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Statistical analysis was performed using selected sets of combined data from the US National Crop Loss Assessment Network and the European Open-Top Chambers Programme to examine the relationships between the occurrences of hourly ambient ozone (O3) concentrations and adverse crop yield responses. The results suggest that the frequency of occurrences of relatively low hourly O3 concentrations (~ < 35 ppb) are not as important as moderate to higher concentrations in eliciting negative crop biomass responses. They also suggestthat daily peak (highest) hourly O3 values (~ > 90 ppb) may not be as critical, most likely because they frequently do not occur during time periods when conditions that promote atmospheric conductivity (O3 deposition) and plant uptake (O3 absorption) are in coherence. [ABSTRACT FROM AUTHOR]