As the article points out, its a geography problem. Basins and valleys accumulate pollutants because the wind doesn't disperse them..
Sometimes, pollutants are transported from one place to another. With fine detection equipment the ozone from China can be detected in California, although it is very small. Or the ozone from San Francisco significantly affects the central valley. Or the mercury emissions in Georgia are definitely affecting NC.
We have "cross state" pollution regs to prevent one state from polluting another