It begins with airborne pollution pushing in from the Bay Area, primarily sourced from transportation, electricity generation and manufacturing (especially refineries). Most of the year it turns right and heads south, into the San Joaquin Valley. Second is locally produced pollution--a combination of vehicles, manufacturing, electricity generation, agriculture and natural sources. Much of the year high pressure is parked above the region, creating an inversion and trapping all the crud.
In winter, it's particluates and in summer, ozone--the Central Valley is a non-attainment region for both. Interestingly, summertime ozone also pushes well into the mountains, creating health hazards and stressing forests.
This will be a big summer for forest fires, so that will be an added pollution source as well.