Purple line is typical summertime conditions in Boulder.
Green line is conditions measured during Flagstaff fire event.

The Flagstaff fire started as the result of a lightning strike less than 2 miles (as the crow flies) from Boulder. The proximity to Boulder and the timing (it began shortly after the Waldo Fire which destroyed so many homes near and in Colorado Springs) resulted in fear of conflagration within Boulder city limits. Fortunately, no homes were burned and most residents were only affected by the smoke which was severe for several days.

This image shows the smoke plume at the start of the fire on June 26, as observed from the NOAA building in Boulder.
This image (from boulder.kdvr.com) shows an overnight exposure of the Flagstaff fire.