In the free troposphere over Tenerife in the summer, O3 concentrations are anti-correlated with major pollutant aerosols (nss-SO=4 and NO-3) and with 210Pb, a tracer for boundary layer sources. In contrast, O3 is highly correlated with 7Be, a product of cosmic ray interactions in the upper troposphere and stratosphere. This suggests that natural O3 sources (i.e. the stratosphere) might be playing an important role. Nonetheless our results do not preclude the possibility that substantial amounts of pollution-related O3 could be transported in the free troposphere. However, to be consistent with our results, the transport mechanisms would have to incorporate efficient processes for the removal of pollutant aerosol species and 210Pb. ¿ American Geophysical Union 1995