Ozone was was regulated based on measurements inside the LA Basin based on the NOx and smog found there and in the NYC area.. Once first regulated, they expanded the regs nationally,, then continued everywhere ationally at ever increasing specs.. (Lower allowed ozone and NOx levels.)
So, like in the naturally “polluted” areas of the Great Smoky Mountains and forests of the Southest US, the high Texas plains have a little bit added Petroleum evaporation and enough natural ozone to put the region out of spec.
Now that you mention it I remember, I think it was back in the Reagan administration, that the EPA was fining some state or city for ozone that turned out to come from VOCs that pine trees put out.