I agree with your thought that these are part time, but for a different reason. I think they'll only be turned on when the temperature/env conditions are right

I agree with your thought that these are part time, but for a different reason. I think they'll only be turned on when the temperature/env conditions are right to form the inversion. Fire the jet up for a day or two to prevent the smog from getting trapped under the inversion layer, then turn the jet off once the conditions subside. None of this 20mm a year business will end up happening.