here is a compilation I made for particulate contamination data for
East Bay in the Bay Area. If you can find data for your area, a
compilation like this can give you new clues. I dumped  raw data for ea
month into excel. I used 0 to separate months from ea other.

This is for particles <2.5 microns (PM2.5). The units are PPM.
source: http://www.baaqmd.gov/

Dec, Jan, and Feb are the worst months

A bad day vs a good day can be a factor of 3-5 difference in PPM.

Different parts of the bay area (distant by ~10 miles) can be as
different as a factor of 3 (not shown). Being in a populated valley
(e.g. Napa) on a bad day is deadly.

Need to check if there is a correlation with the temperature and the
strength of wind. E.g. some winter days were not bad at all, wondering
why.