Calls of a mysterious odor wafting through Delaware County began coming into the Delaware County Office of Emergency Management Monday morning.

Most described it as a burning smell, or a bug spray.

Though the specific source of the odor hasn’t been clarified, there is a reason now why the odor was there, writes Chris O’Donnelll for Fox 29 News.

The foul odor came from a “temperature inversion” according to Delaware County officials.

That was confirmed after an inspection team arrived Tuesday in the county from the Pennsylvania Department of Environmental Protection, according to Regional Communications Manager Stephanie Berardi.

A temperature inversion is a stable air mass where the cooler air is near the earth’s surface and warmer air is on top, according to the National Weather Service.

It usually happens in the late afternoon, early evening and lingers into the next morning for a few hours.

Since warm air rises, air under the inversion cannot escape because it is cooler.   Pollutants from vehicles, wood burning and industry become trapped, triggering the odors and creating poor air quality.

Officials had a tough time pinpointing the source. The smell began to dissipate by Tuesday, and no new reports have been made.

Read more about this weather phenomena and Delaware County’s odor problem at Fox 29.