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(Provided for historical perspective but highly relevant to the December 2020 decision by the EPA to retain the 0.070 ppm secondary ozone standard for vegetation)

The EPA's Staff Paper on ozone was released during the summer of 1996. In the report, EPA reaffirmed for vegetation effects the importance of high hourly average concentrations (i.e., greater than or equal to 0.10 ppm) and the presence of these peaks in the treatments that were used to estimate the 10% yield reduction levels that were used to set a possible secondary standard. The EPA documented the following:


  • ...the modified ambient treatments contained numerous high peaks (O3 equal to or above 0.10 ppm), occurring more frequently than in typical ambient air quality distributions (pg. 194).


  • Lefohn and Foley (1992) note that the high number of hourly O3 concentrations above 0.10 ppm in the NCLAN protocol may prevent generalization of these findings to other types of exposure regimes (pg. 220).


  • Data in published literature still supports the conclusion that cumulative seasonal exposure and higher concentrations are important features of exposure for both crops and trees (pg. 223).


  • Staff continues to believe that the selection of an appropriate averaging time should take into account the cumulative impact from repeated peaks over an entire growing season (pg. 278).


  • Lefohn and Foley (1992) suggest it is important that any exposure index that sets a level of protection based on the response of plants in the NCLAN experiments should recognize the peakiness of the exposure regimes used when attempting to predict biological responses over the range of ambient O3 exposure regimes (pg. 226).

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