Hi Rich,
Good question - Los Angeles is in a geographic basin, we are surrounded by mountains on all sides - except for the side with the pacific ocean, and as such it's a natural trap for pretty much everything. My understanding is that smog is not a new phenomenon, apparently it was present due to the fires from Native American settlements long before the white man settled the area. Because of the geography, it really is possible to surf in the morning and ski in the afternoon - I confess to never having surfed, and it's been a while since I skied, but I could if I wanted to...