If you focus on the grade from the American Lung Association, you will see that the entire Valley received an F, and there is certainly lots of room for improvement in San Joaquin County air pollution.
However, if you look at the underlying data at the Lung Association, you see that San Joaquin County has by far the best air quality in the Valley. Stockton scores better than my old home in Maryland, wheras Fresno and Bakersfield are much worse.
Stockton has some costs imposed on it because it is lumped in with the rest of the Valley for air pollution regulation. It would be beneficial for the northern San Joaquin Valley if the boundaries for air quality regulation were split up.