DHC8 LANDS IN WX BELOW MINIMUMS AFTER HAVING SMOKE IN THE COCKPIT.

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ENTERING HOLDING OVER OM; ILS RWY 27 TLH; DUE TO RVR 1400-1000 FT; WE ENCOUNTERED SMOKE IN THE COCKPIT. WE CLRED THE SMOKE AND DETERMINED IT RESULTED FROM A BLEED AIR PROB AND SHUT BLEEDS OFF. DUE TO DETERIORATING WX IN TLH AND EVERYWHERE ELSE WITHIN 1 HR FLT TIME I ELECTED TO DO A BELOW MINIMUMS LNDG AT TLH. I FELT THAT IF THE PROB RETURNED; LNDG ANYWHERE ELSE WAS GOING TO GET MORE DIFFICULT DUE TO INCREASING FOG. THE FOG IN TLH SEEMED TO BE THICKEST OVER THE GRASS BESIDE THE RWY (OVER TRANSMISSOMETER) AND THE RWY WAS ACTUALLY ONLY COVERED BY A VERY THIN LAYER FOR THE FIRST 3000 FT. WE DID NOT SAY 'DECLARING EMER' AS WE FELT WE WERE BEING TREATED AS AN EMER FROM THE TIME WE RPTED SMOKE. WE BELIEVED FROM OUR 1ST RPT OF SMOKE THAT TLH CONSIDERED OUR FLT TO BE IN AN EMER SIT AND BOTH THEY AND I ACTED ACCORDINGLY. SUPPLEMENTAL INFO FROM ACN 351487: WE FLEW THE ILS RWY 27 APCH WITH THE RPTED RVR AT TOUCHDOWN 1400 FT AND ROLLOUT 1000 FT. AT 200 FT AGL THE APCH LIGHTS WERE CALLED IN SIGHT. AT 150 FT AGL THE RWY AND ENVIRONMENT WERE IN SIGHT AND WE LANDED WITHOUT FURTHER INCIDENT.