Based on material published in the literature and summarized by A.S.L. & Associates, it was recommended that the analysis should be limited to vegetation studies in which the SUM06 or W126 indices had been used to describe ozone exposure. In addition, because of the importance of high hourly average concentrations, A.S.L. & Associates identified the number of hourly average concentrations greater than or equal to 0.10 ppm that were associated with the SUM06 and W126 exposures for each of the experiments. Thus, the three exposure indices used to describe ozone exposures were the

SUM06 - The sum06 index uses an artificial threshold of 0.06 ppm. To calculate the index, all hourly average concentrations over the exposure period equal to and greater than 0.06 ppm are summed;

W126 - Alternatively, the W126 exposure index does not utilize a threshold value, but weights differentially all hourly average concentrations. Significant weighting greater than 0 occurs at all hourly average concentrations above 0.04 ppm. The use of a 0 weighting for hourly average concentrations less than 0.04 ppm was intentionally designed into the W126 so as to reflect near background levels. The W126 has weighting of approximately 1 for all hourly average concentrations equal to and above 0.10 ppm. The weighting of 1 at these concentrations was based on informal discussions with vegetation researchers in California whose vegetation experienced repeated occurrences of hourly values equal to and above 0.10 ppm. The weighting scheme had an inflection point at 0.065 ppm. At the time of the development of the W126, it was recognized that additional research would be needed to verify the weighting scheme. In 1997, Finnan et al. (1997) compared the performance of different ozone indices in exposure-response functions for spring wheat and reported that the best performing index employed a sigmoid function with an inflection point at 0.062 ppm, which was near the W126 weighting.

N100 - The number of hourly average concentrations equal to and greater than 0.10 ppm. The N100 is required to make the W126 exposure index consistent in predicting vegetation injury and damage. Please click here for additional information about the use of the W126 and the N100 exposure indices.

Reference

Finnan, J.M., Burke, J.I., Jones, M.B. (1997). An evaluation of indices that describe the impact of ozone on the yield of spring wheat (Triticum aestivum L.). Atmospheric Environment. 31:2685-2693.