SMA PLT DEPARTS ARPT CLOSED DUE TO NEARBY FIRE FIGHTING ACTIVITY.

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(HAILEY/SUN VALLEY ARPT) WHILE TAXIING OUT TO THE ACTIVE RWY GIVEN BY GND CTL; I WAS INFORMED THAT THE ARPT WAS CLOSED -- DUE TO BRUSH FIRES TO THE W AND SW -- 1 1/2 TO 2 MI DISTANT. THE SKY ABOVE THE ARPT AND TO THE E WAS CLR. THERE WAS SOME HIGHER SMOKE TO THE W AND 3-4 MI TO THE SW; S THERE WAS THIN SMOKE AT OVER 1000 FT ABOVE THE GND. WHILE TAXIING OUT A SINGLE ENG ACFT DEPARTED AND THERE WERE 2 TWINS WAITING TO DEPART. THEY WERE BOTH TALKING WITH THE TWR. THE PRIMARY POINTS MADE BY THE TWR WERE THAT THE ARPT WAS CLOSED AND THEY COULD NOT GIVE DEP APPROVAL. THEY SAID THAT ANY ACFT DEPARTING NOW WOULD DO SO 'AT THEIR OWN RISK;' AND IF THERE WAS ANY CONFLICT WITH ACFT FIGHTING THE FIRES; THEY WOULD BE RPTED. AFTER FURTHER DISCUSSIONS WITH THE TWR; BOTH TWIN ACFT DEPARTED AND I IMMEDIATELY FOLLOWED. I KEPT TO THE E SIDE OF THE VALLEY TO AVOID ANY POSSIBLE PROX TO ANY HELI HAULING WATER FROM THE BIG WOOD RIVER ON THE W SIDE. THERE WERE 2 BORATE TANKERS HOLDING ABOUT 15-20 MI W -- WAITING DROP DIRECTIONS. A FEW MINS AFTER DEP; THE TWR WAS CONTACTED BY AN ARRIVING ACFT. FOR THE FIRST TIME; THE TWR SAID THAT THE ARPT MGR WAS RECORDING 'N' NUMBERS AND WOULD BE SENDING THEM TO BOISE. AFTER A FEW MORE MINS; APPARENTLY; THE ARPT WAS RE-OPENED FOR DEP TO THE N AND ARRS TO THE S. AFTER DEP; I REMAINED IN VFR CONDITIONS AT ALL TIMES AND WAS ABOVE THE LIGHT SMOKE AT 2000 FT AGL. IT SEEMS MOST LIKELY THAT I SHOULD NOT HAVE DEPARTED FROM A CLOSED ARPT. I WAS DEPARTING FOR A 5 HR NON-STOP FLT TO TUCSON; AR. THE FORECAST WAS FOR POSSIBLE INTENSE TSTMS IN MID TO LATE AFTERNOON -- SHORTLY AFTER MY INTENDED ARR TIME. THIS MADE ME UNDULY ANXIOUS TO DEPART. IN RETROSPECT; I MADE A FOOLISH DECISION TO HASTILY DEPART UNDER THE CURRENT CONDITIONS. I COULD HAVE CANCELLED AND MADE THE FLT 2 WKS LATER -- ALTHOUGH APPARENTLY I MIGHT HAVE BEEN ABLE TO LEGALLY DEPART 30 MINS LATER. I'M AFRAID THAT I LET THE OTHER 3 DEPARTING ACFT INFLUENCE MY DECISION TO DEPART. THE TWR IS OPERATED BY A PRIVATE CONTRACTOR. IF THEY MIGHT HAVE SPECIFICALLY STATED THAT DEP AT THE TIME WOULD BE AN FAA VIOLATION; I BELIEVE THAT NONE OF THE 4 ACFT WOULD HAVE DEPARTED. IF THIS SITUATION SHOULD EVER OCCUR TO ME AGAIN; I WILL REMAIN ON THE GND UNTIL I AM DIRECTED BY THE TWR TO DEPART. (I INCIDENTALLY HAVE RECEIVED NO WORD FROM THE FAA AS OF THIS DATE).