Findings (Continued)
In some, but not all, of the cases that were studied, a plausible explanation for the higher O3 values was the presence of upper tropospheric and stratospheric air that was transported down to the surface.
It might be expected that a significant contribution from natural sources is also important in producing O3 levels in the 0.05 – 0.06 ppm or higher range at more southerly locations where stratospheric intrusions also have an important impact on the tropospheric O3 distribution.