The summer air pollution season is off to an early start, with moderately unhealthful air reported in the Baltimore-Washington area yesterday and more pollution forecast for today and tomorrow.
The year's first smoggy day developed after ozone mixed with high temperatures and still air.
Ground-level ozone, a byproduct of fossil fuel burning, comes from industrial smokestacks, automobile tailpipes and many other sources. In hot weather, it "cooks" in a chemical process that forms smog.
Midafternoon ozone levels reached 90 parts per billion yesterday -- high enough to cause breathing problems for asthmatics and the elderly, but far lower than the "code red" pollution levels.
Today's air quality should be about the same, and there could be patches of slightly heavier smog tomorrow, forecasters said.
Ozone experts blamed yesterday's "code yellow" conditions on pollution blown in from coal-burning power plants and smokestacks in the Ohio Valley.
On Monday night, pollution monitors detected higher-than-normal ozone levels over Shenandoah National Park in Virginia. The pollutants moved east early yesterday, said University of Maryland meteorologist Bill Ryan.
The ozone met a weather pattern perfect for forming smog.
"You can think of the atmosphere as a great big box -- and the smaller the box, the more ozone you've got," Ryan said.
The lid on the box: a ridge in the upper atmosphere that created a downward flow, preventing the pollutants from rising. The sides: a breezeless day that kept smog from blowing out to sea.
It's a bit unusual to see climbing ozone levels this early in the season, Ryan said.
But since 1999, when the Maryland Department of Environment moved the start of the ozone season from May 15 to May 1, elevated ozone levels have been common during those weeks, said MDE air quality expert Randy Mozier.
In 1999, May 1 through 13 had code yellow, moderately unhealthful air. Last year, the area endured two code red days in early May, Mozier said. On code red days, most people suffer shortness of breath and burning eyes.