[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?