Before one can characterize
hourly average concentrations in a biologically meaningful way,
it is necessary to understand the relationship between exposure
and vegetation effects. The search for an exposure index that
relates well with plant response has been the subject of intensive
discussion in the research community. Both the magnitude of a
pollutant's concentration and the length of exposure are important
considerations when attempting to develop a realistic exposure
index. Evidence exists in the literature to indicate that the
magnitude of vegetation responses to air pollution is more an
effect of the magnitude of the concentration than the length
of the exposure.
Several different
types of exposure indices have been proposed. Both the 6-h and
7-h long-term seasonal mean ozone exposure parameter have been
used to relate vegetation effects with exposure. The 7-h (0900-1559h)
mean, calculated over an experimental period, was adopted as
the statistic of choice by the U.S. EPA's National Crop Loss
Assessment Network (NCLAN) program. Toward the end of the program,
NCLAN redesigned its experimental protocol and applied proportional
additions of ozone to its crops for 12-h periods.
The use of a long-term
average concentration, such as the 7- or 12-h average, for describing
concentration exposures does not provide accurate descriptions
of exposures that actually occur. For example, some high-elevation
sites exhibit ozone exposure characteristics that are distinctly
different from those observed at lower elevation sites. The long-term
averages calculated at some high-elevation sites tend to be higher
than the long-term averages at lower elevation sites. The higher
long-term averages reflect the lack of hourly average concentrations
near the minimum detectable level and may or may not be biologically
significant.
From the middle
1960s through the middle 1980s, studies published in the literature
identified short-term, high concentration (i.e., episodic) ozone
exposures as important components of agricultural crop effects
and trees. The short-term, high concentration exposures were
identified by many researchers as being more important than long-term,
low concentration exposures.
As additional evidence
began to mount that higher concentrations of ozone should be
given more weight than lower concentrations, concerns about the
use of a long-term average to summarize exposures of ozone began
appearing in the literature. Specific concerns were focused on
the fact that the use of a long-term average failed to consider
the impact of peak concentrations. The 7-h seasonal mean contained
all hourly concentrations between 0900-1559h; this long-term
average treated all concentrations within the fixed window in
a similar manner. An infinite number of hourly distributions
within the 0900-1559h window could be used to generate the same
7-h seasonal mean, ranging from those containing many peaks to
those containing none. It was pointed out in the literature that
it was possible for two air sampling sites with the same daytime
arithmetic mean ozone concentration to experience different estimated
crop reductions.
In the late 1980s,
the focus of attention turned from the use of long-term seasonal
means to cumulative indices (i.e., exposure parameters that sum
the products of concentrations multiplied by time over an exposure
period using a threshold concentration). The use of the cumulative
exposure index with a threshold concentration had some limitations.
Depending upon the threshold concentration used, the parameter
ignored the lower hourly mean concentrations. However, the parameters
appeared to relate ozone exposure with observed functional change
at monitoring sites that experienced (1) repeated high concentration
exposures from day-to-day and (2) relatively short periods between
episodes.
Recognizing the
disadvantage of using a threshold concentration with the cumulative
index, a modification was suggested that applied differential
weighting to the hourly mean concentrations of ozone and summing
over time. Lefohn and Runeckles (1987) proposed a sigmoidal weighting
function that was used in developing a cumulative integrated
exposure index. The sigmoidal weighting function was multiplied
by each of the hourly mean concentrations; thus, the lower, less
biologically effective concentrations were included in the integrated
exposure summation.
The form of the
sigmoidally weighted index was tested using NCLAN data. Lefohn
et al. (1988) showed that exposure indices that weight peak concentrations
of ozone differently than lower concentrations of an exposure
regime can be used in the development of exposure-response functions.
Based on evidence
published in the literature, as well as special analytical studies
sponsored by the U.S. EPA (1996), many in the research community
have concluded that the use of cumulative indices to describe
exposures of ozone for predicting agricultural crop effects appears
to be a more rational approach than the use of long-term seasonal
averages.
Exposure-based metrics are traditionally
used to relate O3 to vegetation response. Recently, flux-based
models have been developed to predict the effects of O3 on vegetation.
Because plant response is more closely related to O3 absorbed
into leaf tissue than to exposure, it is often assumed that flux-based
models offer less uncertainty in predicting vegetation effects
than the use of exposure-based metrics. Lefohn and Musselman
(2005) and Musselman et al. (2006) discussed the advantages
and limitations associated with the use of flux-based models
for predicting vegetation effects. An important aspect associated
with adequately predicting the effects of O3 on vegetation is
identification and quantification of the detoxification processes.
The detoxification processes, including their temporal variability
and relevance, are important and cannot be ignored when predicting
vegetation effects (Musselman et al., 2006; Heath et al., 2009).
While future research
should focus on the use of flux-based indices, it is important
to continue to identify the family of cumulative indices that
best describe the relationship between ozone exposure and vegetation
effects; one needs to be aware that exposure indices will continue
to produce inconsistent results when trying to predict growth
losses. Most exposure indices are insensitive to diurnal periods
of maximum sensitivity of the plant. The sensitivity of vegetation
as a function of the time of day has not been well defined. In
addition, as described in the literature, the distribution patterns
of the hourly average concentrations for some high-elevation
and low-elevation sites are different. Most cumulative-type and
other exposure indices cannot adequately describe some of the
subtle differences in the two different types of exposure regimes.
Besides sensitivity, the majority of exposure indices used today
do not address (1) the amount and chemical form of the pollutant
that enters the target organism (i.e., stomata considerations),
(2) the length of the exposure within each episodic event, or
(3) the time between exposures (i.e., the respite or recovery
time). It is unclear how important sensitivity and the amount
and chemical form of the pollutant that enters the target organism
are in an overall weighting scheme when predicting vegetation
effects. If both the sensitivity of the target organism and the
actual dose that enters the organism are as important as ambient
air pollutant exposure, then a given pollutant exposure will
elicit varying biological responses at different times for the
same crop. Recognizing the limitations of applying exposure indices
as dose surrogates, at this time, the cumulative exposure index
may still be the best family of indices available for relating
exposure and biological response.
Today, many vegetation scientists have
begun to apply a cumulative exposure index that weights the higher
hourly average concentrations more than the mid- and lower-level
values. The U.S. Forest Service and Park Service are using this
index to assess the potential impact of ozone on vegetation.
References
Heath R. L., Lefohn A. S., and 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. and Runeckles V.C. (1987) Establishing
a standard to protect vegetation - ozone exposure/dose considerations.
Atmos. Environ. 21:561-568.
Lefohn, A.S. and Musselman, R.C. (2005)
The Strengths and Weaknesses of Exposure- and Flux-Based Ozone
Indices for Predicting Vegetation Effects. Presented at the Critical
levels of ozone: further applying and developing the flux-based
concept. Obergurgl, Tyrol, Austria. November 15-19, 2005.
Lefohn A.S., Laurence J.A. and Kohut R.J.
(1988) A comparison of indices that describe the relationship
between exposure to ozone and reduction in the yield of agricultural
crops. Atmos. Environ. 22:1229-1240.
Musselman, R.C., Lefohn, A.S., Massman,
W.J., and Heath, R.L. (2006) A critical review and analysis of
the use of exposure- and flux-based ozone indices for predicting
vegetation effects. Atmos. Environ. (Accepted).
U.S. Environmental Protection Agency (1996)
Air quality criteria for ozone and related photochemical oxidants.
Environmental Protection Agency, Office of Air Quality Planning
and Standards, Research Triangle Park, NC. U.S. EPA report no.
EPA/600/P-93/004bF.
