In a decision that could affect air pollution policy nationwide, a federal judge ruled yesterday that California officials can order public agencies to buy cleaner vehicles. Two industry associations challenged a rule by the South Coast Air Quality Management District that requires agencies to purchase low-emission or alternative-fuel cars, buses, and trucks, instead of diesel-powered vehicles. They argued that the new rule violated the U.S. Clean Air Act, which forbids states from putting emissions standards on new vehicles. But U.S. District Judge Florence Cooper said the rule was legit, because it regulated only the purchase of cleaner vehicles by public agencies, not the production or sale of the vehicles by auto manufacturers.