By http://www.aeroinside.com/item/4334/delta-md88-near-knoxville-on-jul-9th-2014-smoke-in-cockpit

A Delta Airlines McDonnell Douglas MD-88, registration N949DL performing flight DL-770 from Orlando,FL to Cincinnati,KY (USA) with 52 passengers and 5 crew, was enroute at FL370 about 25nm southeast of Knoxville,TN (USA) when both flight crew donned their oxygen masks, declared emergency reporting smoke in the cockpit and decided to divert to Knoxville. On approach to Knoxville’s runway 23R the crew advised that the smoke appeared to be dissipating after they had turned off some of the equipment, but continued to use their oxygen masks. The aircraft landed safely on runway 23R and stopped on the runway, the crew removed their oxygen masks and requested emergency services to check the aircraft from the outside for any indications of smoke or abnormality explaining they had a smoke event. Emergency services reported no trace of fire, smoke or heat and advised they would follow the aircraft to the gate. The aircraft taxied to the apron with emergency services in trail. The crew reported 57 people on board to emergency services, the airline reported 41 passengers and 5 crew were on board of the aircraft. A replacement MD-88 reached Cincinnati with a delay of 2:45 hours