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Exposure-Response Relationship

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. (1988) 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 (1988) 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) recently 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, Corvallis, OR.

Lefohn A.S. (1988) A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops [A response to comments by M.A.J. Parry and W. Day and M.R. Ashmore]. Atmos. Environ. 22:2058-2060.

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., Laurence J.A. and Kohut R.J. (1988a) 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.

Lefohn A.S., Laurence J.A. and Kohut R.J. (1988b) A comparison of indices that describe the relationship between exposure to ozone and reduction in the yield of agricultural crops [A response to comments by V.C. Runeckles]. Atmos. Environ. 22:1242-1243.

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. Atmospheric Environment. 40:1869-1888.

Parry M.A.J. and Day W. (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. 22:2057-2058.

Runeckles V.C. (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. 22:1241-1242.

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.

U.S. Environmental Protection Agency (2006) Review of National Ambient Air Quality Standards for Ozone-Assessment of Scientific and Technical Information. Office of Air Quality Planning and Standards, Research Triangle Park, NC. EPA/600/R-05/004af.

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