What was found
WITH RT RECIRC FAN OFF HAD CHEMICAL SMELL SIMILAR TO CHLORINE, THEN HAD HOT ELECTRICAL BURNING SMELL WITH RECIRC FAN BACK ON. THIS CONDITION OCCURRED ON THREE SEPARATE OCCASIONS. COULD NOT ISOLATE SOURCE. DIVERT YJT, EMERGENCY DECLARED, OVERWEIGHT LANDING. *S/D* CHEMICAL AND/OR ELECTRICAL SMELLS COULD NOT BE DUPLICATED ON THE POSITION FERRY FLIGHT TO IAD. IAD MAINTENANCE COULD NOT DUPLICATE THE CONDITION LISTED ABOVE. AIRCRAFT HAS NOT EXPERIENCED A REPORT OF CHEMICAL OR ELECTRICAL SMELL SINCE THIS OCCURANCE. SMELL MAY BE NON-AIRCRAFT RELATED.