ACR MLG TKOF IN RVR RPTED BELOW RWY MINS FOR TKOF.

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HAD BEEN MONITORING ATIS AND TWR. FOG WAS FORMING AT DAWN. RWY 19 TOUCHDOWN RVR WAS 6000 PLUS; DEP WAS 1800 V 1200. TKOF MINS WERE 6;6;6. WHEN WE CALLED TO TAXI; WE WERE GIVEN RWY 9; RVR 1800. MINS ARE 1600. A TURBOPROP COMMUTER WAS LNDG AS WE APCHED THE HOLD SHORT. WHEN THE COMMUTER TAXIED CLR; WE WERE TOLD; CLRED FOR TKOF; RVR 1200. I KNEW THE MINS FOR 19 AND HAD PLANNED TO USE THAT RWY. RWY 9 CAUGHT ME BY SURPRISE; IT WAS A SHORTER TAXI AND GOT US GOING IN THE RIGHT DIRECTION. I ASKED THE FO TO CHK TKOF MINS. I SHOULD HAVE STOPPED AND CHKED THEM MYSELF. WE WERE THE FIRST TO TKOF ON RWY 9. THE LNDG AIRPLANE APPARENTLY DISTURBED THE AIR ENOUGH TO CHANGE THE READING. THE FOG WAS VERY THIN; MAYBE 20 FT THICK. IT DOESN'T CHANGE THE SITUATION. I CALLED THE TWR ON THE LANDLINE LATER. THEY SAID THEY DON'T KNOW WHAT THE MINS ARE; ESPECIALLY FOR THE DIFFERENT CARRIERS. THE FAA SETS THE MINS. HOW COULD THEY NOT KNOW? I FEEL THAT I COULD HAVE BEEN SET UP; GIVEN A TEST AND DIDN'T DO REAL WELL. IT WAS MY OWN FAULT FOR NOT BEING MORE AWARE OF MINS. THIS WAS MY FIRST LOW VISIBILITY TKOF THIS WINTER. I NOW FEEL MORE UP TO SPD.