Arco will build a $250-million plant in France.

Los Angeles-based Atlantic Richfield Co. said its Arco Chemical Europe subsidiary has begun engineering work on the plant at Fos-sur-Mer, with construction expected to begin in mid-1986. The plant will produce 180,000 tons per year of propylene oxide, a chemical used to produce polyurethane foam and other products, and 430,000 tons per year of tertiary butyl alcohol, a solvent used in blending methanol with gasoline.