AMBIENT OZONE AND CROP LOSS - ESTABLISHING A CAUSE-EFFECT RELATIONSHIP

This paper provides the results of a retrospective mathematical analys is of the US NCLAN (National Crop Loss Assessment Network) open-top ch amber data. Some 77% of the 73 crop harvests examined, showed no stati stically significant yield differences between NF (non-filtered open-t op chamber) and AA (chamberless, ambient air) treatments (no easily di scernable chamber effects on yield). However, among these cases only s even acceptable examples showed statistically significant yield reduct ions in NF compared to the CF (charcoal filtered open-top chamber) tre atment. 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, media n and the various percentiles were all higher (greater than or equal t o 3 X ) in the former in contrast to the latter scenario. The combined frequency distribution of hourly O-3 concentrations for the cases wit h yield loss in NF were clearly separated from the corresponding distr ibution with no yield loss, at O-3 concentrations > 49 ppb. Univariate linear regressions between various O-3 exposure parameters and per ce nt yield losses in NF showed that the cumulative frequency of occurren ce 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 r epresented a critical point, since the addition of the frequency distr ibutions of > 87 ppb O-3 did not improve the R(2) values. In fact as t he frequency of hourly O-3 concentrations included in the regression a pproached 50-100 ppb, the R(2) value decreased substantially and the p value increased inversely. Further, univariate linear regressions bet ween the frequencies of occurrence of various O-3 concentrations betwe en 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 (positiv e effect) provided no meaningful statistical relationship (adjusted R( 2) = 0.000) in either category. These results support the basis that a dditional evaluation of the frequency of occurrence of hourly O-3 conc entrations between 50 and 87 ppb for cases with the yield reductions c ould provide a meaningful ambient O-3 standard, objective or guideline for vegetation.