EVASIVE ACTION DSCNT. ACR CREW IS FORCED TO DSND OVER HIGH TERRAIN TO AVOID ACFT USING TCASII.

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DEPARTED ONT ARPT RWY 26L AT XA50 LCL TIME WITH CLRNC TO TURN DIRECT PDZ VOR AND CLB TO 7000 FT (PRADO 4 DEP SID). UPON CONTACTING DEP CTL (ONT DEP) CLRNC CHANGED TO LEVEL AT 4000 FT (WE WERE ALREADY THROUGH 3000 FT WHICH MEANT AN ABRUPT LEVELOFF). WE ALWAYS TRY TO CLB OUT AS FAST AS POSSIBLE BECAUSE OF THE 3 LIGHT ACFT ARPTS (CHINO; CORONA MUNICIPAL; RIVERSIDE MUNICIPAL) AND NUMEROUS LIGHT ACFT IN THE PDZ VOR AREA. ALSO; TYPICALLY DURING DAYLIGHT HRS THE VISIBILITY IS EXTREMELY LIMITED FACING S IN SMOGGY CONDITIONS ABOUT 4 MI VISIBILITY IN HAZE AND SMOKE; BUT LESS FLYING S TOWARD THE SUN. WE PICKED UP NUMEROUS TARGETS ON OUR TCASII AND WERE EXTREMELY BUSY TRYING TO VISUALLY IDENT THEM. ONE TARGET WAS AT OUR ALT AT 12 O'CLOCK; 4 MI WHEN ATC INQUIRED AND TOLD HIM HE SHOULD BE AT 3000 FT INSTEAD OF 4000 FT. ANOTHER WAS THE TFC WE WERE BEING HELD DOWN FOR AT 1 O'CLOCK AT 5000 FT AND ADDITIONAL TFC ON BOTH R AND L WITHIIN 500 FT 2 MI. WHILE TRYING TO KEEP ALL THIS TFC IN SIGHT (HARD TO SEE ANY WITH POOR VISIBILITY) WE GOT AN RA TCASII TO DSND FROM TFC ABOUT 1;30 O'CLOCK AND CLOSE; LESS THAN 1 MI. WE IMMEDIATELY DSNDED TO ABOUT 3100 FT TO STOP THE RA WHICH PUT US ONLY 1600 FT ABOVE THE GND AT PDZ VOR; ELEVATION 1495 FT DODGING TERRAIN AND TFC IN LOW VISIBILITY. THIS IS AN EXTREMELY BAD PLACE TO PUT JET ACFT INCOMPATIBLE WITH LIGHT ACFT TFC CONGESTION IN POOR VISIBILITY. WE SHOULD BE KEPT ON GND INSTEAD OF A LOW LEVELOFF IN THIS AREA. PERHAPS A R TURNOUT TO A CLBING R DOWNWIND BEFORE BEING TURNED TO THIS AREA. IT IS EXTREMELY DANGEROUS TO KEEP JET TFC THIS LOW IN THIS CONGESTED AREA. CTLRS SHOULD DO EVERYTHING POSSIBLE TO NOT LET THIS HAPPEN AGAIN. ALSO; CTLR SEEMED TO HAVE AN ALT AND WAS NOT HELPFUL. THIS CREATES DANGEROUS SITS FOR HIS CESSNAS AS WELL AS AIRLINE TFC.