentrations
are potentially more important than the mid- and low-level hourly
average concentrations in eliciting an adverse effect on vegetation.
As indicated above, actual results in the field collected under
ambient conditions support these findings.
We are continuing our collaboration with researchers from
around the world in developing flux-based ozone models, which
include the quantification of defense mechanisms, to describe
the relationship between exposure, dose, vegetation effects,
and the standard-setting process. However, until research results
are readily available that explore the "neutralizing capacity"
that defense mechanisms may have on ozone update, any attempt
to ozone establish standards or critical levels based on flux,
without including defense mechanisms in flux-based models, will
result in serious inconsistent predictions that are therefore
not reliable.
As indicated in Musselman et al. (2006), at this time,
the exposure-based indices appear to be to be the only practicable
measure for use in relating ambient air quality to vegetation
response. At its August 24-25, 2006 meeting in Durham, North
Carolina, EPA's Clean Air Scientific Advisory Committee (CASAC)
recommended that the W126 exposure index,
as described by Lefohn and Runeckles (1987), become the secondary
ozone standard to protect vegetation. CASAC proposed that the
W126 exposure index would be integrated
over a 3-month growing season period measured daily from 0800
to 1959 hr. In June 2007, the EPA Administrator proposed the
W126 exposure index as the secondary ozone standard. On March
12, 2008, the EPA Administrator made the final decision on the
human health and vegetation ozone standards. EPA revised the
8-hour "primary" ozone standard, designed to protect
public health, to a level of 0.075 parts per million (ppm). The
previous standard, set in 1997, was 0.08 ppm. 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; Federal Register, 2008), the EPA Administrator made
the secondary ozone standard the same as the primary 8-hour average
standard (0.075 ppm).
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