SYR LCL CTLR EXPERIENCED OPERROR WHEN ISSUING TKOF CLRNC FROM CLOSED RWY.

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AT SYR ARPT; NY; I WAS WORKING LCL AND GND CTL COMBINED. AN AEROSTAR CALLED AND REQUESTED TO TAXI. I CLRED THE PLT TO TAXI TO RWY 28; THE CURRENT ACTIVE RWY. WHEN HE REACHED A XING RWY 33; HE REQUESTED A DEP FROM THIS RWY; WHICH HAPPENED TO BE CLOSED FOR ABOUT 30 MINS PRIOR FOR MAINT. THIS RWY CLOSURE WAS BROADCAST ON THE ATIS. WHEN HE REQUESTED RWY 33; I RECLRED THE PLT TO TAXI TO RWY 33; THEN HE CALLED READY; AND I CLRED HIM FOR TKOF RWY 33. I HAVE THESE CONCLUSIONS WHICH COULD HELP PREVENT THIS ERROR FROM OCCURRING AGAIN IN THE FUTURE. AS A CTLR; I WAS RESPONSIBLE TO NOT ALLOW ANY OPS ON THIS RWY. I WAS BRIEFED CORRECTLY BY THE PAST CTLR; AND HAD A RED RWY CLOSURE STRIP IN MY STRIP BAY. THOSE TOOLS WERE UTILIZED; BUT WHEN THE PLT REQUESTED THIS RWY; I SPONTANEOUSLY ISSUED THIS CLRNC. MORE AWARENESS BY MYSELF; AND ADDITIONAL CTLR STAFFING SO THE POS WASN'T COMBINED DURING A TIME PERIOD WHICH MAY HAVE MODERATE TFC MAY DECREASE THE POTENTIAL OF A FUTURE ERROR. INCREASED AWARENESS BY THE PLT; SINCE HE HEARD THE ATIS STATING THE RWY CLOSURE WOULD ALSO DECREASE RECURRENCE. ALSO; A BARRIER SIGN PUT UP BY ARPT PERSONNEL; WHICH IRONICALLY SHOWED UP AFTER THIS SITUATION; WOULD HAVE ALMOST DEFINITELY FLAGGED THE PLT THAT THE RWY WAS CLOSED.