Data collected from the late 1980s to 1997 provided a measure of how smog is modified after leaving the pollution zone in industrial USA and Canada. CO, a long-lived marker for anthropogenic pollution, is measured with O3 to help establish source of the O3. O3:CO ratios observed at Sable Island showed strong positive correlations in summer indicating increased levels of anthropogenic pollution in summer. Figure: D. Parrish.