Maryland air quality today

One monitor decides Baltimore’s ozone status. The Edgewood site in Harford County, on the western shore of the Chesapeake between the Bush and Gunpowder rivers, carries the highest eight-hour ozone design value in the Baltimore nonattainment area, which makes it the monitor the designation turns on. Its position is the whole point: northeastern Maryland sits exactly where a particular piece of local weather does its work.

That weather is the bay breeze. Water stays cooler than land through the warm months, so a shallow onshore flow develops in the afternoon and pushes air that has been sitting over the Chesapeake — including emissions carried out of Baltimore earlier — back onto that one shoreline while the rest of the state is spared. On June 25, 2009, ozone at Edgewood rose as the breeze strengthened to a daily maximum eight-hour value of 109 parts per billion. A regional problem can compress into a strip a few miles wide here, which is why a single statewide Maryland figure explains so little.

The state is also long in the wrong direction for averaging. Garrett County in the far west is Appalachian plateau; the eastern shore is flat farmland and tidal marsh at sea level; the Baltimore-Washington corridor between them holds most of the population and most of the traffic. The Maryland Department of the Environment publishes a daily air quality forecast, which is a reasonable response to a state that changes character every eighty miles.