WASHINGTON, D.C. -- The United States Supreme Court ruled on April 28 that a Southern California air quality management district may have gone too far in imposing its own anti-smog rules for city buses, airport shuttles, and other vehicles. Justices voted 8-1 on the case involving the South Coast Air Quality Management District. The nation's high court sided with oil companies and diesel engine manufacturers who claimed that local pollution rules conflict with national standards. The San Francisco-based Ninth U.S. Circuit Court of Appeals upheld the tougher local rules, but the decision was voided by the high court. The Supreme Court sent the case back to California to consider the issues. Supreme Court Justice Antonin Scalia, writing for the court, said that the emissions rules appear to be blocked by the federal Clean Air Act. “If one state or political subdivision may enact such rules, then so may any other; and the end result would undo Congress's carefully calibrated regulatory scheme,'' he wrote. Justice David H. Souter filed the only dissent. Souter, of New Hampshire, said he disagreed that the Clean Air Act “prohibits one of the most polluted regions in the United States from requiring private fleet operators to buy clean engines that are readily available on the commercial market.'' The rules apply to Los Angeles, San Bernardino, Riverside and Orange counties, which together have the nation's worst air-quality problem. The restrictions were imposed in 2000 and apply to fleets of vehicles such as buses, waste haulers and others. The Clean Air Act gives states some authority to set their own rules. At issue in the case were local standards.