What was found
FLT 4028 - AFTER TAKEOFF, CREW SMELLED SOMETHING BURNING. CAPTAIN TURNED AUTOPILOT ON AND TURNED ON COCKPIT READING LIGHTS AND SAW THIN WHITE SMOKE. F/A REPORTED BURNING SMELL, BUT SAW NO SMOKE IN CABIN AFTER INSPECTION. CREW UNABLE TO IDENTIFY SOURCE OF BURNING SMELL. CAPTAIN TURNED RECIRCULATING AND GASPER FANS OFF AND CLOSED EYEBALL VENTS. CREW DECLARED EMERGENCY AND RETURNED TO ATL WHERE AIRCRAFT LANDED SAFELY. MAINTENANCE REMOVED AND REPLACED THE LEFT PACK GROUND COOLING FAN. OPERATIONAL CHECKS WERE GOOD. AIRCRAFT RETURNED TO SERVICE. (M)