This paper provides the results of a retrospective mathematical analysis of the US NCLAN (National Crop Loss Assessment Network) open-top chamber data. Some 77% of the 73 crop harvests examined, showed no statistically significant yield differences between NF (non-filtered open-top chamber) and AA (chamberless, ambient air) treatments (no easily discernable chamber effects on yield). However, among these cases only seven acceptable examples showed statistically significant yield reductions in NF compared to the CF (charcoal filtered open-top chamber) treatment. An examination of the combined or cumulative hourly ambient O-3 frequency distribution for cases with yield loss in NF compared to a similar match of cases without yield loss showed that the mean, median and the various percentiles were all higher (greater than or equal to 3 X ) in the former in contrast to the latter scenario. The combined frequency distribution of hourly O-3 concentrations for the cases with yield loss in NF were clearly separated from the corresponding distribution with no yield loss, at O-3 concentrations > 49 ppb. Univariate linear regressions between various O-3 exposure parameters and per cent yield losses in NF showed that the cumulative frequency of occurrence of O-3 concentrations between 50 and 87 ppb was the best predictor (adjusted R(2) = 0.712 and p = 0.011). This analysis also showed that the frequency distribution of hourly concentrations up to 87 ppb O-3 represented a critical point, since the addition of the frequency distributions of > 87 ppb O-3 did not improve the R(2) values. In fact as the frequency of hourly O-3 concentrations included in the regression approached 50-100 ppb, the R(2) value decreased substantially and the p value increased inversely. Further, univariate linear regressions between the frequencies of occurrence of various O-3 concentrations between 50 and 90 ppb and: (a) cases with no yield difference in NF and (b) cases with yield increase in NF compared to the CF treatment (positive effect) provided no meaningful statistical relationship (adjusted R(2) = 0.000) in either category. These results support the basis that additional evaluation of the frequency of occurrence of hourly O-3 concentrations between 50 and 87 ppb for cases with the yield reductions could provide a meaningful ambient O-3 standard, objective or guideline for vegetation.
