[thinkpad20]

The number for Chicago doesn't strike me as accurate. I'm in the area right now and it's a clear day. Plus, the levels for nearby areas in Chicago are far lower (80 or so) and the historical levels are much lower. Looks like a blip. That said, I'm sure Chicago is not the clearest city in general.

[tokenadult]

I live upwind of Chicago (by prevailing winds) in Minnesota, and we have read in our weather reports that forest fires in Canada have added a lot of particulate matter to our air. The sunsets here have been spectacularly red in recent days. I suppose checking those city-by-city reports of measured pollution for long-term trends and recent spikes might help figure out how unusual Chicago's current conditions are, as you thoughtfully suggest.

[buro9]

A quick Google and Wikipedia visit says that pm2.5 fine particulates of the kind reported as being extremely high in some parts of Chicago, are not visible except as a light haze over the city.

It appears to be true that you can have a bright mostly clear day and very high levels of fine particulate pollution.

This haze is part of what contributes to smog when combined with other more visible (course particulates) pollution. But these fine particulates don't appear to be the product of fireworks as suggested below, and are more likely to be the product of the coal power stations.

The measure for Chicago is probably skewed by one of the measuring stations being nearer to, or down-wind from, a coal power station.

[maxerickson]

Why do you assume that the sampling methodology and calibration is ridiculous?