NCLAN results and their application to the standard-setting process: protecting vegetation from surface ozone...

The current form of the standard is not appropriate for protecting vegetation from O3 exposures. As an alternative to the current form of the standard, it has been suggested in the literature that a maximum cumulative 3-month SUM06 O3 exposure index be used as the form of a secondary standard to protect agricultural crops. However, applying this index may result in inconsistent protection for vegetation. It appears that cumulative indices will have to be combined with other parameters to accurately quantify the occurrence of high hourly average concentrations. This paper describes the characterization of the hourly O3 exposures in selected National Crop Loss Assessment Network (NCLAN) experiments and discusses the application of the results to the standard-setting process. Our results indicated that, in most cases, the NCLAN experimental data we analyzed appeared to support the observation that the repeated occurrences of hourly average O3 concentrations of 0.10 ppm and higher result in adverse effects on vegetation. For the NCLAN experiments, the characterized distributions reflected the ability of the high hourly average concentrations to affect crop yield reduction. Prior to suggesting a new form of the secondary standard, it will be important to carefully characterize the specific regimes responsible for affecting vegetation and identify the important components of those regimes responsible for the effects. By applying this approach, it should be possible to limit the occurrence of inconsistent results when applying a new form of the secondary standard.