The air in SF is very clean. The prevailing winds come from the very-wide pacific ocean and blow all the pollution across the Bay into Oakland and Berkeley.

It does however have a fog problem, because those nice clean prevailing winds are quite cold and moisture condenses rise when encountering the hills along the coast. This is in fact the major source of water for the coastal giant sequoias.

"Coast redwoods (Sequoia sempervirens) are distributed along a narrow band of California's northern coast. During the summer these red giants take advantage of the fog to capture water out of the air—and summer is the critical growing season for the trees, despite being California's dry season."

http://www.scientificamerican.com/article/fog-that-nourishes...