A HELI PLT ENCOUNTERS ICE FOG AFTER TKOF. PLT DECLARED AN EMER TO RETURN.

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DEPARTED ARPT ON AN SVFR CLRNC; CEILING 300 FT; VISIBILITY 1.5 SM. TOLD TO EXPEDITE DEP FOR IFR TFC. FLEW OVER RIVER; SE OF AIRFIELD; TO INTERCEPT SHORELINE AND PROCEED EBOUND ALONG SHORELINE. NOTICED THAT MOST OF CITY WAS COVERED BY DARK AIR; RESEMBLED A SNOW SHOWER. BUT THERE WAS NO SNOW; SO IT MUST HAVE BEEN SMOG. MAINTAINED LOW PWR AND AIRSPD AT 150 FT. INTENDED TO RE-EVALUATE FLT UPON REACHING SHORELINE. NEVER MADE IT. WENT INADVERTENT IMC BEFORE GETTING THERE. NOW WE WERE OVER AREA SURROUNDED BY DWELLINGS; PWR LINES; CONSTRUCTION CRANES; AND STORAGE TANKS. IMMEDIATELY TURNED TOWARD FIELD BUT UNABLE TO REACQUIRE. TURNED SW AND CLBED TO 300 FT; EXECUTED LCL INADVERTENT IMC PROC. AT APPROX 1 SM S OF FIELD; GAINED VISUAL VIEW OF SURFACE ICE. DSNDED BUT UNABLE TO SEE ANYTHING AHEAD. LANDED AND ADVISED ATC. AFTER ABOUT 20 MINS; THE VISIBILITY IMPROVED SO THAT I COULD SEE THE SHORELINE; UTILITY POLES; TANKS; ETC. I IMMEDIATELY ADVISED ATC AND REQUESTED CLRNC. IT WOULD TAKE ME 1 MIN TO TAKE OFF AND LAND ON THE S RAMP. ATC UNABLE; HAD 7 IFR ACFT. AFTER ABOUT ANOTHER 20 MINS; THE VISIBILITY BEGAN TO DETERIORATE AGAIN; CONTRARY TO FORECAST. I ADVISED ATC AND TOLD THEM I NEEDED TO DEPART NOW OR I MIGHT BE STUCK ON THE ICE. I HAD TO STAY AT FULL RPM; AND THERE WAS NO ASSURANCE THAT THE WX MIGHT IMPROVE. THEY COULD NOT ASSIST; SO I DECLARED AN EMER AND TOOK OFF FOR THE APCH END OF RWY X (RWY Y WAS ACTIVE). I CROSSED THE UTILITY POLE AND COULD NOT SEE THE RWY OR NEAREST HANGAR (LESS THAN 0.2 NM DISTANT). I FOLLOWED THE SHRUBS UNTIL I COULD SEE THE RAMP; THEN LANDED AND CANCELED. NO HAZARD TO OTHER ACFT. NEVER DECLARED AN EMER BEFORE IN 28 YRS OF AVIATION EXPERIENCE. ICE; FOG; VISUAL RESTR WAS INTENSE AND SET UP RAPIDLY.