Maine air quality today

Maine’s highest coastal ozone hour often falls in the evening, which is backwards from how the pollutant behaves in most places. Maine DEP’s explanation is transport: ozone and its precursors move out of the New York metropolitan region, across Connecticut and Massachusetts and into the Gulf of Maine, and the timing of the peak progresses west to east with the air mass. Near the New Hampshire border it tends to arrive midafternoon. Near Mount Desert and beyond, early to late evening. Cooler water keeps that pollution in a shallow layer near the surface and pulls the wind onshore, which is why DEP’s alerts are regularly issued for the coast alone.

Elevation splits the day again. DEP added a high-elevation region to its forecast for sites like the ozone monitor on Cadillac Mountain in Acadia National Park, where levels do not fall off after sunset the way they do nearer the ground and frequently peak after 10 p.m., while lower sites are already dropping.

The Bureau of Air Quality does something many state agencies skip. Its meteorologists submit a forecast to EPA’s AirNow every day of the year rather than only through ozone season, and distribute it by a 24-hour toll-free hotline and through EnviroFlash email and text alerts. Maine has few metros to begin with, so a short list in the table above is the state’s settlement pattern and our build queue arriving at the same answer, not evidence that the ground between those metros was checked.