Illinois air quality today

The monitors that decide Illinois’s ozone status sit where the air is coldest. Lake Michigan stays cool well into early summer while the land beside it heats, and the contrast drives a lake breeze running roughly perpendicular to the shoreline. Air that leaves the Chicago area out over the water keeps reacting in sunlight above a shallow stable layer that will not let it mix downward, then returns onshore in the afternoon. That is why near-shore sites can post higher ozone than points well inland on the same day, and why a hot inland reading is a poor guide to what the lakefront is doing.

The regulatory geography follows the wind rather than the state line. The Chicago nonattainment area for the 2015 ozone standard is written as Illinois-Indiana-Wisconsin, one of several areas ringing the lake designated for it. The 2017 Lake Michigan Ozone Study, a multi-agency campaign involving EPA, NASA, NOAA and the Lake Michigan Air Directors Consortium, put two aircraft and a research vessel alongside the ground network, because a fixed monitor on land cannot describe air that spends half its day offshore.

Illinois is also long. From the lakefront to the Ohio River at Cairo the state crosses several hundred miles of farmland, coalfield and river industry, none of which shares that mechanism. The Bureau of Air at the Illinois EPA handles monitoring, permitting and enforcement across all of it. We have written up Chicago and nothing else in the state, so Rockford, Peoria, Springfield and the Illinois side of the St. Louis metro go unexplained here.