That 'cloud of dust' is just as likely the edge of the smog.
The north winds typically enter the basin thru Cajon and Newhall Passes.
When they are sufficiently strong the San Gabriel and Pomona Valleys will be cleared, in time.
When not, the edge of smog may hang roughly on the I5 alignment on the east edge of the SFV,
or near the Arroyo Seco, leaving Pasadena still and smoggy.
Hmm I left on AMtrak to Salt Lake City 35 years ago on a gloomy , dirty pre-thanksgiving afternoon at LAUPT and crossed into the Cajon Pass Wind Zone around Mira Loma- where I could see the wind driven brush fires already ravaging the hills above Ono