Smoke can sure hang in the air for long periods of time . Here in Texas each spring we have about a week of similar days with smoke coming up from Mexico when the farmers down there are burning off the fields prior to planting their spring crops. Those day are definitely days for being inside.
I'm surprised that kind of pollution wasn't addressed in any of our treaties with Mexico.
It only happens once a year for a about 4 or 5 days, not really worth a treaty in the grand scheme of things when there are more important issues to be discussed I am pretty sure.