Piedmont Natural Gas said it found the source of the smell that spread around Charlotte Thursday morning. A company destroyed tanks of mercaptan, the chemical used to add the sour smell to odorless natural gas, PNG said.  

The company said there was no natural gas leak in its system.

The fire department said the city was getting a lot of 911 calls about the smell of gas. The city asked people not to call 911 unless there's an emergency.

"There is no need to report this order. Unless you have a medical emergency or believe the gas is coming from your home or building - please call 911," the fire department said on Twitter.

The smell prompted the county to evacuate the courthouse in Uptown Charlotte, but the building has since been cleared by the fire department, according to the Mecklenburg County Sheriff's Office.

The smell of gas spread around the city through a weather event called "inversion," which is trapping air close to the ground.

"A temperature inversion occurs when a thin layer of cool air develops under a warmer layer of air. This can occur in the lowest levels of the atmosphere on a mostly clear night," said Spectrum News 1 Meteorologist Lee Ringer.

"Fog or smoke if there's a fire nearby can be trapped near the ground by an inversion until the atmosphere mixes out, which typically happens through the daytime hours," he said.

The city sent an emergency alert to cellphones Thursday morning: "Charlotte Fire Department is experiencing a high 9-1-1 call volume related to natural gas odor in the city. A weather inversion is making the odor easier to smell, and we are continuing to investigate the source of the odor. There is no need to report this odor. If you have a medical emergency, then call 9-1-1."