This is largely because the chemicals that form ozone are blown inland during the day, and are trapped against the mountains, as a previous commenter suggested.
You can confirm this by checking out the data provided on the AQMD website. You will see consistently higher levels of ozone in Pasadena in the afternoons. It is higher in summer because strong sunlight helps to create more ozone.
When you see news reports of the smog levels it appears it's always worse in the Valley then in the basin. It seems to be worse in Summer (no clouds to make it acid rain). I also think it just gets trapped up against the mountains.
I think it's worse.
Look, I don't know. Of course, I live and work in Glendale, right smack between Pasadena and the Valley. All I can say is, even being "above" the masses, in Glendale we call it "the Glendale grime"
the AQMD recently reported that all of the basin is "unhealthful".
Sometimes it is a matter of terms.
I'd like to share more but the Tulia mavens won't let me.