Which respirators and filters do I buy for protection against Yellowstone volcanic ash?

If a volcanic-ash cloud were to fly over Chicago, I would pretty much stay indoors all the time, EXCEPT for the following:

1) I would periodically go outside, climb up to the roof, and sweep the ash off the roof.

2) I would periodically go to stores to buy food (assuming food is available).

I would need to wear a respirator only when going out to sweep the ash or to buy food. So, how many disposable respirators should I buy? For how long will the ash cloud hover over Chicago?

Tie piss-soaked underwear around your face. The ammonia will cut the stench of volcanic ash and rotting bodies while affording you a natural stimulant as you traverse the barren wasteland.