Data obtained from
the National Crop Loss Assessment Network (NCLAN) offer the opportunity
to develop meaningful ozone exposure-response relationships for
agricultural crops. Retrospective studies have been performed
using NCLAN data (Lefohn et al., 1988a; Lee et al., 1988; Lee
et al., 1989). Lefohn et al. (1988a), using wheat and soybean
data sets, compared the use of several exposure indices in describing
the relationship between ozone and reduction in agricultural
crop yield. NCLAN investigators normally fumigated for only a
7-h period each day. Recognizing that the experimental crops
were exposed to ozone during the period that artificial fumigations
were not applied, Lefohn et al. (1988a) estimated the 17-h period
exposure outside the daily 7-h fumigation window by using air
quality data recorded at the ambient plots. Thus, all cumulative
exposure indices used in their analysis were determined for a
daily 24-h period.
Two of the indices
used by Lefohn et al. (1988a) were determined using a sigmoidally-weighted
function as proposed by Lefohn and Runeckles (1987). One of the
two sigmoidally-weighted functions used by Lefohn et al. (1988a)
in their analysis was designed with an inflection point of approximately
0.06 ppm. Unlike the seasonal average index, the cumulative indices
performed well when data were combined over a two-year period.
Lefohn et al. (1988a) reported that while none of the exposure
indices consistently provided a best fit with the models tested,
their analysis indicated 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.
During the course
of their work, the authors demonstrated the advantages of using
cumulative indices when comparing vegetation results obtained
over different "time-of-exposure" periods. The results
reported by Lefohn et al. (1988a) elicited comments from Runeckles
(1988), Parry and Day (1988), and Ashmore (1988). In response,
Lefohn et al. (1988b) and Lefohn (1988a) stressed that the use
of long-term seasonal or experimental means has serious flaws
because the specific exposure duration component for each experiment
is decoupled from the parameter. The use of long-term exposure-period
means averages out the "time-of-exposure" element and
thus, makes it difficult to compare results obtained from different
experiments operated under different air pollutant exposure durations.
In their analysis, the authors illustrated the weakness of using
a 7-h exposure-period mean when winter wheat data were combined
for 1982 and 1983. Lefohn et al. (1988a) quantitatively showed
that the cumulative index performed better than the 7-h average
when the data were combined for the 2 years. The winter wheat
experiment performed in 1982 was operated over a 71-day period
while the winter wheat experiment performed in 1983 lasted only
36 days. Although the cumulative indices used in the experiment
appeared to take into consideration the differences in the length
of ozone exposure over the 2-year period, the long-term mean
was unable to differentiate between the 71- and 36-day "time-of-exposure"
periods. Lefohn (1988a) concluded that the analysis showed that
a long-term average concentration cannot adequately describe
the total exposure that a plant receives. Further discussion
concerning the development of exposure-response relationships
for vegetation can be found in the U.S. EPA (1996) in Section
5.5. Drs. A.S. Lefohn and E.H. Lee were the co-authors of this
very important section. An updated version of the EPA (1996)
discussion can be found in Chapter 9 of the U.S. EPA (2006).
Drs. A.S. Lefohn, R. C. Musselman, and W. J. Massman were the
authors of the exposure- and dose-response section of Chapter
9. Musselman et al. (2006) published a comprehensive critical
review on the subject of the effects of ozone exposure- and dose-response
on vegetation. The article is an important addition to the field
of the effects of surface ozone on vegetation.
References
Ashmore M.R. (1988) A comparison of indices
that describe the relationship between exposure to ozone and
reduction in the yield of agricultural crops [Comments on article
by Lefohn et al. (1988a)]. Atmos. Environ. 22:2060-2061.
Lee E.H., Tingey D.T. and Hogsett W.E.
(1988) Evaluation of ozone exposure indices in exposure-response
modeling. Environ. Pollut. 53:43-62.
Lee E.H., Tingey D.T. and Hogsett W.E.
(1989) Interrelation of Experimental Exposure and Ambient Air
Quality Data for Comparison of Ozone Exposure Indices and Estimating
Agricultural Losses. Contract No. 68-C8-0006, U.S. Environmental
Protection Agency, Corvallis Environmental Research Laboratory,
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Lefohn A.S., Laurence J.A. and Kohut R.J.
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Lefohn A.S., Laurence J.A. and Kohut R.J.
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Musselman R.C., Lefohn
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Parry M.A.J. and Day W. (1988) A comparison
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