A few points to echo wdolson's fine essay:
I live in (cough, cough) Fresno. Some of the air pollution also comes from China, believe it or not. The prevailing wind blows east across the Pacific and through Altamont Pass or the delta into the Valley. Farming and trucking also contribute to our poor air quality. Farmers still use old, decrepit diesel tractors and pump motors. Seasonal truckers of ag products also have 1980s diesel motors in many of their tractors. Almond, walnut and pistachio harvest consist of shaking trees, sweeping the nuts into rows and then scooping them up into gondolas behind trucks for hauling to the huller/processor. A lot of dust is in the air from August through October.
The Sacramento-San Joaquin Valley is a U-shaped valley (formed by glaciation) that extends from Shasta Lake on the north to the Grapevine on the south. We are surrounded by mountains: The Sierra Nevada Range to the east, the transverse ranges to the south and the coastal ranges to the west. The north is a mixture of the Cascade Range and Trinity Alps. The only exit for our heat is through the delta. And since the prevailing winds are from the west, the pollution cannot escape. It just gets pushed further north and south. As a result, when the temperatures rise in late April-early May, we get an inversion layer that hovers about 1,200-1,500 feet above the valley floor, beneath the elevations of the surrounding mountains. It rises slightly at night when the sun sets, but typically anything that is in the air remains trapped within this inversion layer. Cities closer to the delta do receive some respite from the pollution and sweltering heat, but the further one is from the delta, the worse the temperatures and air pollution are.
On those rare winter days after a good rainstorm, and there is a zephyr to keep the particulate matter moving about, the snow-capped Sierra Nevada mountains are truly a beautiful sight. We might be able to see the mountains from our home 20-30 days per year. The rest of the year they are obscured by all the crap in the air.