A tentative understanding reached last week between Santa Barbara County and Exxon Corp. over a proposed $2-billion oil and gas processing plant at Las Flores Canyon fell apart after county supervisors called for tougher air pollution controls. Exxon had agreed to 181 separate conditions. But the board’s 3-2 vote in favor of extra controls led Exxon to threaten to conduct all operations in federal waters outside the county’s jurisdiction by doubling the size of an existing offshore storage and oil treatment plant. “You can stick by your agreement or you can stick it in your ear,” Exxon environmental manager Don Cornett told supervisors before walking out of the hearing. He said the company would move immediately to obtain federal approval for expanding the offshore facility. The California Coastal Commission has previously said such an expansion would violate the local coastal plan. But Commerce Secretary Malcolm Baldrige can override the commission.